Showing papers in "Behavioral Ecology in 2004"

A farewell to Bonferroni: the problems of low statistical power and publication bias

Abstract: Recently, Jennions and Moller (2003) carried out a metaanalysis on statistical power in the field of behavioral ecology and animal behavior, reviewing 10 leading journals including Behavioral Ecology. Their results showed dismayingly low average statistical power (note that a meta-analytic review of statistical power is different from post hoc power analysis as criticized in Hoenig and Heisey, 2001). The statistical power of a null hypothesis (Ho) significance test is the probability that the test will reject Ho when a research hypothesis (Ha) is true. Knowledge of effect size is particularly important for statistical power analysis (for statistical power analysis, see Cohen, 1988; Nakagawa and Foster, in press). There are many kinds of effect size measures available (e.g., Pearson’s r, Cohen’s d, Hedges’s g), but most of these fall into one of two major types, namely the r family and the d family (Rosenthal, 1994). The r family shows the strength of relationship between two variables while the d family shows the size of difference between two variables. As a benchmark for research planning and evaluation, Cohen (1988) proposed ‘conventional’ values for small, medium, and large effects: r 1⁄4.10, .30, and .50 and d 1⁄4.20, .50, and .80, respectively (in the way that p values of .05, .01, and .001 are conventional points, although these conventional values of effect size have been criticized; e.g., Rosenthal et al., 2000). The meta-analysis on statistical power by Jennions and Moller (2003) revealed that, in the field of behavioral ecology and animal behavior, statistical power of less than 20% to detect a small effect and power of less than 50% to detect a medium effect existed. This means, for example, that the average behavioral scientist performing a statistical test has a greater probability of making a Type II error (or b) (i.e., not rejecting Ho when Ho is false; note that statistical power is equals to 1 2 b) than if they had flipped a coin, when an experiment effect is of medium size (i.e., r 1⁄4 .30, d 1⁄4 .50). Here, I highlight and discuss an implication of this low statistical power on one of the most widely used statistical procedures, Bonferroni correction (Cabin and Mitchell, 2000). Bonferroni corrections are employed to reduce Type I errors (i.e., rejecting Ho when Ho is true) when multiple tests or comparisons are conducted. Two kinds of Bonferroni procedures are commonly used. One is the standard Bonferroni procedure, where a modified significant criterion (a/k where k is the number of statistical tests conducted on given data) is used. The other is the sequential Bonferroni procedure, which was introduced by Holm (1979) and popularized in the field of ecology and evolution by Rice (1989) (see these papers for the procedure). For example, in a recent volume of Behavioral Ecology (vol. 13, 2002), nearly one-fifth of papers (23 out of 117) included Bonferroni corrections. Twelve articles employed the standard procedure while 11 articles employed the sequential procedure (10 citing Rice, 1989, and one citing Holm, 1979). A serious problem associated with the standard Bonferroni procedure is a substantial reduction in the statistical power of rejecting an incorrect Ho in each test (e.g., Holm, 1979; Perneger, 1998; Rice, 1989). The sequential Bonferroni procedure also incurs reduction in power, but to a lesser extent (which is the reason that the sequential procedure is used in preference by some researchers; Moran, 2003). Thus, both procedures exacerbate the existing problem of low power, identified by Jennions and Moller (2003). For example, suppose an experiment where both an experimental group and a control group consist of 30 subjects. After an experimental period, we measure five different variables and conduct a series of t tests on each variable. Even prior to applying Bonferroni corrections, the statistical power of each test to detect a medium effect is 61% (a 1⁄4 .05), which is less than a recommended acceptable 80% level (Cohen, 1988). In the field of behavioral ecology and animal behavior, it is usually difficult to use large sample sizes (in many cases, n , 30) because of practical and ethical reasons (see Still, 1992). When standard Bonferroni corrections are applied, the statistical power of each t test drops to as low as 33% (to detect a medium effect at a/5 1⁄4 .01). Although sequential Bonferroni corrections do not reduce the power of the tests to the same extent, on average (33–61% per t test), the probability of making a Type II error for some of the tests (b 1⁄4 1 2 power, so 39–66%) remains unacceptably high. Furthermore, statistical power would be even lower if we measured more than five variables or if we were interested in detecting a small effect. Bonferroni procedures appear to raise another set of problems. There is no formal consensus for when Bonferroni procedures should be used, even among statisticians (Perneger, 1998). It seems, in some cases, that Bonferroni corrections are applied only when their results remain significant. Some researchers may think that their results are ‘more significant’ if the results pass the rigor of Bonferroni corrections, although this is logically incorrect (Cohen, 1990, 1994; Yoccoz, 1991). Many researchers are already reluctant to report nonsignificant results ( Jennions and Moller, 2002a,b). The wide use of Bonferroni procedures may be aggravating the tendency of researchers not to present nonsignificant results, because presentation of more tests with nonsignificant results may make previously ‘significant’ results ‘nonsignificant’ under Bonferroni procedures. The more detailed research (i.e., research measuring more variables) researchers do, the less probability they have of finding significant results. Moran (2003) recently named this paradox as a hyper-Red Queen phenomenon (see the paper for more discussion on problems with the sequential method). Imagine that we conduct a study where we measure as many relevant variables as possible, 10 variables, for example. We find only two variables statistically significant. Then, what should we do? We could decide to write a paper highlighting these two variables (and not reporting the other eight at all) as if we had hypotheses about the two significant variables in the first place. Subsequently, our paper would be published. Alternatively, we could write a paper including all 10 variables. When the paper is reviewed, referees might tell us that there were no significant results if we had ‘appropriately’ employed Bonferroni corrections, so that our study would not be advisable for publication. However, the latter paper is Behavioral Ecology Vol. 15 No. 6: 1044–1045 doi:10.1093/beheco/arh107 Advance Access publication on June 30, 2004

Vocal performance influences female response to male bird song: an experimental test

Abstract: Female songbirds are thought to assess males based on aspects of song, such as repertoire size or amount of singing, that could potentially provide information about male quality. A relatively unexplored aspect of song that also might serve as an assessment signal is a male’s ability to perform physically challenging songs. Trilled songs, such as those produced by swamp sparrows (Melospiza georgiana), present males with a performance challenge because trills require rapid and precise coordination of vocal tract movements, resulting in a trade-off between trill rate and frequency bandwidth. This trade-off defines a constraint on song production observed as a triangular distribution in acoustic space of trill rate by frequency bandwidth, with an upper boundary that represents a performance limit. Given this background on song production constraints, we are able to identify a priori which songs are performed with a higher degree of proficiency and, thus, which songs should be more attractive to females. We determined the performance limit for a population of swamp sparrows and measured how well individual males performed songs relative to this limit (‘‘vocal performance’’). We then compared female solicitation responses to high-performance versus lowperformance versions of the same song type produced by different males. Females displayed significantly more to highperformance songs than to low-performance songs, supporting the hypothesis that females use vocal performance to assess males. Key words: bird song, female choice, indicator mechanism, Melospiza georgiana, motor constraint, sexual selection, vocal performance. [Behav Ecol 15:163–168 (2004)]

Rodent foraging is affected by indirect, but not by direct, cues of predation risk

Abstract: We used foraging trays to determine whether oldfield mice, Peromyscus polionotus, altered foraging in response to direct cues of predation risk (urine of native and nonnative predators) and indirect cues of predation risk (foraging microhabitat, precipitation, and moon illumination). The proportion of seeds remaining in each tray (a measure of the giving-up density [GUD]) was used to measure risk perceived by mice. Mice did not alter their GUD when presented with cues of native predators (bobcats, Lynx rufus, and red foxes, Vulpes vulpes), recently introduced predators (coyotes, Canis latrans), nonnative predators (ocelots, Leopardus pardalis), a native herbivore (white-tailed deer, Odocoileus virginianus), or a water control. Rather, GUD was related to microhabitat: rodents removed more seeds from foraging trays sheltered beneath vegetative cover compared with exposed trays outside of cover. Rodents also removed more seeds during nights with precipitation and when moon illumination was low. Our results suggest that P. polionotus used indirect cues rather than direct cues to assess risk of vertebrate predation. Indirect cues may be more reliable than are direct scent cues for estimating risk from multiple vertebrate predators that present the most risk in open environments. Key words: foraging, giving-up densities, Peromyscus polionotus; predator recognition; prey behavior; risk assessment; Savannah River Site. [Behav Ecol 15:433–437 (2004)]

The relation between dominance and exploratory behavior is context-dependent in wild great tits

Abstract: Individual differences in personality affect behavior in novel or challenging situations. Personality traits may be subject to selection because they affect the ability to dominate others. We investigated whether dominance rank at feeding tables in winter correlated with a heritable personality trait (as measured by exploratory behavior in a novel environment) in a natural population of great tits, Parus major. We provided clumped resources at feeding tables and calculated linear dominance hierarchies on the basis of observations between dyads of color-ringed individuals, and we used an experimental procedure to measure individual exploratory behavior of these birds. We show that fast-exploring territorial males had higher dominance ranks than did slow-exploring territorial males in two out of three samples, and that dominance related negatively to the distance between the site of observation and the territory. In contrast, fast-exploring nonterritorial juveniles had lower dominance ranks than did slow-exploring nonterritorial juveniles, implying that the relation between dominance and personality is contextdependent in the wild. We discuss how these patterns in dominance can explain earlier reported effects of avian personality on natal dispersal and fitness. Key words: boldness, dispersal, dominance, exploration, fitness, Parus major, personality. [Behav Ecol 15:1023–1030 (2004)]

The cost of dispersal: predation as a function of movement and site familiarity in ruffed grouse

Abstract: This work was funded by the Ohio Department of Natural Resources, Division of Wildlife and supported by the Department of Evolution, Ecology, and Organismal Biology and the Aquatic Ecology Laboratory at the Ohio State University.

Hatchery selection promotes boldness in newly hatched brown trout (Salmo trutta): implications for dominance

Abstract: Hatchery selection promotes boldness in newly hatched brown trout (Salmo trutta): implications for dominance

Host social behavior and parasitic infection: a multifactorial approach

Abstract: I examined associations between several components of host social organization, including group size and gregariousness, group stability, territoriality and social class, and gastrointestinal parasite load in African bovids. At an intraspecific level, group size was positively correlated with parasite prevalence, but only when the parasite was relatively host specific and only among host species living in stable groups. Social class was also an important predictor of infection rates. Among gazelles, territorial males had higher parasite intensities than did either bachelor males or females and juveniles, suggesting that highly territorial individuals may be either more exposed or more susceptible to parasites. Associations among territoriality, grouping, and parasitism were also found across taxa. Territorial host genera were more likely to be infected with strongyle nematodes than were nonterritorial hosts, and gregarious hosts were more infected than were solitary hosts. Analyses also revealed that gregariousness and territoriality had an interactive effect on individual parasite richness, whereby hosts with both traits harbored significantly more parasite groups than did hosts with only one or neither trait. Overall, study results indicate that multiple features of host social behavior influence infection risk and suggest that synergism between traits also has important effects on host parasite load. Key words: Bovidae, group size, group living, parasite prevalence, parasite richness, strongyle nematodes, territoriality. [Behav Ecol 15:446–454 (2004)]

Scent-marking displays provide honest signals of health and infection

Abstract: Males of many species produce scent marks and other olfactory signals that function to intimidate rivals and attract females. It has been suggested that scent marks provide an honest, cheat-proof display of an individual’s health and condition. Here we report several findings that address this hypothesis in wild-derived house mice (Mus musculus domesticus). (1) We exposed males to female odor, which induces an increase in testosterone, and found that sexual stimulation significantly increased the males’ scent-marking and the attractiveness of their scent marks to females. (2) We challenged sexually stimulated males with a nonreplicating strain of bacteria (Salmonella enterica C5TS) to activate immunity and found that this significantly decreased the males’ scent-marking and the attractiveness of their scent marks to females. (3) We collected scent marks from infected and sham-infected males when they were sexually stimulated or not, and we found that females could significantly discriminate the scent marks of infected versus control males, but only when the males were sexually stimulated. Taken together, our results indicate that male mice modulate their scent-marking display depending on their health and perceived mating opportunities. Increased scent marking enhances males’ attractiveness to females, scent marks provide an honest indicator of bacterial infection (and perhaps immune activation), and females are able to assess the health of males more easily when males mark at a high rate.

Plumage coloration, not length or symmetry of tail-streamers, is a sexually selected trait in North American barn swallows

Abstract: Sexual adornments often vary markedly across a species' range, which presumably is owing to differences in local environmental conditions and the associated selection pressures, such as natural versus sexual selection or the relative signaling value of different ornamental traits. However, there are only a few reported examples in which the information content of mating signals varies geographically, and even fewer in which a set of secondary sexual traits serves different signaling functions in different populations. Classic studies of sexual selection in the European barn swallow (Hirundo rustica rustica) demonstrate that elongate tail-streamers provide several reproductive advantages to males and females and are used as reliable signals of mate quality. Here, we show that tail-streamers do not appear to confer these same benefits in a population of barn swallows from North America (Hirundo rustica erythrogaster). Instead, ventral plumage coloration, which is more exaggerated in North American swallows compared with their European counterparts, predicts patterns of assortative mating and annual reproductive success in H. r. erythrogaster. These observations support the idea that ornamental traits can serve different functions among animal populations and suggest that geographic variation in different sexual signals may facilitate population divergence, which may ultimately lead to speciation. Copyright 2004.

The role of conformity in foraging when personal and social information conflict

Abstract: There is currently considerable interest in the interplay between personal and social information in decision-making processes. Two experiments are presented exploring the relative use of prior personal information and subsequent social information in foraging decisions of guppies. Experiment 1 tested the assumption that when the use of information acquired through personal experience is not costly, conflicting social information will be ignored. The assumption was confirmed because, when given a choice between feeding at two food patches, at one of which they had previously seen conspecifics feed, individual fish with prior experience of feeding at the alternative site chose the alternative, whereas fish with no prior experience chose the site at which their conspecifics had fed. Experiment 2 tested theoretical predictions that when the use of information acquired through personal experience is potentially costly, conflicting social information will be weighed more heavily than will personal information. The prediction was confirmed because, when given a choice between feeding at two food patches, one at which they had previously seen conspecifics feed and one behind a visual barrier, individual fish with prior experience of feeding behind the barrier chose the site at which their conspecifics had fed. These findings suggest that conformity can promote social learning in naive individuals, but prior experience can insulate individuals from conformity provided the costs of relying on that experience are small. In addition, the experiments highlight the fact that personal and social information are not always weighed equally.

Extended parental care and delayed dispersal: northern, tropical, and southern passerines compared

Abstract: Using modern comparative methods, we found that both time to independence and time with parents were significantly longer in southern hemisphere and tropical birds than in northern hemisphere ones. These differences held even after removing Australian passerines or cooperatively breeding species, and they do not depend on habitat, diet, or migration pattern. In southern hemisphere and tropical regions, both cooperative breeding and non-cooperative parents continue to feed their young for a similar length of time, but cooperative breeders allow them to stay longer in their natal territory after they become nutritionally independent. Nevertheless, the young of non-cooperative species stay longer with their parents than do the young of non-cooperative species in the temperate northern hemisphere. The fact that extended periods of post-fledging parental care are widespread among passerines provides further empirical support for the view that life histories of southern and tropical birds are ‘slow,’ with small clutches, extended parental care, and long lifespan; parents take care of fewer young for longer. These results support recent theoretical models that predict that high adult survival and low turnover of territory owners generally favor natal philopatry. We suggest that the reasons why some species (with or without cooperative breeding) exhibit natal philopatry and others do not lie in the balance between productivity and survival of adults and of retained or dispersing offspring. Key words: Life history, delayed dispersal, parental care. [Behav Ecol 15:831–838 (2004)]

The control of nest climate in bumblebee (Bombus terrestris) colonies: interindividual variability and self reinforcement in fanning response

Abstract: Interindividual variability in response to environmental stimuli is believed to have a major impact on collective behaviors in social insects. The present study presents a detailed investigation of the variability in individual fanning behavior underlying the collective control of nest climate in bumblebee (Bombus terrestris) colonies. Four colonies were repeatedly exposed to increasing temperature and CO2 levels. The response threshold of each worker (defined as the mean stimulus intensity at which a worker responded by fanning) was determined. Temperature response thresholds of 118 workers and CO2 response thresholds of 88 workers were analyzed. Workers differed in their response thresholds. Some consistently responded to low stimulus intensities, others consistently responded to high stimulus intensities. No consistent correlation between temperature and CO2 thresholds was found within individuals. Response thresholds of fanning bees decreased over successive trials, providing empirical support for the idea of specialization through individual threshold reinforcement. In addition to variability in individual response thresholds, workers of a colony differed in two other parameters of responsiveness: response probability (the probability of responding to a stimulus once it exceeded an individual’s response threshold) and response duration (the persistency with which fanning was performed once an individual responded). The results of the present study suggest that response threshold, response probability and response duration are important independent parameters of individual responsiveness in the collective control of nest climate in bumblebee colonies. Key words: division of labor, nest climate, reinforcement, response thresholds, selforganization. [Behav Ecol 15:120–128 (2004)]

Attractiveness of women's body odors over the menstrual cycle: the role of oral contraceptives and receiver sex

Abstract: It is a long held assumption that women have concealed ovulation, which means that men do not know when women's menstrual cycles are in their most fertile phase. Recent empirical results have provided evidence that ovulation may not be totally concealed from pair-bonded males, but the generality and the mechanisms of the finding demand further study. To examine the possible adaptive value of the phenomenon, it is necessary to study whether the ability to detect ovulation is confined to males. We studied these questions in an experiment in which male and female raters rated the sexual attractiveness and intensity of T-shirts' odors worn by 42 women using oral contraceptives (pill users) and by 39 women without oral contraceptives (nonusers). Males rated the sexual attractiveness of nonusers highest at midcycle. However, female raters showed only a nonsignificant trend for this relationship. Neither sex rated attractiveness of the odors of pill users according to their menstrual cycle. The results indicate that men can use olfactory cues to distinguish between ovulating and nonovulating women. Furthermore, the contrasting results between pill users and nonusers may indicate that oral contraceptives demolish the cyclic attractiveness of odors. Together, these findings give more basis for the study of the role of odors in human sexual behavior. Copyright 2004.

Testosterone, immunocompetence, and honest sexual signaling in male red grouse

Abstract: The expression of sexual ornaments has been suggested to reliably indicate individual quality, such as the ability to cope with parasites and diseases. The Immunocompetence Handicap Hypothesis (IHH) states that testosterone-dependent ornaments honestly signal such quality because of physiological costs associated with testosterone, such as impaired immune function. We tested predictions of the IHH both correlatively and experimentally in red grouse Lagopus lagopus scoticus. Male grouse exhibit supra-orbital red combs whose size is testosterone-dependent. We found that comb size was not correlated to infection intensity by two parasites (coccidia and the nematode Trichostrongylus tenuis), but it was significantly positively correlated with condition and T-cell-mediated immunity (the ability to mount a primary inflammatory response). We manipulated testosterone by means of implants and re-caught males after a month to investigate the effects on comb size, condition, immunity, and parasite load. Males implanted with testosterone had increased comb size, lost more condition, and had lower T-cell-mediated immunity than control males. Increased testosterone also resulted in a significant increase in coccidia infection intensity but had no effect on T. tenuis burden. The results are consistent with predictions of the IHH and suggest that comb size honestly indicates immunocompetence and males’ ability to cope with certain parasites. Females could thus benefit from choosing mates based on the expression of this sexual trait. Key words: Immunocompetence Handicap Hypothesis, parasite, red grouse, sexual ornament, T-cell-mediated immunity, testosterone. [Behav Ecol 15:930–937 (2004)]

Modification of the visual background increases the conspicuousness of golden-collared manakin displays

Abstract: Effective visual communication requires signals that are easy to detect, transmit, receive, and discriminate. Animals can increase the probability that their visual signals would be detected by evolving signals that contrast with their visual background. Animals can further enhance this contrast by behaviorally modifying the existing visual background. Male golden-collared manakins (Manacus vitellinus) clear leaf litter from the ground to form courts, which are used as display arenas. Using reflectance measures of the signal (male plumage) and the visual background (cleared court and adjacent litter), the irradiance measures of ambient light during display, and published measures of photoreceptor sensitivity of a Passerine, we test the hypothesis that court-clearing augments the contrast between male plumage and the visual background. We find that the chromatic and brightness contrasts of golden patches used during courtship are greater against the cleared court than against adjacent litter. In addition, we find that cleared courts provide a less variable background for these color patches, resulting in displays that consistently contrast the visual background. These results suggest that behavioral modification of the visual background may act to increase the conspicuousness of colorful male plumage during display, providing an explanation for why golden-collared manakins, and possibly other species, build or clear display courts. Key words: chromatic contrasts, court-clearing, Manacus vitellinus, manakins, signaling, visual signals. [Behav Ecol 15:1003–1010 (2004)]

Helpers increase long-term but not short-term productivity in cooperatively breeding long-tailed tits

Abstract: Helpers at the nest in the cooperative breeding system of long-tailed tits Aegithalos caudatus exhibit kin preference in their helping behavior. The aim of this study was to use multivariate analyses to investigate whether helpers accrue indirect fitness benefits through their cooperation by increasing the productivity of relatives. All birds started each season breeding independently in pairs, but birds that failed in their own breeding attempt often redirected their care to help another pair provision their offspring. About half of all broods had one or more helpers, 86% of which were male. Provisioning rates increased and there was a corresponding increase in the mass of nestlings within broods as the number of helpers increased. Helpers had no significant short-term effect on productivity because nest predation, nestling survival, and brood size were unaffected by the presence of helpers. However, in the long term helpers had a highly significant effect on the recruitment of fledglings, the positive effect of helpers being linear within the range of helper numbers that we observed. We found no evidence to suggest that these results were confounded by the effects of individual or habitat quality. We conclude that long-tailed tits accrue indirect fitness benefits by helping kin. Nevertheless, the inclusive fitness benefit from helping is substantially lower than that of independent breeding, showing that helpers are making the best of a bad job. Key words: Aegithalos caudatus, cooperative breeding, inclusive fitness, kin selection, long-tailed tit. [Behav Ecol 15:1–10 (2004)]

Ultraviolet reflectance affects male-male interactions in the blue tit (Parus caeruleus ultramarinus)

Abstract: Several animal species have been shown to use phenotypic traits to assess the competitive ability of opponents and adjust their aggressiveness depending on the likelihood to win the contest. In birds, these phenotypic traits usually involve patches of colored feathers. The benefit to harbor honest signals of male quality is the avoidance of wasteful aggressive interactions. Recent work has shown that ultraviolet (UV) plumage reflectance is an important signal used by females during mate choice. Surprisingly, however, the role of UV signaling on intrasexual selection has been neglected. In the present study, we aimed to test whether UV reflectance of crown feathers was used as a signal of male competitive ability during male-male interactions. Breeding male blue tits (Parus caeruleus ultramarinus) were exposed during the female egg-laying period to blue tit taxidermic mounts with either control or reduced UV reflectance of crown feathers. In agreement with the prediction that UV reflectance advertises male quality, we found that breeding blue tits behaved less aggressively toward the UV-reduced decoy. To our knowledge, this is the first experimental evidence suggesting a role for UV signaling on intrasexual selection. Key words: animal contests, male-male competition, sexual selection, sexual signals. [Behav Ecol 15:805–809 (2004)]

Extrapair paternity, migration, and breeding synchrony in birds

Abstract: To understand interspecific patterns in the strength of sexual selection, variation in the costs and benefits of exercising mate choice needs to be evaluated. One manifestation of sexual selection in birds is the occurrence of greatly variable levels of extrapair paternity (EPP). A proposed general explanation for this variation is that the benefits to females in seeking extrapair copulations vary in a predictable manner according to the degree of breeding synchrony because females are better able to assess potential extrapair partners when males are simultaneously in breeding condition. This hypothesis predicts a latitudinal trend in EPP because birds tend to breed more synchronously away from the equator. Expanding on previous geographically and taxonomically restricted tropical/temperate comparisons, we used phylogenetically independent standardized linear contrasts to show that this positive relationship persists when all bird species for which EPP estimates currently exist are considered. However, if a third factor covaries with latitude in the same way as breeding synchrony and EPP, this relationship need not be causal. Migration could also account for latitudinal variation in the benefits to females of pursuing EPP, if migration is associated with (1) hasty or (2) inaccurate mate choice, (3) facilitated assessment of male quality through the condition-dependence of arrival time, or (4) increased genetic variance in male quality. We show that migration distance is positively related to the proportion of EPP and that migration can statistically explain the latitudinal trend in EPP, even when confounding factors are simultaneously controlled. Hence, alternative explanations for latitudinal variation in EPP may be feasible, and careful intraspecific tests are needed to assess their relative importance and their implications for geographical variation in life-history evolution. Key words: breeding synchrony, comparative analysis, extrapair paternity, migration, sexual selection. [Behav Ecol 15:41–57 (2004)]

Behavioral dynamics between caring males and females in a beetle with facultative biparental care

Abstract: In families in which both parents care for multiple offspring, the amount of care a parent provides can be simultaneously influenced by multiple social interactions (i.e., parent-parent and parent-offspring). In this study, we first tested for sex differences in the parents’ contribution to care and then used path analysis to address the simultaneous impact of parent-parent and parent-offspring interactions on male and female care in the burying beetle, Nicrophorus vespilloides. In this species, both parents provision their offspring predigested carrion from a vertebrate carcass, and the larvae beg for food from their parents. We found that females were more involved in direct care for the larvae and spent more time than did males provisioning the larvae with food. By using path analysis, we found a negative relationship between male and female provisioning, suggesting that parents adjust their behavior to that of their mate. Furthermore, we found that both social interactions (i.e., larval begging) and nonsocial factors (i.e., brood size) significantly influenced male provisioning, but had no significant effect on female provisioning. We suggest that the difference in the relative contribution of the two sexes to the care of the offspring explains why only males seemed to adjust their care to variation in social and nonsocial factors. For example, females may be less able to adjust their care to variation in larval begging and brood size because they were already working near their maximum capacity. Key words: begging, biparental care, burying beetles, parent-offspring conflict, sexual conflict. [Behav Ecol 15:621–628 (2004)]

Relative allocation to horn and body growth in bighorn rams varies with resource availability.

Abstract: Males may allocate a greater proportion of metabolic resources to maintenance than to the development of secondary sexual characters when food is scarce, to avoid compromising their probability of survival. We assessed the effects of resource availability on body mass and horn growth of bighorn rams (Ovis canadensis) at Ram Mountain, Alberta, Canada over 30 years. The number of adult ewes in the population tripled during our study, and the average mass of yearling females decreased by 13%. We used the average mass of yearling females as an index of resource availability. Yearling female mass was negatively correlated with the body mass of rams of all ages, but it affected horn growth only during the first three years of life. Yearly horn growth was affected by a complex interaction of age, body mass, and resource availability. Among rams aged 2–4 years, the heaviest individuals had similar horn growth at high and at low resource availability, but as ram mass decreased, horn growth for a given body mass became progressively smaller with decreasing resource availability. For rams aged 5–9 years, horn growth was weakly but positively correlated with body mass, and rams grew slightly more horn for a given body mass as resource availability decreased. When food is limited, young rams may direct more resources to body growth than to horn growth, possibly trading long-term reproductive success for short-term survival. Although horn growth of older rams appeared to be greater at low than at high resource availability, we found no correlation between early and late growth in horn length for the same ram, suggesting that compensatory horn growth does not occur in our study population. Young rams with longer horns were more likely to be shot by sport hunters than those with shorter horns. Trophy hunting could select against rams with fast-growing horns. Key words: bighorn sheep, body mass, horn size, Ovis canadensis, population density, reproductive strategy, resource allocation, sexual selection. [Behav Ecol 15:305–312 (2004)]

Immunocompetence and resource holding potential in the damselfly, Calopteryx virgo L

Abstract: It is generally believed that resource holding potential reliably reflects male quality, but empirical evidence showing this is scarce. Here we show that the outcome of male-male competition may predict male immunocompetence in the territorial damselfly, Calopteryx virgo (Odonata: Calopterygidae). We staged contests between 27 pairs of males and found that winners of the contests showed higher immunocompetence, measured as encapsulation response, compared with that of losers. Furthermore, the winners had larger fat reserves. We also collected 29 males that had not been used in staged contests, and found that in these males encapsulation response correlated positively with an individual’s fat reserves. Both immunocompetence and resource holding potential seem to depend on energy reserves, suggesting a trade-off between parasite resistance and energetically costly territorial behavior. The results suggest that the outcome of male-male contest can be used to predict male quality in terms of immune defense. Key words: damselfly, encapsulation rate, fat, hemocyte, immunocompetence, male-male competition, resource holding potential. [Behav Ecol 15:169–173 (2004)]

Immune challenge mediates vocal communication in a passerine bird: an experiment

Abstract: Secondary sexual characters may have evolved in part to signal resistance to parasites. Avian song has been hypothesized to be involved in this process, but the role of parasites in modulating acoustic communication systems in birds remains largely unknown, owing to lack of experiments. We studied the relationship between parasitism, testosterone, song performance, and mating success in male collared flycatchers (Ficedula albicollis) by experimentally challenging their immune system with a novel antigen. We predicted that a challenge of the immune system would reduce song performance, and that this reduction would be conditional on the size of a visual sexual signal, the forehead patch that was previously found to reflect resistance. An antagonistic linkage between testosterone and immune function would predict that a challenge of the immune system should suppress testosterone level. An immunological treatment by sheep red blood cells (SRBCs) triggered a decrease in body mass, testosterone level, and song rate, but other song traits were not significantly affected by the antigen challenge. Initial testosterone level was associated with forehead patch size and all song traits except song rate. SRBC injection caused stronger reduction in song rate among males with smaller forehead patches, and the change in song rate was also predictable by song features such as strophe complexity and length. We show that song rate and other song characteristics may be important cues in male-male competition and female choice. These results suggest that parasite-mediated sexual selection has contributed in shaping a complex acoustic communication system in the collared flycatcher, and that testosterone may play an important role in this process. Parasitism may drive a multiple signaling mechanism involving acoustic and visual traits with different signal function. Key words: bird song, collared flycatcher, immunocompetence, parasites, secondary sexual characters, testosterone. [Behav Ecol 15:148–157 (2004)]

The effects of habitat- and diet-based cues on association preferences in three-spined sticklebacks

Abstract: A number of recent articles have investigated the potential of familiarity preferences to influence group membership in freeranging animals. However, it is not clear to which extent individual recognition or a more general recognition of a group odor is responsible for familiarity preferences. First, we tested the sensory basis of the recognition of familiars in stickleback. When allowed to choose between a familiar and a nonfamiliar stimulus shoal on the basis of both visual and chemical communication, visual communication only, and chemical communication only, the preference of focal fish for familiars was shown to depend on the presence of chemical cues. We subsequently investigated the effects of recent habitat and diet on shoaling preferences in the three-spined stickleback (Gasterosteus aculeatus). Experimental fish were divided into four treatment groups consisting of two water treatments (saline and freshwater) and two diet treatments (Daphnia spp. and bloodworm). Focal fish subsequently showed significant association preferences for groups of nonfamiliar fish that had undergone the same water or diet treatment as themselves. These data indicate that individual recognition is not a prerequisite for the expression of association preferences.

Protandry and sexual dimorphism in trans-Saharan migratory birds

Abstract: Earlier arrival to reproductive sites of males relative to females (protandry) is widespread among migratory organisms. Diverse mechanisms have been proposed that may select for protandry, including competition for limiting resources (e.g., territories) or mates. In species with large variation in male reproductive success, such as polygamous species and those with intense sperm competition, early arriving males may accrue a fitness advantage because they acquire more mates or have larger chances of paternity. Comparative studies of birds have shown that sexual size dimorphism (SSD) is positively associated with the level of polygyny, whereas intense sperm competition is associated with sexual dichromatism (SD). Positive correlations between protandry and SSD or SD can therefore be expected to exist across avian species. Because large males are predicted to be better able to cope with adverse ecological conditions early in the breeding season, selection for protandry, in turn, may have a correlated response on SSD among migratory species breeding in boreal latitudes. Although previous studies of birds have analyzed the association between SSD and protandry, none has analyzed SD in relation to protandry. Here we analyze the association between protandry during spring migration, SSD, and SD in 21 trans-Saharan monogamous migratory bird species. The difference in median migration dates between females and males, reflecting protandry, was positively associated with SD but not with SSD. Because dichromatism is positively related to sperm competition across species, present results are consistent with predictions derived from sexual selection hypotheses for the evolution of protandry mediated by sperm competition. Key words: arrival date, bird migration, comparative analysis, sexual dichromatism, sexual selection, sperm competition. [Behav Ecol 15:592–601 (2004)]

Multiple selection pressures influence Trinidadian guppy (Poecilia reticulata) antipredator behavior

Abstract: Animals alter their behavior to avoid a variety of different types of predators. Trinidadian guppies (Poecilia reticulata) have been an important system for examining the evolution of antipredator behavior because geographically isolated populations experience different amounts of aquatic predation. Although the influence of aquatic predators has been well documented, selective pressures from other types of predators are less well understood. We examined the response of wild-caught individuals from upstream and downstream populations to an aquatic predator and a simulated aerial predator. As previously documented, fish from the downstream population responded more strongly to the aquatic predator than did fish from the upstream population, inspecting for longer periods of time. Guppies also exhibited a strong behavioral response to the potential aerial predator. Although both populations displayed a similar magnitude of response, they differed in the specific behaviors used. Upstream fish tended to freeze on the tank bottom, whereas downstream fish tended to hide under shelter. Field observations suggest that these strategies are related to habitat features specific to each site. The behaviors used against aerial predators differed substantially from the behaviors used against aquatic predators, suggesting that different types of predators exert conflicting selection pressures. This research demonstrates the importance of considering multiple selection pressures acting on an organism when trying to understand the evolutionary history of behavioral and morphological traits. Key words: aerial predators, aquatic predators, fish, guppy. [Behav Ecol 15:673–678 (2004)]

Reproductive promiscuity in the splendid fairy-wren: effects of group size and auxiliary reproduction

Abstract: Extrapair fertilizations complicate our understanding of cooperative breeding in a number of ways. For example, auxiliaries may reduce the costs of seeking extrapair fertilizations for breeding males or females, and auxiliary males may themselves seek copulations with the breeding female in their own group. We employed microsatellite markers to examine patterns of parentage in the cooperatively breeding splendid fairy-wren (Malurus splendens melanotus). Our study population exhibited a relatively high level of extrapair paternity (42% of 386 offspring) with considerable annual variation (range = 24–52%). Across years the proportion of offspring sired by extrapair males was significantly correlated with the average number of auxiliaries per group. Furthermore, the proportion of extrapair young within a brood was related to group composition; groups with multiple auxiliaries were twice as likely as groups with zero or one auxiliary to contain extrapair young. Most offspring were sired by dominant breeding males, but auxiliary males sired approximately 25% of all extrapair young (10% of all offspring), and about half of these were cases in which the auxiliary male sired offspring in his own group. Within-group sirings by auxiliary males were most common after replacement of the breeding female, and they also appeared to be more likely when the auxiliary was not related to the breeding male. Thus, the presence of auxiliary males increased the likelihood that females would produce extrapair young, and although incest avoidance mechanisms usually prevent within-group copulations by auxiliary males, a conflict of interest among group males arises when a new female joins the group. Key words: cooperative breeding, extrapair reproduction, Malurus splendens, microsatellites, reproductive skew, splendid fairy-wren. [Behav Ecol 15:907–915 (2004)]

Flock density, social foraging, and scanning: an experiment with starlings

Abstract: Social foraging differs from individual foraging because it alters both resource availability and the forager’s behavior. We examined responses of starlings to the presence of conspecifics by manipulating foraging-group density experimentally, while ensuring that each subject’s foraging opportunities were unchanged. To do this, we used individuals foraging simultaneously in four bottomless enclosures placed at various separations in natural foraging grounds. We measured foraging and scanning intensity and qualitative aspects of scanning of focal individuals. Additionally, we examined the temporal distribution of scanning between individuals. The focal individual analysis showed that (1) food-searching activity increased, while time spent scanning, time off the ground and scanning bout length decreased with flock density; (2) food finding per unit of searching effort increased with density; (3) head orientation during scanning was sensitive to companions’ proximity: heads pointed away from the companions at close distance, toward them at intermediate distance, and was random farther away. The analysis of the (temporal overlapping in scanning) temporal distribution of scanning for the group showed that scanning was significantly synchronized when companions were adjacent to each other but was not significantly different from random at further separations. We conclude that behavioral responses of individuals to the presence of others generate important changes in foraging performance even in the absence of physical interference and, more generally, that assessing the mechanisms that control the behavior of group members at different flock densities offers a way to understand the functional and ecological significance of foraging aggregations. Key words: conspecific scanning, flock density, neighbor distance, social foraging, starlings, Sturnus vulgaris, vigilance. [Behav Ecol 15:371–379 (2004)]

Patterns of energy acquisition by a central place forager: benefits of alternating short and long foraging trips

Abstract: In some seabirds, foraging trips have been defined as either long or short, with the length of time spent traveling to the foraging area apparently a critical feature in determining foraging trip length. Using logger technology, together with complimentary data from published studies, we investigated traveling and foraging times in 18 free-living Adelie Penguins Pygoscelis adeliae, which were foraging for chicks. Most deep, foraging dives were distributed around the center of the foraging trip. This central tendency was particularly apparent if the cumulative amount of undulations in the depth profile (indicative of prey capture) was considered during deep dives; values started to increase before 20.9% and ceased after 67.2% of the dives had occurred. This concentration of the feeding activity in the middle of the foraging trip indicates that birds traveled to and from a prey patch whose location varied little over the birds' trips. These data form the basis for a simple model that uses traveling and foraging times together with projected rates of prey ingestion and chick and adult gastric emptying to determine that there are occasions when, to optimize rates of prey ingestion while at sea for both adults and chicks, birds should conduct foraging trips of bimodal lengths.

Male dominance and immunocompetence in a field cricket

Abstract: Female preference for dominant males has been found in many species, and it is generally thought that winners of male-male competition are of superior quality. Success in contests probably depends on male condition and overall health. Thus, females could avoid infection and gain genetic benefits in terms of more viable offspring by mating with dominant males. In the present study, we tested whether dominant males of the Mediterranean field cricket, Gryllus bimaculatus, had higher immunocompetence than did their subordinates in experimental trials. We found that dominant males had better immune defense, as indicated by significantly higher encapsulation rate and lytic activity, than did subordinate males of the same size. Dominant males were also more successful in obtaining matings, but this was measured nonindependently of dominance status. Our results suggest that a male’s dominance status and success in fights may indicate his immunocompetence to females. Key words: dominance, female choice, Gryllus bimaculatus, immunocompetence, male-male competition. [Behav Ecol 15:187–191 (2004)]

Sex-specific growth rates in zebra finch nestlings: a possible mechanism for sex ratio adjustment

Abstract: Wild and captive zebra finches (Taenopygia guttata), like several other species, produce a male-biased sex ratio at fledging when food is scarce. This is due to primary sex-ratio adjustment and female-biased nestling mortality. Given that young females fledging at low body masses have been shown to have low fecundity as adults, lower returns to parents from producing female offspring in conditions of restricted food has been raised as a functional explanation (Trivers and Willard’s hypothesis of adaptive sexual investment; 1973). However, an alternative, mechanistic hypothesis is that under restricted conditions female chicks are more costly to produce. In consequence, lower returns to parents under these conditions would happen earlier in the life of female offspring rather than later. To test this hypothesis, I hand-reared chicks on a food gradient. In the absence of parentoffspring and sib-sib interactions, final body mass and growth rates for females were lower in conditions of restricted food. For males, final body mass and growth rates did not differ with food condition. Low female growth rates in food-restricted conditions might be one potential mechanism causing female-biased mortality in birds. More importantly, this result is the strongest evidence yet of female offspring experiencing higher marginal fitness benefits from additional food than males and it has implications for primary and secondary sex-ratio adjustment. Also, as this mechanism has been shown in the absence of parentoffspring interactions, significant questions can now be raised as to how parental and offspring behavior interact in their effects on secondary sex-ratio adjustment. Key words: hand-rearing, sex allocation theory, sex-biased mortality, sex ratio, sex-specific growth rates, zebra finches. [Behav Ecol 15:174–180 (2004)]