Showing papers in "Behavioral Ecology in 2008"

Fitness consequences of personality: a meta-analysis

Abstract: The study of nonhuman personality capitalizes on the fact that individuals of many species behave in predictable, variable, and quantifiable ways. Although a few empirical studies have examined the ultimate consequences of personality differences, there has been no synthesis of results. We conducted a formal meta-analysis of published studies reporting fitness consequences of single personality dimensions to identify general trends across species. We found bolder individuals had increased reproductive success, particularly in males, but incurred a survival cost, thus, supporting the hypothesis that variation in boldness was maintained due to a ‘‘trade-off’’ in fitness consequences across contexts. Potential mechanisms maintaining variation in exploration and aggression are not as clear. Exploration had a positive effect only on survival, whereas aggression had a positive effect on both reproductive success and, not significantly, on survival. Such results would suggest that selection is driving populations to become more explorative and aggressive. However, limitations in meta-analytic techniques preclude us from testing for the effects of fluctuating environmental conditions or other forms of selection on these dimensions. Results do, however, provide evidence for general relationships between personality and fitness, and we provide a framework for future studies to follow in the hopes of spurring more in-depth, long-term research into the evolutionary mechanisms maintaining variation in personality dimensions and overall behavioral syndromes. We conclude with a discussion on how understanding and managing personality traits may play a key role in the captive breeding and recovery programs of endangered species. Key words: behavioral syndrome, fitness, personality, reproductive success, survival. [Behav Ecol]

Time for some a priori thinking about post hoc testing

Abstract: Researchers are commonly in a situation, often after an experiment, where they want to compare the central tendency of some measure across a number of groups. If the number of groups is simply 2, then there is little controversy as to the appropriate analysis, with normally a t-test or a nonparametric equivalent being adopted. If the number of groups is greater than 2, most elementary statistical textbooks suggest performing an analysis of variance (ANOVA) to test the null hypothesis that all the groups are the same and, if this null hypothesis is rejected, implementing some post hoc testing to identify which groups are significantly different from which other groups. However, as readers and reviewers of scientific papers in behavioral science, we have noted a great diversity of approaches when comparing more than 2 groups often with little or no justification for the adoption of a specific approach. Hence, our aim in this note is to briefly survey current practice in this regard and to provide clear guidance on how such testing might most appropriately be carried out in different instances.

Foraging behavior of egg parasitoids exploiting chemical information

Abstract: Female parasitic wasps seek hosts for their offspring often in a dynamic environment. Foraging egg parasitoids rely on a variety of chemical cues originating from the adult host, host products, or the host plant rather than from the attacked host stage?the insect egg itself. Besides pupae, insect eggs are the most inconspicuous host stage attacked by parasitic wasps. To overcome the problem of low detectability of host eggs, egg parasitoids have evolved several strategies such as exploiting long-range kairomones of the adult hosts, for example, host aggregation and sex pheromones, plant synomones induced by egg deposition or host feeding, or short-range contact cues derived from the adult host or the host plant. Moreover, egg parasitoids have evolved the ability to use chemical espionage in combination with hitchhiking on the adult host (phoresy) to compensate their limited flight capability and to gain access to freshly laid host eggs. Here, we provide a comprehensive overview on the variety of host-foraging strategies of egg parasitoids exploiting chemical signals. Furthermore, the use of such infochemicals is discussed with respect to the wasps? dietary breadth and their ability to learn

What is the magnitude of the group-size effect on vigilance?

Abstract: Vigilance has been predicted to decrease with group size due to increased predator detection and dilution of predation risk in larger groups. Although earlier literature reviews have provided ample support for this prediction, an increasing number of studies have failed to document a decline in vigilance with group size. In addition, support for this prediction has been based thus far on the P value of the relationship between vigilance and group size rather than on a quantitative assessment of effect magnitude. Here, I use a meta-analysis of empirical relationships between vigilance and group size in birds published in the last 35 years to provide a reassessment of the group-size effect on vigilance. Nearly one-third of all published relationships between vigilance and group size were not significant (n = 172). Results from the meta-analysis indicate weak to moderate negative correlations between group size and time spent vigilant (n = 43), scan frequency (n = 29), or scan duration (n = 20). The magnitude of the relationship was stronger in studies that controlled the amount of food available to birds. A funnel plot of the relationship between correlation coefficients and sample size failed to reveal an obvious publication bias. Although the meta-analysis results generally support the prediction that vigilance should decline with group size, a large amount of variation in vigilance remains unexplained in avian studies.

Fuel stores, fuel accumulation, and the decision to depart from a migration stopover site

Abstract: Birds usually migrate by alternating flight bouts and stopovers for refueling. The decision when to leave a stopover place is of paramount importance for the success of migration. Despite its importance, little is known about which factors influence the departure decision. Using capture–recapture data of passerines from 3 stopover sites, we investigate by capture–recapture models whether this decision is dependent on actual fuel stores and fuel deposition rates. Individuals that accumulated fuel stores at medium rates departed later than individuals that either lost fuel stores during their stopover or able to increase their fuel stores quickly. This pattern was consistent among all sites. The departure decision was not dependent on actual fuel stores at 2 stopover sites. At the site facing an ecological barrier, emigration probability increased with increasing fuel stores, indicating that birds wait until they reach a threshold of fuel stores before departing. There was a positive correlation between departure fuel load and fuel deposition rate at all sites, which is in accordance with the time-minimizing hypothesis. These results suggest that the decision to depart from a stopover site is based on rather simple behavioral rules: birds that lose fuel stores or that increase fuel stores at a high rate would leave a site quickly, whereas birds increasing fuel stores at intermediate rates would stay for a longer time. The departure decision is shaped also by the position of the stopover site in relation to the next one and may be affected further by environmental factors. Key words: capture–recapture model, fuel deposition rate, passerine, take-off probability, trans-Sahara migrant. [Behav Ecol]

Adaptive phenotypic plasticity in an island songbird exposed to a novel predation risk

Abstract: Avian nest site selection and levels of parental care require assessments of numerous fitness costs and benefits. Nest site selection in open cup-nesting species is considered a relatively conservative trait; most species and genera are confined to nesting within particular vegetation strata. The nesting stratum further determines risk to nest predation, the principal cause of reproductive failure. We document predator-induced plasticity in nest site placement and levels of parental care in orange-crowned warblers (Vermivora celata) on an island lacking avian nest predators. We show a shift from ground nesting, characteristic of mainland populations, to off-ground nesting that appears adaptive relative to higher predation levels of ground nests. By altering the perceived nest predation risk via experimental introduction of a model avian predator prior to nest building, we demonstrate that warblers shift nest sites to more concealed ground locations. Moreover, warblers differentially adjust nest visits to feed nestlings in the presence of the predator: reducing feeding more at less concealed off-ground nests than at more concealed ground ones. Both shifts in nest site placement and feeding rate adjustments suggest adaptive phenotypic plasticity in response to increased perceived predation risk, providing evidence that birds continuously assess variation in the fitness costs and benefits of their behavioral decisions.

Female reproductive synchrony predicts skewed paternity across primates

Abstract: Recent studies have uncovered remarkable variation in paternity within primate groups. To date, however, we lack a general understanding of the factors that drive variation in paternity skew among primate groups and across species. Our study focused on hypotheses from reproductive skew theory involving limited control and the use of paternity ‘‘concessions’’ by investigating how paternity covaries with the number of males, female estrous synchrony, and rates of extragroup paternity. In multivariate and phylogenetically controlled analyses of data from 27 studies on 19 species, we found strong support for a limited control skew model, with reproductive skew within groups declining as female reproductive synchrony and the number of males per group increase. Of these 2 variables, female reproductive synchrony explained more of the variation in paternity distributions. To test whether dominant males provide incentives to subordinates to resist matings by extragroup males, that is, whether dominants make concessions of paternity, we derived a novel prediction that skew is lower within groups when threat from outside the group exists. This prediction was not supported as a primary factor underlying patterns of reproductive skew among primate species. However, our approach revealed that if concessions occur in primates, they are most likely when female synchrony is low, as these conditions provide alpha male control of paternity that is assumed by concessions models. Collectively, our analyses demonstrate that aspects of male reproductive competition are the primary drivers of reproductive skew in primates. Key words: limited control, paternity, primates, reproductive skew, reproductive synchrony. [Behav Ecol]

What best explains vigilance in elk: characteristics of prey, predators, or the environment?

Abstract: We quantified the vigilance levels of elk (Cervus elaphus) preyed on by wolves (Canis lupus) in Yellowstone National Park, between January and May in 2005 and 2006, and used Akaike’s information criterion to compare a set of 38 regression models for vigilance levels. These models combined up to 9 predictor variables of 3 types: characteristics of the prey group (herd size and composition), characteristics of the predator (wolf pack size, distance away, and the presence/absence of a kill), and characteristics of the local environment (distance to woodland edges, snow depth, and snow cover). The set of models spanned a range of complexity from simple univariate models to complex combinations with up to 3 variables of each type. Complex models incorporating the characteristics of the wolf pack, the structure of the elk herd, and the environmental conditions had higher information content than simple models. Although univariate models of vigilance detect significant relationships, they have low information content relative to multivariate models. These results show that elk assesses factors of several types when assessing risk and deciding how much time to allocate to vigilance. In particular, we found that all well-supported models of vigilance included several ‘‘prey’’ variables and several ‘‘predator’’ variables. This result highlights the need to consider information about predators when trying to explain variation in vigilance levels in prey. Key words: antipredator behavior, Canis lupus, Cervus elaphus, elk, predation risk, vigilance, wolf. [Behav Ecol]

A mechanism for cryptic female choice in chinook salmon

Abstract: Female mate choice after copulation or spawning is cryptic when a female differentially influences the fertilization success of sperm from different males. We tested whether ovarian fluid could act as a potential mechanism of cryptic female choice (CFC) in chinook salmon (Oncorhynchus tshawytscha) by comparing how ovarian fluid from each of 7 females affected the sperm behavior of 11 different males. Using computer-assisted sperm analysis, we measured sperm velocity, motility, longevity, and linearity from the ejaculates of each male activated in the ovarian fluid from each female. Mean sperm swimming speed, path trajectory, and longevity differed significantly among males, and within males depended on the female’s ovarian fluid in which it was activated. Most important, the pattern of within-male variation in these traits also varied significantly among males in response to different females’ ovarian fluids. As sperm velocity is known to be a prime determinant of fertilization success in externally fertilizing fishes. This finding suggests that variation in the composition of female ovarian fluid may be a mechanism for CFC, whereby females differentially enhance the swimming speed of sperm from different males. Thus, female ovarian fluid can alter relative male fertilization success when there is intense sperm competition, as there is in this and other group spawning fish species. Key words: chinook salmon, cryptic female choice, Oncorhynchus tshawytshca, ovarian fluid, sperm traits, sperm velocity. [Behav Ecol]

Competitor density cues for habitat quality facilitating habitat selection and investment decisions

Abstract: The theory of species coexistence predicts avoidance between species that compete for similar resources. Recent studies, however, have suggested that facilitation is also possible if competitor density provides information about resources. Optimal solution to trade-off between competition and facilitation is predicted to occur at intermediate competitor densities. We tested this hypothesis by experimentally creating a density range of resident tit species (Parus spp.), and measured the response of a competitively subordinate migratory bird, the collared flycatcher (Ficedula albicollis) in terms of habitat preference (settlement order and density), offspring investment (clutch size and primary sex ratio of offspring), and reproductive success (number and condition of nestlings). We show that most habitat choice and investment decisions of flycatchers were unimodally related to tit density, whereas reproductive success decreased linearly with increasing density. Flycatchers thus made mismatched investment decisions at the artificial tit densities because manipulation disassociated the natural correlation between habitat quality and population density. Apparently low and high tit densities were perceived as indication of poor quality habitat in terms of low amount or quality of resources/high mortality risk and high costs of competition, respectively. This demonstrates that competitor density can be used in assessing overall habitat quality in habitat selection and offspring investment decisions, integrating information on resources and competition.

Conspicuousness, not eye mimicry, makes "eyespots" effective antipredator signals

Abstract: Many animals bear colors and patterns to reduce the risk of predation from visually hunting predators, including warning colors, camouflage, and mimicry. In addition, various species possess paired circular features often called ‘‘eyespots,’’ which may intimidate or startle predators preventing or postponing an attack. Most explanations for how eyespots work assert that they mimic the eyes of the predators own enemies. However, recent work has indicated that spots may reduce the risk of predation based purely on how conspicuous they are to a predator’s visual system. Here, we use a field technique involving artificial prey marked with stimuli of various shapes, numbers, and sizes, presented to avian predators in the field, to distinguish between the eye mimicry and conspicuousness theories. In 3 experiments, we find that the features which make effective antipredator wing markings are large size and higher numbers of spots. Stimuli with circles survived no better than those marked with other conspicuous shapes such as bars, and changing the spatial construction of the spots to increase the level of eye mimicry had no effect on the protective value of the spots. These experiments support other recent work indicating that conspicuousness, and not eye mimicry, is important in promoting avoidance behavior in predators and that eyespots on real animals need not necessarily, as most accounts claim, mimic the eyes of other animals. Key words: antipredator, conspicuousness, eyespots, mimicry, predation, vision. [Behav Ecol]

Female mating biases for bright ultraviolet iridescence in the butterfly Eurema hecabe (Pieridae)

Abstract: Exaggerated male-limited coloration is widespread among butterflies, yet convincing demonstrations of intraspecific mating preferences for signal brightness and/or chromaticity are relatively rare in this group. Here, I couple behavioral experiments involving manipulations of ambient light environments and male reflectance patterns with observation of wild mating patterns to investigate visual mating biases in the large grass yellow (Eurema hecabe). Males in this species possess exaggerated, limited-view ultraviolet (UV) iridescence across most of their dorsal wing surface that has putative sexual signaling function. In the first experiment, conducted in small (0.7‐m 3 ) cages, individuals were significantly less likely to copulate when the UV portion of natural ambient illumination (i.e., 300‐400 nm) was strongly reduced. In 2 subsequent experiments, conducted under fullspectrum sunlight in small and large (5 3 6 3 4 m) cages, males with their UV signal artificially dulled by 25% consistently copulated with fewer, and smaller, females than sham-control individuals. Importantly, the manipulated levels of UV brightness in these experiments fall well within the naturally occurring bounds of variation in male UV reflectance. These findings therefore unanimously support the presence of a UV signal‐based female bias. In apparent contrast, comparison of 161 in-copula and 188 free-flying males from a high-density field assemblage revealed that copulating males were significantly older and henceforth actually possessed (subtly) less UV bright wings. Copulating male UV brightness was, however, positively related to the size of their mate, which echoes the experimental findings and may represent a signature of mutual mate choice. I discuss these results in light of the full complexities of the butterfly mating system and the potential signaling value of iridescent coloration in butterflies and animals generally. Key words: coloration, female mate choice, Lepidoptera, ornamentation, sexual selection, signaling, structural color. [Behav Ecol]

Accuracy of song syntax learning and singing consistency signal early condition in zebra finches

Abstract: Birdsong is a sexually selected and culturally transmitted multidimensional signal. Sexually selected traits are generally assumed to indicate condition. In oscine songbirds, song is learned early in life. The developmental stress hypothesis proposed that poor early developmental condition can adversely affect song learning. The quality and accuracy of learned song features could thus indicate male quality to conspecifics. Surprisingly, studies testing this hypothesis to date mostly compared adult males’ song repertoires without looking at song imitation. The few that did reported inconsistent effects and analyzed a limited number of song features. Here, we examined the effects of early condition (by brood size manipulation) on learned song in zebra finches, Taeniopygia guttata, in comparing both the number of specific elements copied from an adult song tutor and a great number of previously neglected syntax-, complexity-, and performance-related song features. The treatment did not significantly affect average number of imitated elements, the standard measure of quality of song imitation in this species. However, developmental condition had 2 significant main effects on adult song: birds from large broods (i.e., of poor early condition) in comparison to birds from small broods copied syntactical dependencies of song elements from the song motif of their tutor less accurately and had less consistent sound duration between song motifs. These findings support the developmental stress hypothesis. We discuss how this sheds light on the potential role of such long-term signals of male developmental condition in female mate choice and potential constraints underlying condition-dependent expression of song features.

Predator-induced reductions in nest visitation rates are modified by forest cover and food availability

Abstract: Bird parents can alert predators to the location of their nest. One mitigating option is that parents reduce their nest visitation rate in exchange for a lower predation risk. Here, using field data and experiments, we show that Siberian jay Perisoreus infaustus parents adjust feeding visit rates depending on an interaction of 3 factors: predator activity, nest concealment, and food availability. The rate of nest visits increased with the degree of nest concealment; yet, this relationship was modified by the presence of corvid predators. As the vegetation became more dense, parents at sites with high corvid activity disproportionately increased their feeding visit rates when compared with birds at sites with low corvid activity. We experimentally assessed how nesting cover affects this response of parents to the presence of corvids by using an Eurasian jay Garrulus glandarius model. Parents nesting at open sites ceased nest visits, whereas those nesting in dense forest continued feeding, albeit at a lower rate. Cover may thus not fully compensate for the effect of predator activity on feeding visit rates. However, offspring exposed to high predator activity might still receive the same amount of food because parents may adjust load sizes to compensate. This idea was confirmed by an experiment showing that in areas of high predator activity, food-supplemented birds significantly decreased nest visits when compared with nonsupplemented birds. These results indicate that some bird species can employ multiple nest-defense strategies to reduce predator-attracting nest visits; yet, these strategies may carry fitness consequences through reduced offspring quality.

The confusion effect—from neural networks to reduced predation risk

Abstract: The confusion effect is often cited as an antipredatory benefit of group living and has been demonstrated by numerous studies across a range of taxa. However, there have been relatively few studies examining the mechanisms behind the effect and no experimental test of its supposed theoretical basis (information degradation in neural networks) using a natural predator-prey pairing. In agreement with other studies, we demonstrate that attack success of the three-spined stickleback (Gasterosteus aculeatus L.) is reduced by an increase in Daphnia magna group size. Neural network models attempt to explain this trend with multiple prey inducing poor neural mapping of target prey, thus leading to an increase in the spatial error of each attack. We explicitly tested this prediction and demonstrate that the decrease in attack success by sticklebacks does correspond to an increase in spatial targeting error with larger prey group size. Finally, we show that the number of targets, rather than the density or area occupied by the group, has the greatest effect on reducing the rate of attack. These results are discussed in the context of the information processing constraints of predators, the ultimate cause of the confusion effect.

The size of the red wing spot of the American rubyspot as a heightened condition-dependent ornament

Abstract: We investigated an ornamental trait known to reflect male fighting ability and tested whether it shows heightened condition dependence compared with nonornamental traits in the American rubyspot (Hetaerina americana). Adult males bear red wing spots, the size of which is sexually selected: large-spotted and fatter males are more successful in territorial competition and obtain more matings than are nonterritorial males. First, to see whether spot area may signal fighting ability at a particular age (to discriminate animals that are unlikely to compete), we investigated the age at which males engaged more in fighting and compared their fat reserves and muscle mass at 3 ages (young, middle aged, and old) and territorial status. Middle-aged males showed the highest fat and muscle values, engaged more in fighting, and were predominantly territorial. Second, we looked for traits not shaped by sexual selection: we compared red chroma and brightness of spot and thorax, spot area, muscle mass, and fat reserves in winner and loser males after a territorial contest. The only difference was that winners had larger spot areas and higher fat reserves. Finally, an immune challenge-based experiment was performed during the development of spot area and its color properties (chroma and brightness). Compared with a control (unchallenged) group, the results revealed that area decreased, brightness increased, and there was no change in red chroma, muscle mass, and fat reserves in challenged animals. Thus, spot area is a stress-sensitive, energy-reflecting trait that is likely to be used for communication during territorial competition in these damselflies. Key words: American rubyspot, energetic signaling, heightened condition dependence. [Behav Ecol]

Female praying mantids use sexual cannibalism as a foraging strategy to increase fecundity

Abstract: Several hypotheses have been proposed to explain the evolution of sexual cannibalism. The foraging strategy hypothesis states that sexual cannibalism may arise as an adaptive foraging strategy, providing females with the nutrients to increase future fecundity. Yet, very few studies have found that nourishment through cannibalism translates into increased fecundity. One explanation for this may be that most of these investigations have concentrated on sexually dimorphic spider species with tiny males that do not significantly increase female body mass on consumption. The current study focuses on the praying mantid, Pseudomantis albofimbriata, a moderately size dimorphic species with relatively large males. Cannibalistic females of such species may be more likely to gain nutritional benefits from male consumption, which translate into increased fecundity. Here, cannibalistic females substantially improved their body condition and subsequently produced heavier egg cases than their noncannibalistic counterparts. An additional prediction of the foraging strategy hypothesis is that sexual cannibalism will increase with decreasing female condition. We found that the prevalence of sexual cannibalism in this system was indeed affected by female body condition; females in poor condition were more likely to consume their potential mates than females in good condition. Additional analysis of the data refuted the relevance of each of the remaining hypotheses for this species, providing clear evidence for the foraging strategy hypothesis as an explanation for the maintenance of sexual cannibalism in this species.

Costs of group size: lower developmental and reproductive rates in larger groups of leaf monkeys

Abstract: Feeding competition is suggested as a major factor constraining group size in social foragers. It has, however, been challenging to demonstrate consequences of reduced energy gain in terms of fitness, possibly because social foragers may compensate negative effects of scramble competition via adjustments in time budgets. Herbivorous animals are particularly interesting in this context because their fibrous diet and slow digestion process may make it difficult to adjust time budgets. Here we investigate infant development and reproductive rates in Phayre’s leaf monkeys (Trachypithecus phayrei) at Phu Khieo Wildlife Sanctuary, Thailand. The diet of the species consists of 39.0% leaves (maximum 81.2% per month). Our analysis is based on data for 3 groups (185 group months) of different sizes (mean 11.4, 18.3, and 25.8 individuals, respectively). Infant development was significantly slower in the large group, in which infants changed to the adult coat later than in the medium-sized group (20.3 vs. 26.3 weeks) and were older when weaned (18.3, 19.7, and 21.4 months, respectively). The interbirth interval after a surviving infant significantly increased with group size (21.3, 22.8, and 24.5 months) while rearing success did not differ (77.8%, 76.5%, and 82.4%, survival to 2 years). Thus, infants in the large group developed more slowly were weaned later and females reproduced more slowly. With similar infant survival rates, these different reproductive rates indicate fitness differences across groups. As in other herbivores, these group-specific differences may reflect scramble competition for food or differences in habitat quality. Key words: infant development, infant mortality, interbirth interval, reproductive costs, scramble competition, Trachypithecus phayrei. [Behav Ecol]

Flight distance and population trends in European breeding birds

Abstract: Flight distance reflects the risk that individual animals are willing to take when approached by a potential predator, as shown by a negative relationship between susceptibility to predation and flight distance. Species with long flight distances should more often suffer from disruption of their activities by potential predators, including humans, than species with short distances, resulting in declining reproductive success and hence declining population size of such species if disturbance happens more often. Long flight distances thus suggest that individuals need large amounts of space for their body size, resulting in the prediction that species with long flight distances should have a higher frequency of declining populations than species with short flight distances, when human impact on natural habitats increases. Bird species with long flight distances had declining population trends across Europe in a comparative study of 56 species, even when controlling statistically for other factors associated with population declines, with effect sizes ranging from 0.36 to 0.58 (Pearson’s r). These findings suggest that standardized measures of flight distance provide reliable information about the population consequences of risk-taking behavior by individuals and the susceptibility of different species to increased levels of disturbance. Key words: bird census, birds, population consequences, risk taking. [Behav Ecol]

Philopatry in prairie voles: an evaluation of the habitat saturation hypothesis

Abstract: Natal philopatry, or delayed dispersal of sexually mature offspring, may be due to ecological constraints on dispersal. In this study, we manipulated the population density of prairie voles (Microtus ochrogaster) living in experimental outdoor enclosures to test a prediction from the habitat saturation hypothesis that philopatry and subsequent group formation in this cooperatively breeding mammal is affected by the availability of suitable territories. We detected a significant, positive relationship between the proportion of offspring remaining philopatric and density, with females being more philopatric than males at all densities. This increase in philopatry led to a significant increase in the proportion of social units that were groups as well as a significant increase in group size. These results provide the strongest evidence of a causal effect of density on dispersal and group formation in a mammal. Our findings suggest that habitat saturation is at least a partial explanation for philopatry in prairie voles. However, we cannot eliminate the possibility that other variables, such as benefits accrued from remaining philopatric, may also be factors contributing to philopatry. Nonetheless, these results show that changes in ecological conditions can influence social structure within a population, leading to group formation and a social milieu conducive to the evolution of cooperative breeding.

The influence of sex and sociality on parasite loads in an African ground squirrel

Abstract: Male-biased parasite loads, which are common in vertebrates, could be a consequence of sexual selection, and for species that group, the costs of parasites could vary with group size or social structure. We examined sex-biased parasitism and the influence of group size on parasite loads in Cape ground squirrels (Xerus inauris), a highly social species that occurs in the arid regions of southern Africa. Group size did not affect numbers of ectoparasites or endoparasites. Males carried 3 times as many ectoparasites as females, but females harbored nearly 3 times as many endoparasites as males. Age class did not affect parasite loads in females, but in males, adults carried more ectoparasites than juveniles. Allogrooming was performed primarily by females, but no sex difference was found in autogrooming. Males in the subadult age class are becoming scrotal (indicating an increase in sex hormones) but typically remain in the natal group until adulthood, maintaining home range sizes comparable to adult females. Our results suggest that sexual selection does influence parasite loads in this species; increased androgen levels may reduce ectoparasite resistance in males, and smaller home ranges may increase females’ exposure to endoparasites. Allogrooming could reduce ectoparasite loads of the group and mitigate the costs of grouping. Key words: ectoparasite, endoparasite, fleas, group size, grooming, lice, sex-biased parasitism, sexual selection, ticks. [Behav Ecol]

Risk taking by singing males

Abstract: The distance at which an individual flees from a potential predator represents a measure of risk taking. If individuals are engaged in another activity that might affect fitness, trade-offs between the fitness benefits of flight and the other activity should determine the nearest distance of approach by a predator. In a comparative analysis of birds, flight distance represented a reliable measure of risk of predation by the sparrowhawk Accipiter nisus that increased with decreasing flight distance across species. To test the hypothesis that singing males adjusted their risk taking to the costs and benefits of early flight, we compared the flight distance of singing and nonsinging birds to an approaching human observing with a binocular. Singing birds on average fled at a greater distance than nonsinging birds, implying that singing birds took small risks. We used a standardized measure of difference in flight distance between singing and nonsinging individuals to investigate factors affecting interspecific variation in risk taking. Species that used more exposed song posts (sites used for singing) took smaller risks than species with less exposed song posts. Species that suffered from higher levels of parasitism as reflected by the prevalence of Plasmodium, but not by 3 other genera of blood parasites, took greater risks during singing compared with nonsinging activities. Likewise, species with high circulating levels of natural antibodies, and hence a history of natural selection caused by bacteria took relatively greater risks during singing than species with few natural antibodies. These findings suggest that risks taken by singing birds have been molded by natural and sexual selection, and that risk taking represents a compromise between the costs and benefits of flight from a potential predator.

Heterospecific eavesdropping in a nonsocial species

Abstract: Many species respond to heterospecific alarm calls, and the majority are social taxa and possess complex alarm calls themselves. Thus, the ability to respond to heterospecific alarm calls may be facilitated by possessing both these traits. Gunther’s dik-dik (Madoqua guentheri) is a monogamous, territorial, and nonsocial miniature antelope with a simple vocal repertoire. Dik-diks are highly vulnerable to predation and could benefit from eavesdropping on heterospecific alarm calls. We broadcast recordings of white-bellied go-away bird (Corythaixoides leucogaster) alarm calls to dik-diks to test whether they could respond to heterospecific alarm calls. On hearing a go-away bird alarm call, dik-diks increased their likelihood to run to cover, decreased their foraging activity, increased their rate of head turning, and increased their period of vigilance compared with a nonthreatening bird song. Thus, predation risk alone, in the absence of complex sociality or complex communicative abilities, may be sufficient to drive the evolution of heterospecific eavesdropping. Key words: associative learning evolution, eavesdropping, dik-dik, heterospecific alarm call. [Behav Ecol 19:1041–1046 (2008)]

Host shifts favor vibrational signal divergence in Enchenopa binotata treehoppers

Abstract: For specialized herbivorous insects, shifts to novel host plants can have dramatic evolutionary consequences. If mating traits diverge, assortative mating can develop between ancestral and novel host populations and facilitate speciation. Mating signals may diverge under a variety of scenarios. Signal differences may be a consequence of divergence in correlated traits, such as body size. If local communication environments differ, mating signals may also diverge through selection for enhanced transmission. We tested these hypotheses using 2 closely related species in the Enchenopa binotata complex of treehoppers. Each member of this complex specializes on a different host plant species. Their communication modality may make signal divergence likely after a host shift: like many plant-dwelling insects, Enchenopa communicate using substrate-borne vibrations for which the plant itself is the transmission channel. Each species' mating signal is a relatively pure tone, and differences between species in signal frequency are critical for mate recognition. Whereas no support was found for a correlated selection hypothesis, we found support for a signal transmission hypothesis: both species use a signal frequency that transmits well in their contrasting communication environments, suggesting that host shifts may favor signal divergence and ultimately behavioral isolation.

Response to conspecific and heterospecific alarm calls in mixed-species bird flocks of a Sri Lankan rainforest

Abstract: The flow of information about predators in mixed-species groups is likely to influence the structure and organization of these communities. To map this flow, it is necessary to demonstrate in a playback experiment that animals use the information provided in heterospecific calls. We investigated the response of birds to conspecific and heterospecific calls in a mixed-species flock system of a Sri Lankan rainforest. We played back the aerial alarm calls of the species that produce the most alarms, the Orange-billed Babbler (Turdoides rufescens) and the Greater Racket-tailed Drongo (Dicrurus paradiseus), as well as natural choruses of alarm calls of several species, to 4 species of birds. We found that babblers moved quickly away from the playback speaker after hearing either babbler or drongo alarm calls. Ashy-headed Laughingthrushes (Garrulax cinereifrons) responded similarly to drongo alarm calls. Response by other species was less clear: Malabar Trogons (Harpectes fasciatus) stayed perched for longer during alarms of several species; drongos showed no difference in response to any treatment. We conclude that birds are listening to heterospecifics and suggest that their response is influenced by several factors, including their foraging technique. The fact that the babbler, the gregarious leading species of the flocks, gains information from another species is one of the first indications that such a “nuclear” species can benefit from its participation in mixed-species flocks, and the response of at least 2 species to drongo alarms emphasizes the importance sentinel species such as drongos may play in flock communities.

Multiple proximate and ultimate causes of natal dispersal in white-tailed deer

Abstract: Proximate and ultimate causes of dispersal in vertebrates vary, and relative importance of these causes is poorly understood. Among populations, inter- and intrasexual social cues for dispersal are thought to reduce inbreeding and local mate competition, respectively, and specific emigration cue may affect dispersal distance, such that inbreeding avoidance dispersal tends to be farther than dispersal to reduce local competition. To investigate potential occurrence of multiple proximate and ultimate causes of dispersal within populations, we radio-marked 363 juvenile male white-tailed deer (Odocoileus virginianus) in 2 study areas in Pennsylvania. Natal dispersal probability and distance were monitored over a 3-year period when large-scale management changes reduced density of adult females and increased density of adult males. Most dispersal (95‐97%) occurred during two 12-week periods: spring, when yearling males still closely associate with related females, and prior to fall breeding season, when yearling males closely associate with other breeding-age males. Following changes to sex and age structure that reduced potential for inbreeding and increased potential for mate competition, annual dispersal probability did not change; however, probability of spring dispersal decreased, whereas probability of fall dispersal increased. Spring dispersal distances were greater than fall dispersal distances, suggesting that adaptive inbreeding avoidance dispersal requires greater distance than mate competition dispersal where opposite-sex relatives are philopatric and populations are not patchily distributed. Both inbreeding avoidance and mate competition are important ultimate causes of dispersal of white-tailed deer, but ultimate motivations for dispersal are proximately cued by different social mechanisms and elicit different responses in dispersers. Key words: demography, dispersal probability, dispersal rate, proximate cause, sex-biased dispersal, sociobiology. [Behav Ecol]

Plumage color and reproduction in the red-backed fairy-wren: Why be a dull breeder?

Abstract: Males of many species can breed in distinct alternative phenotypes; for example, in many birds some males breed in dull plumage while others breed in bright plumage. Because females often appear to prefer brighter males, it is unclear why some males breed in dull plumage. Males in dull plumage might enjoy enhanced within-pair reproductive success if they can gain access to better breeding territories, or they might have relatively high extrapair reproductive success if they are better able to intrude on the territories of other males. To test these possibilities, we examined the reproductive consequences of plumage color in the red-backed fairy-wren (Malurus melanocephalus), a species in which males can breed in either bright plumage or dull plumage or serve as nonbreeding auxiliaries. Male plumage color was distributed bimodally and was loosely associated with age, such that some males molted into bright plumage a year or more earlier than others. Both male phenotypes were cuckolded at similar rates, but bright males sired significantly more extrapair young than did dull males, and this effect was independent of age. Thus, 1-year-old males who bred in dull plumage had low seasonal reproductive success compared with same-aged males who bred in bright plumage. These results suggest that males may not reap any fitness benefits by breeding in dull coloration, compared with breeding in bright plumage, but rather may be constrained to breed in suboptimal plumage by the timing of plumage acquisition.

Predictors of resistance to brood parasitism within and among reed warbler populations

Abstract: In the coevolutionary arms race between avian brood parasites and their hosts, several adaptations have evolved on both sides, the most prominent and important host defense being rejection of the parasitic egg. In the present study, we investigated possible predictors of egg rejection in 14 populations of reed warblers Acrocephalus scirpaceus across Europe differing in risk of parasitism by the common cuckoo Cuculus canorus, providing a test of factors associated with geographic variation in host resistance to parasitism. In a binomial general linear mixed model procedure, we quantified the possible influence of host clutch size, cuckoo parasitism in population (yes/no), height of the nest above ground, height of vegetation in the vicinity of the nest and distance to nearest vantage point on rejection of an experimentally added nonmimetic cuckoo sized egg. In addition, we entered ‘‘population’’ into the models as a random factor. Rejection rate varied significantly among populations (range 4.8–68.9%). The most parsimonious model, based on selection by the Akaike information criterion, included cuckoo parasitism in the population (yes/ no) and host clutch size; rejection rate was the highest in parasitized populations, and individuals laying larger clutches were the best rejecters. Furthermore, rejecters tended to breed in higher vegetation than acceptors. These findings suggest that spatial variation in the level of host resistance to brood parasitism may depend on current and/or past selection pressure due to the parasite and individual differences linked to abilities for egg rejection. Key words: Acrocephalus scirpaceus, brood parasitism, clutch size, coevolution, common cuckoo, Cuculus canorus, egg rejection, metapopulation, reed warbler. [Behav Ecol]

Availability of nonpigmentary antioxidant affects red coloration in gulls

Abstract: Red, orange, and yellow carotenoid-based colorations displayed by fishes and birds may function as honest sexual signals of the bearer's quality. However, the mechanisms underlying the expression of these traits and the information they convey are still controversial. Because carotenoids are antioxidants and carotenoid-based pigmentation is bleached as a consequence of oxidative processes, it has been suggested that the pigmentation may signal antioxidant status. We tested this hypothesis in the yellow-legged gull (Larus michahellis), a seabird that exhibits a carotenoid-based red spot on the lower mandible. The availability of a nonpigmentary antioxidant (i.e., vitamin E) to the gulls was modified before egg laying by means of a supplementary feeding experiment. During the incubation period, breeding pairs were captured to assess the intensity of the color and the size of the red bill spots. We measured the plasma level of lipid peroxidation, total antioxidant capacity, and carotenoids. We found that males that received vitamin E supplements had larger red spot than control birds but that color intensity was not affected by the supplements. Moreover, we found that only those plasma carotenoids involved in the red coloration were affected by the antioxidant supplementation, suggesting an active mechanism to increase red coloration. Overall, our results provide experimental evidence for the hypothesis that carotenoid-based coloration reflects the bearer's antioxidant status in male gulls.

Mating system, sexual dimorphism, and the opportunity for sexual selection in a territorial ungulate

Abstract: In mammals, species with high sexual size dimorphism tend to have highly polygynous mating systems associated with high variance in male lifetime reproductive success (LRS), leading to a high opportunity for sexual selection. However, little information is available for species with weak sexual size dimorphism. In a long-term study population, we used parentage analysis based on 21 microsatellite markers to describe, for the first time, variance in male lifetime breeding success (LBS) of roe deer, a territorial ungulate where males weigh less than 10% more than females. LBS ranged from 0 to 14 (mean = 4.54, variance = 15.5), and its distribution was highly skewed, with only a few males obtaining high LBS and many males failing to breed or siring only one fawn. As predicted for polygynous species with low sexual size dimorphism, the standardized variance in male LBS was low (I m = 0.75) and was only slightly higher than the standardized variance in female LRS (I f = 0.53), suggesting a low opportunity for sexual selection. The I m value reported here for roe deer is much lower than values reported for highly dimorphic ungulates such as red deer (I m > 3). We suggest that, along a continuum of opportunity for sexual selection, roe deer occupy a position closer to monogamous and monomorphic territorial ungulates than to highly polygynous, sexually dimorphic ungulates with dominance rank-based mating systems such as harems or roving mating systems.