Showing papers in "Behavioral Ecology in 2005"

Determining the fitness consequences of antipredation behavior

Abstract: Any animal whose form or behavior facilitates the avoidance of predators or escape when attacked by predators will have a greater probability of surviving to breed and therefore greater probability ...

Context dependence of personalities: risk-taking behavior in a social and a nonsocial situation

Abstract: Individuals of many species differ consistently in their behavioral reaction to mild novel challenges. Suites of these behaviors are referred to as behavioral syndromes or personalities. Personality traits are often phenotypically and genetically correlated. Therefore, animal personalities are generally considered as broad characteristics, with one underlying genetical and physiological mechanism that is expressed across situations and contexts. Because there are carryover effects between situations, animals are not entirely flexible in their behavior in each situation. This may cause behaviors to seem nonadaptive in isolated situations. To test whether individuals with different personalities could react differently to changes in their environment, we studied context dependence of personalities in the great tit (Parus major). We tested birds categorized as either fast or slow explorers for their latency to come back to a feeding table after a mild startle (risk-taking behavior) in a nonsocial followed by a social context. We found that the relation between exploratory behavior and risk-taking behavior depended on the social context. Females in general returned later in the social test, while male reaction to the presence of a conspecific was dependent on their behavioral type. Slow males thereby reacted to the behavior of the companion and fast males did not. These results show that although personalities have a rigid structure the relation between personality traits is context dependent. These results are discussed in the perspective of the adaptive significance and maintenance of personalities. [KEYWORDS: Behavioral syndromes ; boldness ; exploration ; Parus major ; personality ; risk taking]

Quorum sensing by encounter rates in the ant Temnothorax albipennis

Abstract: Emigrating colonies of the ant Temnothorax (formerly Leptothorax) albipennis can choose the best of several nest sites, even when the active ants organizing the move do not compare sites. This collective ability depends on a quorum rule used by ants assessing a candidate site. Only when the site’s population has surpassed a threshold do they switch from slow recruitment of fellow active ants by tandem runs to rapid transport of the majority of the colony. Here, I show that ants perceive the achievement of a quorum through their rate of direct encounters with nest mates at the site. When ants in a crowded site were prevented from tactile contact with nest mates, they recruited by tandem runs, as though to an empty nest. Furthermore, when the encounter rate was raised independent of population, by reducing the size of the candidate nest, ants started to transport at a significantly lower population. The switch occurred at the same encounter rate regardless of nest size, whether the rate was measured as the mean over the entire visit or as the inverse of the latency until the first encounter. Because encounter rate reflects the density of nest mates and thus varies with nest size as well as population, the ants’ collective decision-making algorithm may be robust to the exact population at which the switch to transport occurs. Ants cease monitoring quorum presence after switching to transport, coincident with an abrupt shortening of visit duration by approximately 2 min, which may be interpreted as the time required for quorum detection. Key words: ants, collective decision making, nest-site selection, quorum sensing, Temothorax albipemis. [Behav Ecol 16:488–496 (2005)]

A taste for novelty in invading house sparrows, Passer domesticus

Abstract: One of the central questions in invasion biology involves why some introductions succeed and others fail. Although several correlates of invasion success have been identified, patterns alone cannot identify the mechanisms underlying the invasion process. Here, we test the hypothesis that one predictor of invasion success, behavioral flexibility, is different between invading and established populations of the same species of bird. We predicted that neophobia (fear of novelty), a surrogate of behavioral flexibility, would be weaker in an actively invading population (28 years resident; Colon, Republic of Panama) of house sparrows (Passer domesticus) compared to a population that had been resident for more than 150 years (Princeton, New Jersey, USA). To test this hypothesis, we compared latency to consume novel foods and phobia of novel objects between populations when both were kept under similar environmental conditions in captivity. As predicted, birds from the 150-year-old population took significantly longer to approach and consume novel foods than birds from the 28-year-old population. Responses to novel objects were not different between populations however; both populations fed more readily near some novel objects, which to our knowledge is the first such occurrence in a wild vertebrate species. Overall, a predilection for trying new foods and being attracted to novel objects may in part explain how this species has so successfully invaded new areas. Key words: foraging, invasive species, neophobia, range expansion. [Behav Ecol]

Spinner dolphins in a remote Hawaiian atoll: social grouping and population structure

Abstract: Spinner dolphins (Stenella longirostris) commonly use inshore island and atoll habitats for daytime rest and social interactions and forage over deep waters at night. In Hawaii, they occur througho ...

Density distribution and size sorting in fish schools: an individual-based model

Abstract: In fish schools the density varies per location and often individuals are sorted according to familiarity and/or body size. High density is considered advantageous for protection against predators and this sorting is believed to be advantageous not only to avoid predators but also for finding food. In this paper, we list a number of mechanisms and we study, with the help of an individual-based model of schooling agents, which spatial patterns may result from them. In our model, schooling is regulated by the following rules: avoiding those that are close by, aligning to those at intermediate distances, and moving towards others further off. Regarding kinship/familiarity, we study patterns that come about when agents actively choose to be close to related agents (i.e., ‘active sorting’). Regarding body size, we study what happens when agents merely differ in size but behave according to the usual schooling rules (‘size difference model’), when agents choose to be close to those of similar size, and when small agents avoid larger ones (‘risk avoidance’). Several spatial configurations result: during ‘active sorting’ familiar agents group together anywhere in the shoal, but agents of different size group concentrically, whereby the small agents occupy the center and the large ones the periphery (‘size difference model’ and ‘active sorting’). If small agents avoid the risk of being close to large ones, however, small agents end up at the periphery and large ones occupy the center (‘risk avoidance’). Spatial configurations are also influenced by the composition of the group, namely the percentage of agents of each type. Furthermore, schools are usually oblong and their density is always greatest near the front. We explain the way in which these patterns emerge and indicate how results of our model may guide the study of spatial patterns in real animals. Key words: assortment, density distribution, fish, self-organization, school form, spatial structure. [Behav Ecol]

Optimal group size and seasonal stress in ring-tailed lemurs (Lemur catta)

Abstract: Adaptive explanations for social grouping assume that there are fitness consequences associated with group size, and individuals maintain membership in groups of favorable size to maximize fitness. Here I examine fecal cortisol concentrations as a hormonal measure of stress to assess the relative well-being of Lemur catta in groups of different size and in seasons of normal and low tamarind fruit availability. I test the hypotheses that there is an optimal group size at which cortisol is lowest and that optimal group size changes in food-scarce conditions. I collected 799 fecal samples from 87 individuals in seven free-ranging L. catta groups at Berenty Reserve, Madagascar, over a 1-year period (August 1999--July 2000) and determined fecal cortisol concentrations using a radioimmunoassay. Expressing these as residuals from monthly population means to control for temporal fluctuations in cortisol concentration, I calculated mean fecal cortisol levels for each animal in seasons of normal and low tamarind fruit abundance and over the entire year. Overall, females exhibited lowest mean cortisol levels in groups of intermediate size, suggesting that there are benefits to maintaining membership in these groups. Females in groups that were atypically large or small for their habitat type had higher mean cortisol levels than typical groups. Cortisol levels increased in food-scarce conditions for larger groups, suggesting that intergroup competitive advantages do not outweigh intragroup feeding competition at this time. Group size may be optimized for long-term average conditions, and short-term stresses may intermittently alter the costs associated with group size. Copyright 2005.

Effects of predation pressure on the cognitive ability of the poeciliid Brachyraphis episcopi

Abstract: Variable levels of predation pressure are known to have significant impacts on the evolutionary ecology of different populations and can affect life-history traits, behavior, and morphology. To date, no studies have directly investigated the impact of predation pressure on cognitive ability. Here we use a system of replicate rivers, each with sites of high- and low-predation pressure, to investigate how this ecological variable affects learning ability in a tropical poeciliid, Brachyraphis episcopi. We used a spatial task to assess the cognitive ability of eight populations from four independent streams (four high- and four low-predation populations). The fish were required to locate a foraging patch in one of four compartments by utilizing spatial cues. Fish from areas of low-predation pressure had shorter foraging latencies, entered fewer compartments before discovering the reward patch and navigated more actively within the maze, than fish from high-predation sites. The difference in performance is discussed with reference to forage patch predictability, inter- and intraspecific foraging competition, geographic variation in predation pressure, boldness--shyness traits, and brain lateralization. Copyright 2005.

Attention-altering signal interactions in the multimodal courtship display of the wolf spider Schizocosa uetzi

Abstract: Complex signals are common throughout the animal kingdom, consisting of one or more signals in one or more sensory modalities presented within a single display. I tested an efficacy-based backup hypothesis of complex signal function using the bimodal courtship signaling wolf spider Schizocosa uetzi. This hypothesis predicts that the visual and vibratory courtship displays function as backups to each other in the presence of environmental variability. I compared mating frequencies across four environmental treatments in which the visual and vibratory environments were manipulated independently in a 2 3 2 design with visual treatments of light/dark (i.e., visual signal present/absent) and vibratory treatments of filter paper substratum/ granite substratum (i.e., vibratory signal present/absent). Results did not match the predictions of an efficacy-based backup hypothesis. The vibratory environment affected mating frequency, with more mating occurring in the vibration-present treatments compared to the vibration-absent treatments, but the visual environment had no effect on mating frequency. A second experiment was then conducted to test for an inter-signal interaction. Using the video-playback technique, I presented females with manipulated video sequences simultaneous with a controlled vibratory signal to test the hypothesis that the presence of a vibratory signal alters a female’s response to the visual signal. In the presence of a vibratory courtship signal, females were more receptive to more visually ornamented males. This increased receptivity to increased visual ornamentation was not seen in a previous study conducted on S. uetzi in the absence of a vibratory signal, suggesting a potential inter-signal interaction. In a third experiment, I tested whether a female’s visual attention was altered by the vibratory signal by examining female response to a visual ‘‘predator’’ while exposed to all possible combinations of male courtship signals. Females were more likely to get caught, and thus less likely to notice a predatory visual stimulus when exposed to a courtship vibration, supporting the hypothesis that the vibratory signal alters a female’s visual attention. Key words: attention-altering, complex signals, inter-signal interaction, limited attention, multimodal signals, spiders, video playback. [Behav Ecol]

Predator-induced plasticity in nest visitation rates in the Siberian jay (Perisoreus infaustus)

Abstract: Predator-induced plasticity in nest visitation rates in the Siberian jay (Perisoreus infaustus).

You can run—or you can hide: optimal strategies for cryptic prey against pursuit predators

Abstract: We consider the optimal behavior of a cryptic prey individual as it is approached by a predator searching for prey. Although the predator has not yet discovered the prey, it has an increasing likelihood of doing so as it gets closer to the prey. Further, the closer the predator is to the prey when it discovers it, the more likely the predator will be to capture the prey. These arguments suggest that the prey should flee before the predator discovers it. However, the act of fleeing will alert the predator to the presence of the prey and trigger an attack that might not have occurred otherwise. We capture these conflicting outcomes in a mathematical model, which we then use to predict the optimal behavior of the prey and predator. We argue that the optimal strategy for the prey is either to run as soon as they detect a predator approaching or to only flee in response to having been detected by the predator. Running as soon as the predator is detected is associated with low predator search speeds, a low nonpredation cost to running, a large advantage to the prey in initiating chases rather than reacting, limited ability to spot the predator at distance, a high ability to spot prey by the predator, and a high probability that chases will be successful. The optimal strategy for the predator depends on whether its current trajectory is taking it closer to or further from the prey. In the latter case, the predator should attack immediately on discovering the prey; in the former case, it should delay its attack until it reaches the point on its current trajectory where distance to the prey is minimized. Key words: antipredator strategies, coursing predators, crypsis, fleeing, flight, predation, predator-prey interactions, prey detection. [Behav Ecol]

Microbial infection affects egg viability and incubation behavior in a tropical passerine

Abstract: Many avian species initiate incubation before clutch completion, which causes eggs to hatch asynchronously. This influences brood competitive dynamics and often results in nestling mortality. The prevailing hypotheses contend that parents incubate early because asynchronous hatching provides fitness benefits to parents or surviving offspring. An alternative idea is that early incubation is the best of a bad job because of the costs of delaying incubation to the viability of first-laid eggs. To explore this, we examined the potential for microbial infection, and the relative effects of infection and suboptimal development temperatures on the viability of pearly-eyed thrasher (Margarops fuscatus) eggs. We exposed newly laid eggs for 5 days at either end of a tropical altitudinal gradient and cleaned shells of half the eggs to reduce microbial growth. Uncleaned eggs were infected more than were cleaned eggs, and infection was greater for eggs exposed at the cool, humid site than at the hot, less humid site. Parentally incubated eggs, however, were not infected, suggesting that incubation limits infection. The consequence of exposure to infection and high ambient temperatures was a dramatic reduction in viability; cleaned eggs held at the cool site had the highest hatching success, which was significantly greater than for uncleaned eggs at this site and for cleaned eggs held at the hot site. This provides the first evidence that microbes can infect unincubated eggs of a wild bird, and that infection and ambient temperature act independently to reduce hatching success. These factors could affect avian life-history strategies in diverse habitats. Key words: ambient temperature, egg viability, hatching asynchrony, onset of incubation, saprophytic microorganism, trans-shell transmission. [Behav Ecol]

Why do male Callosobruchus maculatus harm their mates

Abstract: The importance of cryptic female choice, i.e. female post-copulatory influence over male reproductive success, in driving the evolution of male traits remains controversial. The main aim of this thesis was to understand the post-copulatory consequences of sexual interactions and the importance of cryptic female choice in two species of beetle.Males of the red flour beetle Tribolium castaneum use their legs to rub the lateral edges of the female elytra during mating. When manipulating female perception of this behaviour, I found that females preferentially use the sperm of males with vigorous leg rubbing when they mate with more than one male. Leg rubbing also appeared to increase female rate of oviposition. Females do not seem to gain any indirect benefits by preferring males with an intense leg rubbing behaviour since this behaviour was found to have very low narrow sense heritability and did not appear to be condition dependent in its expression.Males of the bruchid beetle Callosobruchus maculatus have spiny genitalia that harm their mates. Females kick males during copulation and when prevented from kicking, suffered reduced lifetime offspring production as a consequence of more extensive injuries. Males were not able to delay female remating, increase rate of oviposition or increase sperm precedence by inflicting relatively severe injuries to non-kicking females. Hence, the injuries appear to be side effects of male efforts to remain in copula. When copulation duration was manipulated, ejaculate size and female lifetime offspring production increased with the length of copulation. Females reduced their mating rate when they had access to water, suggesting that they obtain water from the large ejaculates and trade-off their need for additional water against the costs of mating. Males may then reduce the benefits of remating by providing their mates with a large amount of water. Females did not increase their remating propensity to avoid inbreeding when they had mated to brothers. Together, these studies reveal the complexity of sexual interactions and the importance of post-copulatory processes for the fitness of both males and females.

The heterospecific habitat copying hypothesis: can competitors indicate habitat quality?

Abstract: According to the ‘‘habitat copying’’ hypothesis, animals use the reproductive performance of conspecifics to assess habitat suitability and choose their future breeding site. This is because conspecifics share ecological needs and thus indicate habitat suitability. Here, we propose the ‘‘heterospecific habitat copying’’ hypothesis, which states that animals should use public information (i.e., information derived from the performance of others) from con- and heterospecifics sharing ecological needs. In a correlational approach we test some assumptions and predictions of this hypothesis with a data set from two sympatric bird populations, rollers (Coracias garrulus) and kestrels (Falco tinnunculus), using the same nest-boxes and exploiting similar food resources. Since kestrels are residents and breed earlier, we assumed that they are dominant over rollers for nest-box acquisition. The environment appears to be patchy for both species and temporally predictable for kestrels only. Two results suggest that the use of heterospecific public information in breeding habitat selection may be at work: (1) an increase in the reoccupancy probability by kestrels of previous roller nests with increasing nest success, and (2) an increase in roller breeding population with increasing local kestrel success. Most of the other observed patterns could be explained by alternative mechanisms such as natal philopatry, breeding fidelity, conspecific attraction, intraspecific habitat copying, and the effect of interspecific competition.

The role of wind in passerine autumn migration between Europe and Africa

Abstract: Large ecological barriers such as oceans and deserts have considerably shaped the migratory strategies of birds. The ecological barriers posed by the Alps, the Mediterranean Sea, and the Sahara seem to prevent most long-distance migrants from flying on a direct southward course from Europe to Africa. Migratory routes toward southwest and southeast prevail. These two flyways differ with respect to topography, refueling possibilities, and wind conditions. Aiming at a better understanding of the evolution of both flyways in spite of differing conditions, we studied potential survival of passerine birds on their first autumn migration from northern Europe to tropical Africa by means of a computer simulation. Considering real wind conditions at 850 mb (approximately 1500 m above sea level), the survival rates of birds with southeasterly (SE) migratory directions were much higher than those of birds with southwesterly (SW) directions. With the possibility to choose the altitude (from four levels) with the most favorable wind, both SE and SW migrants had similar high survival, but only with refueling opportunities in northwest (NW) Africa for SW migrants. Our results suggest that the southwestern flyway depends on the selection of days, but especially altitudes, with favorable wind conditions and on refueling opportunities in NW Africa. The SE flyway is privileged by the frequent favorable wind conditions for crossing the eastern Mediterranean Sea and the Egyptian desert, where refueling sites are almost absent. Both autumn migration routes would be unlikely without wind assistance. Copyright 2005.

Learning affects mate choice in female fruit flies

Abstract: Learning in the context of mate choice can influence sexual selection and speciation. Relatively little work, however, has been conducted on the role of learning in the context of mate choice, and this topic has been mostly ignored in insects even though insects have served as a prime model system in research on sexual selection and incipient speciation. Extending recent work indicating apparently adaptive learning in the context of sexual behavior by male fruit flies (Drosophila melanogaster), I tested for the effect of learning on mate choice by female fruit flies. Compared to young virgin females that experienced courtship by large males, young virgin females that experienced courtship by small males were more likely to mate with small and large males in a test conducted a day after the experience phase. These results, which are the first clear empirical demonstration of learning in the context of mate choice by female insects, lay the foundation for research on the role of learning in insect sexual selection and speciation. Key words: courtship, Drosophila, fruit flies, learning, mate choice, speciation. [Behav Ecol]

Mortality by moonlight: predation risk and the snowshoe hare

Abstract: Optimal behavior theory suggests that prey animals will reduce activity during intermittent periods when elevated predation risk outweighs the fitness benefits of activity. Specifically, the predation risk allocation hypothesis predicts that prey activity should decrease dramatically at times of high predation risk if there is high temporal variation in predation risk but should remain relatively uniform when temporal variation in predation risk is low. To test these predictions we examined the seasonably variable response of snowshoe hares to moonlight and predation risk. Unlike studies finding uniform avoidance of moonlight in small mammals, we find that moonlight avoidance is seasonal and corresponds to seasonal variation in moonlight intensity. We radiocollared 177 wild snowshoe hares to estimate predation rates as a measure of risk and used movement distances from a sample of those animals as a measure of activity. In the snowy season, 5-day periods around full moons had 2.5 times more predation than around new moons, but that ratio of the increased predation rate was only 1.8 in the snow-free season. There was no significant increase in use of habitats with more hiding cover during full moons. Snowshoe hares’ nightly movement distances decreased during high-risk full-moon periods in the snowy season but did not change according to moon phase in the snow-free season. These results are consistent with the predation risk allocation hypothesis. Key words: moonlight, moon phase, movement, predation rate, predation risk, prey behavior. [Behav Ecol]

Mutual ornamentation, age and reproductive performance in the European starling

Abstract: Ornamental traits expressed in both sexes are widespread among birds. Many studies have indicated that male ornaments develop through sexual selection. Female ornaments may be correlated effects of selection on males or have signaling functions in their own right. In the facultatively polygynous starling Sturnus vulgaris, both sexes possess iridescent throat feathers, which are actively displayed during courtship by males. This paper investigates the reproductive correlates of throat feather ornaments in both sexes. Bird-perceived hue and components of feather length covaried strongly, and a summary variable (the first principal component [PC1]) was extracted using Principal component analysis. Sex and age-related differences were found for PC1. Positive assortative mating was found with respect to age and PC1. However, the relative influences of ornamentation and age on breeding variables were hard to separate, so effects may be related to other age-related variables. This provides a cautionary note for studies of ornamentation where age is unknown. However, we argue that throat feathers in starlings may signal age and age-related quality measures in both sexes. Older females with higher PC1 scores bred earlier and laid larger clutches with higher hatching success; older males with higher PC1 scores had higher chances of becoming polygynous and attracting high reproductive quality females. PC1 showed no relationship with parental care in either sex. Direct reproductive benefits for males and indirect genetic benefits for either sex may drive mate choice based on these age-related characters. However, only experimental manipulation can determine whether ornamentation signals quality variation within age classes. Key words: avian color vision, mutual sexual selection, ornamentation, plumage coloration, reproductive performance. [Behav Ecol]

The evolution of egg rejection by cuckoo hosts in Australia and Europe

Abstract: Exploitation of hosts by brood parasitic cuckoos is expected to stimulate a coevolutionary arms race of adaptations and counteradaptations. However, some hosts have not evolved defenses against parasitism. One hypothesis to explain a lack of host defensesisthatthelife-historystrategiesof somehostsreducethecost ofparasitism totheextent thatacceptingparasiticeggsinthe nest is evolutionarily stable. Under this hypothesis, it pays hosts to accept cuckoo eggs if (1) the energetic cost of raising the cuckoo is low, (2) there is time to renest, and (3) clutch size is small. We parasitized the nests of host and nonhost species with nonmimetic model eggs to test whether the evolution of egg recognition by cuckoo hosts could be explained by life-history variables of the host. The most significant factor explaining rates of rejection of model eggs was whether or not a species was a cuckoo host, with hosts rejecting model eggs at a higher rate than nonhosts. Egg-rejection rates were also explained by visibility within the nest and by cuckoo mass. We found little support for the life-history model of egg rejection. Our results suggest that parasitism is always sufficiently costly to select for host defenses and that the evolution of defenses may be limited by proximate constraints such as visibility within the nest. Key words: brood parasitism, coevolution, cowbirds, cuckoos, life-history strategies. [Behav Ecol]

Mating rate and fitness in female bean weevils

Abstract: Females of most animal taxa mate with several males during their lifespan. Yet our understanding of the ultimate causes of polyandry is incomplete. For example, it is not clear if and in what sense ...

Reproductive asynchrony and population divergence between two tropical bird populations

Abstract: High-latitude vertebrates generally breed seasonally and synchronously as the primary environmental cue used to time seasonal processes is photoperiod. Investigations of tropical vertebrates have also documented seasonal reproduction, but it is unclear how synchronous reproduction is, both within and between populations. In this study, we investigated whether seasonal reproduction in a tropical species is synchronous between two populations in close proximity and, if not, whether asynchrony is correlated with genetic and cultural differentiation. We describe two equatorial populations of rufous-collared sparrows (Zonotrichia capensis), at the same latitude and separated by 25 km, that each breed seasonally but out of phase with each other. This asynchronous reproductive phenology is associated with local weather patterns and is independent of photoperiod. At a finer scale, reproductive timing is more highly synchronized within monogamous pairs than within the population as a whole. Associated with the difference in reproductive phenologies, we document that males in each population sing different song dialects. Using microsatellite DNA analysis, we found limited gene flow and significant genetic differentiation between the two populations. From these results we hypothesize that cultural and genetic differentiation between populations, which can be greater in tropical populations than temperate ones, can be associated with locally adapted reproductive phenologies. Key words: biodiversity, bird, dialects, DNA, estrogen, evolution, microsatellite, migration rate, reproduction, song, speciation, testosterone, tropics, Zonotrichia capensis. [Behav Ecol]

Effects of extra-pair and within-pair reproductive success on the opportunity for selection in birds

Abstract: The number of mates, their fecundity, and the number of extra-pair fertilizations can all affect male reproductive success in biparental species. Extra-pair mating in birds has been of particular interest, because it could generate strong levels of sexual selection even when a species is socially monogamous. We examined how extra-pair fertilizations affect the opportunity for selection in the sexually dimorphic common yellowthroat (Geothlypis trichas) and the sexually monomorphic house wren (Troglodytes aedon). We were able to identify sires for at least 95% of all nestlings, and, thus, we were able to make a nearly complete accounting of male reproductive success. Although extra-pair fertilizations were common in yellowthroats (26% of young) it contributed little (21%) to the total variance in male reproductive success. Most of the variance in reproductive success (58%) was attributable to the male’s within-pair success, which was influenced primarily by the number of young produced by each mate and the proportion of within-pair young sired. Despite a moderate level of extra-pair fertilizations (10% of young) in house wrens, almost all of the variance in male reproductive success (97%) was attributable to within-pair success, particularly the number of social mates. Although extra-pair fertilizations generally increase the variance in male reproductive success, withinpair reproductive success may be the major source of variation in male reproductive success. Thus, sexual dimorphism in monogamous birds may be influenced more by the number of mates and their fecundity than by extra-pair matings. Key words: extra-pair paternity, sexual selection, common yellowthroat, house wren. [Behav Ecol]

Paternity in mallards: Effects of sperm quality and female sperm selection for inbreeding avoidance

Abstract: Postcopulatory processes might play an important role in sexual selection. In theory, fertilization success could be controlled by females via selection of particular sperm within their reproductive tract, or it could be determined by sperm competition per se. In practice, these two mechanisms are difficult to disentangle. To assess the relative importance of both mechanisms we used artificial insemination in combination with measurements of sperm quality (swimming speed and motility) in mallards. In this species, females often lack behavioral control over copulations and hence may use postcopulatory mechanisms to optimize their reproductive output. One important factor affecting female fitness may be selection of genetically compatible males. To investigate the influence of sperm quality and parental relatedness on paternity we inseminated 12 groups of related females with a sperm mixture containing equal numbers of sperm from a brother and from an unrelated male. Paternity was independent of the relatedness of the siring male to the female but was significantly affected by long-term sperm swimming speed and motility. No interaction between relatedness and sperm quality on paternity was observed. These results suggest that female mallards are not able to select sperm on a purely genetic basis and emphasize the importance of sperm quality in gaining paternity. Copyright 2005.

Experimental evidence for helper effects in a cooperatively breeding cichlid

Abstract: Neolamprologus pulcher is a cooperatively breeding cichlid fish, in which helpers stay in their natal territory and help with brood care, territory defense, and maintenance. In this study we investigated helper effects by an experimental group size reduction in the field. After this manipulation, focal helpers in reduced groups tended to feed less, and small helpers visited the breeding shelter significantly more often than same-sized helpers in control groups. No evidence was found that remaining helpers compensated for the removed helpers by increasing territory defense and maintenance behavior. Breeders, however, did show a lower defense rate, possibly caused by an increase in brood care effort. Survival of fry was significantly lower in removal than control groups, which provides the first experimental proof in a natural population of fish that brood care helpers do effectively help. The data suggest that in small, generally younger, helpers, kin selection may be an important evolutionary cause of cooperation. Large helpers, however, who are generally older and less related to the breeders than small helpers are suggested to pay to be allowed to stay in the territory by helping. All group members benefit from group augmentation. Key words: cooperative breeding, group size reduction, helping behavior, Lake Tanganyika cichlids, reproductive success. [Behav Ecol]

The evolution of vocal alarm communication in rodents

Abstract: On encountering a predator, many species emit potentially risky vocalizations known as alarm calls. We evaluated the relative importance of two adaptive hypotheses on the evolution of calling: (1) communicating to predators, which may function by deterring pursuit and hence increasing individual survival, and (2) an alternative nepotistic hypothesis for alarm calling whereby callers obtain direct and indirect fitness by warning relatives. Focusing on 209 species of rodents, we found significant associations between diurnality and alarm calling, living socially and alarm calling, and diurnality and sociality. Diurnality, however, accounted for nearly three times as much variation in whether or not a species alarm called than did sociality. Phylogenetic tests revealed that the evolution of diurnality preceded the evolution of alarm calling, and that the evolutions of diurnality and sociality were unrelated. Our results are consistent with the hypothesis that alarm communication evolved to communicate to predators. If so, then nepotistic benefits, although important for the maintenance of alarm calling in some rodents, may be relatively less important in its evolution. Copyright 2005.

Constraints on control: factors influencing reproductive success in male mandrills (Mandrillus sphinx)

Abstract: Over the last decade, paternity analysis using molecular markers has revealed that observed mating systems do not necessarily correlate with reproductive systems and thus cannot provide reliable information about male reproductive success (RS). This is especially true for primate species with a complex multimale-multifemale social organization, such as mandrills (Mandrillus sphinx). Using molecular markers for the measurement of individual RS and a comprehensive data set comprising 193 offspring from 27 birth cohorts over a 20-year period of sampling, we investigated the social, genetic, and demographic factors that may influence the probability of paternity by dominant male mandrills, living in a semi--free-ranging colony. We observed a significant skew in RS towards dominant males, with their probability of paternity increasing as the number of adult males in the group increased, and when they were closely related to subordinate adolescent males. Conversely, the probability of dominant males siring infants decreased when the number of simultaneously tumescent females increased. Fewer offspring were sired by dominant males when female partners were closely related to them and when the relatedness between dominant and subordinate adult males increased. These two last points suggests particularly that mechanisms of kin recognition are operating to avoid the costs of inbreeding and may also reflect the lower costs to dominant males of losing conception opportunities to more closely related subordinate adult males. This study is, to our knowledge, one of the first in primates to use an integrative approach and multivariate analysis to show that multiple factors are involved in determining the probability of paternity by dominant males. Copyright 2005.

Evidence for the signaling function of egg color in the pied flycatcher Ficedula hypoleuca

Abstract: A recent hypothesis proposes that the bright colors, especially blue and green, of many avian eggs may function as signals of female or offspring phenotypic quality or condition to males in species with biparental care, inducing them to allocate more effort to their offspring. The pigment determining blue and green egg colors is an antioxidant whose availability for eggshell coloring may be limited. To test the signaling function on a species with blue eggs, the pied flycatcher Ficedula hypoleuca, we measured egg color with a spectrophotometer on the day of laying and obtained two principal components from their reflectance spectra that together explained 99% of variation and represented shell lightness, and hue and saturation, respectively. We also measured female immunocompetence during the nestling period through the response to phytohemagglutinin as a measure of cell-mediated immunity and the response to a tetanus vaccination as a measure of humoral immunity. The total amount of immunoglobulins in blood of females and of nestlings before fledging was also estimated. Mean within-clutch egg darkness was positively associated with both measures of female immunocompetence, while better female condition was associated with colors tending away from intermediate and toward short wavelengths. Ageing female laid lighter eggs. The mean within-brood level of nestling IgY was also associated with mean within-clutch egg colors tending away from intermediate and toward short wavelengths. Mean egg darkness decreased linearly during the laying sequence, suggesting pigment limitation. Males were observed frequently visiting nests during the laying period, allowing them to observe eggs before the start of incubation. These results support the signaling hypothesis for explaining bright colors of avian eggs. Copyright 2005.

Specific color sensitivities of prey and predator explain camouflage in different visual systems

Abstract: In situations of aggressive mimicry, predators adapt their color to that of the substrate on which they sit for hunting, a behavior that is presumed to hide them from prey as well as from their own predators. Females of few crab-spider species encounter such situations when lying on flowers to ambush pollinators. To evaluate the efficiency of spider camouflage on flowers, we measured by spectroradiometry adult female Thomisus onustus and marguerite daisies, Leucanthemum vulgare. We compared chromatic contrast (color used for short-range detection) of each pair of spider and flower to detection thresholds computed in the visual systems of both Hymenopteran prey and passerine bird predator. We also computed achromatic contrast (brightness) used for long-range detection. In both visual systems, each individual spider was efficiently matching the precise color of the flower center on which it was hunting. Being significantly darker than flowers, crab-spiders could in theory be detected at long range by either predator or prey using achromatic contrast. However, long-range detection is unlikely, owing to small spider size. Spiders also generated significant chromatic and achromatic contrasts to both Hymenoptera and bird when moving on flower periphery. Our study is the first to identify which photoreceptors of both prey and predator are involved in camouflage. The analysis suggests more research on bird predation and vision to determine to which extent bird predators effectively constrain spider crypsis.

Flexible social structure of a desert rodent, Rhombomys opimus: philopatry, kinship, and ecological constraints

Abstract: We tested hypotheses based on philopatry, kinship, and ecological constraints to explain sociality in a semifossorial desert rodent, the great gerbil, Rhombomys opimus. Data were collected in the field in Uzbekistan in the spring and fall of 1996 and 1998–2004. Population densities fluctuated dramatically with high turnover in both males and females to reveal that dispersal and social structure were density dependent. Fewer gerbils dispersed at higher densities and members of family groups dispersed together. A majority of females lived in groups at high densities, but as population densities declined, proportionally more females were solitary. DNA analysis revealed that group-living females were genetically similar, whereas solitary females visited by the same male, as well as adult males and females in the same family group, were usually not genetically similar. Reproductive success as measured by the number of emergent pups and survival of juveniles during the summer drought was not related to group size or whether females were philopatric. A majority of females in family groups reproduced, and all females engaged in cooperative behaviors. We accepted three hypotheses to explain fluctuations in group formation in the great gerbil: variation in food abundance and distribution, habitat saturation, and kinship. We conclude that great gerbils are facultatively social. Flexible social behavior may be adaptive in unpredictable desert conditions. Females live solitarily under conditions of limited food and high mortality that disrupt social behavior and group formation and share territories with female kin under favorable conditions for survival and reproduction when kin groups can be maintained. Males adjust to the distribution of females. Key words: desert, gerbil, philopatry, Rhombomys opimus, social. [Behav Ecol]

The mechanisms of interference competition: two experiments on foraging waders

Abstract: Models of population dynamics that include interference competition have often been applied to foraging waders and less so to other foragers, even though these models are, in principle, generally applicable. At present, however, it is still unclear whether interference competition is of importance for foraging waders. To support this idea experimental evidence and knowledge of the mechanisms underlying interference effects are required. We experimentally determined the relationship between forager density and foraging success in two wader species: the red knot (Calidris canutus) and the ruddy turnstone (Arenaria interpres). With each of the two species, we conducted an experiment consisting of 300 one-min trials. In these trials we scored the behavior and the foraging success of focal individuals at specific combinations of bird and prey density. Irrespective of prey density, individuals of both species discovered fewer prey items at higher bird densities. Despite this, only in turnstones did intake rates decline with increasing bird density. Knots compensated for a lower prey-discovery rate by rejecting fewer prey items at higher bird densities. In knots, bird density had a complex, nonmonotonic effect on the time spent vigilant and searching. In turnstones the main effect of increased bird density was a reduction in the prey-encounter rate, that is, the reward per unit search time. Effects on the time spent vigilant and the time spent searching were less pronounced than in knots. Thus, the mechanistic basis of the effects of bird density was complex for each of the two species and differed between them. Key words: Arenaria interpres, behavioral mechanisms, Calidris canutus, density dependence, exploitation competition, social dominance. [Behav Ecol]