Behav Ecol Research Articles

Estrous synchrony in a nonseasonal breeder: adaptive strategy or population process?

The idea that female mammals can manipulate the duration of each other’s estrus in an effort to influence the degree of synchrony between their periods of sexual receptivity is a persistent and popularly held one. It is frequently cited as proof of pheromonal communication in humans and often invoked by models of female reproductive strategies more generally. Yet, to date, no tests of the evolutionary arguments put forward by proponents of the phenomenon have been undertaken. We addressed this deficit with an analysis of the reproductive demography of wild female chacma baboons, where variance in the temporal distribution of female receptivity is known to occur. Specifically, we tested the predictions that this variance will reflect female attempts to minimize 1) the risks of being monopolized by a single male or 2) the intensity of interfemale competition for males. Using model comparison, we found no evidence that male number or operational sex ratio had any influence on the distribution of female receptivity, the number of females in estrus, or the duration of female sexual swellings. Indeed, when modeling estrous overlap and cycling female number, we found that a simple nondeterministic model provided the best fit. We conclude, therefore, that variance in the temporal distribution of female receptivity is indicative of nothing more than a population process and that socially mediated synchrony is not a tangible adaptive phenomenon. Key words: female receptivity, population process, sexual selection, socially mediated synchrony. [Behav Ecol]

Mating systems in cooperative breeders: the roles of resource dispersion and conflict mitigation

Within animal societies, the ecological and social underpinnings of mating system variation can be related to resource dispersion, sexual conflict between breeders, and the effects of non-breeders. Here, we conducted a broad-scale investigation into the evolution of mating systems in the cooperatively breeding cichlid, Neolamprologus pulcher, a species that exhibits both monogamy and polygyny within populations. Using long-term field data, we showed that polygynous groups were more spatially clustered and held by larger competitively superior males than were monogamous groups, supporting the role of resource dispersion. To explore the role of sexual conflict, we forced polygynous males to become experimentally monogamous (EM) in the field. EM males spent more time on their remaining territory than naturally polygynous males but otherwise did not change behaviorally or physiologically. Females mated to EM males performed more submissive acts, and in a forced choice experiment, females did not preferentially associate with the larger of two unmated males. Females may therefore incur an unexpected cost from mating monogamously with a large and competitively superior male, a cost that mitigates sexual conflict over the mating system. Helpers were more closely related in monogamous groups but did not behave differently under monogamy or polygyny. Helpers therefore seem neither to be affected by nor affect the mating system of breeders. Our results demonstrate the roles of resource availability and conflict mitigation in determining the mating system, and highlight the importance of experimental manipulation for revealing hidden costs of hypothetical mating patterns. Key words: cooperative breeding, mating system, monogamy, polygyny, resource dispersion, sexual conflict. [Behav Ecol]

Interspecific dominance relationships and hybridization between black-capped and mountain chickadees

Black-capped chickadees (Poecile atricapillus) and mountain chickadees (P. gambeli) are ecologically segregated due to differences in habitat preference. However, forestry practices in northwestern Canada have created a mosaic of coniferous (mountain chickadee habitat) and deciduous forest patches (black-capped habitat), which might explain cases of observed regional sympatry between these 2 closely related species. In Poecile species, social hierarchies amongst conspecific individuals influence lifehistory parameters such as mate choice. As a result, interspecific social hierarchies might drive hybridization between these 2 closely related species. By conducting field observations and aviary experiments, we demonstrated that black-capped chickadees are dominant over mountain chickadees. Using a combination of species-specific phenotypes (plumage), mitochondrial DNA (mtDNA) to assess maternal genotype, and microsatellite markers, we confirmed that genetic mixing occurs within our contact zone but that the pattern of parentage appears directional. All but one of the adult hybrids was phenotypically identified as mountain chickadee and had mountain chickadee mtDNA. Furthermore, all nestlings where microsatellites detected mixedspecies ancestry were from mountain chickadee nests with both attending parents having mountain chickadee phenotypes. All mtDNA from these nestlings was mountain chickadee except for one individual, and in all cases, these nestlings showed genetic patterns of having arisen through extrapair copulations between female mountain and male black-capped chickadees. Our results suggest that hybridization may result from males of the mountain chickadees having lower expression of a preferred trait (dominance) than the black-capped chickadees. Key words: black-capped and mountain chickadees, dominance hierarchies, extrapair paternity, hybridization. [Behav Ecol]

Priming men with different contest outcomes modulates their dominance perceptions

It has recently been proposed that dominance perceptions in men function, at least in part, to reduce the potential costs of within-sex competition for resources, and that sensitivity to cues of men’s dominance is greatest among men who would incur the largest costs if they engaged in competition with rivals indiscriminately (i.e., low-dominance men). Consistent with these proposals, we found that men randomly allocated to priming conditions in which they imagined losing confrontations with other men subsequently demonstrated greater sensitivity to dominance cues when assessing the dominance of men’s faces than did men randomly allocated to priming conditions in which they imagined winning confrontations. No equivalent effect occurred for perceptions of men’s trustworthiness, suggesting that the priming effect observed for dominance judgments may be somewhat specific to competition-related perceptions. Collectively, these findings suggest that men’s perceptions of other men’s dominance are facultative, changing in response to contextual cues, such as the outcomes of recent confrontations. Such responses could play a critical role in calibrating men’s dominance sensitivity according to their recent experiences and are consistent with findings from experiments with other species in which the outcome of prior confrontations modulated competition-related behaviors. Key words: aggression, dominance, fighting experience, within-sex competition. [Behav Ecol]

Pack social dynamics and inbreeding avoidance in the cooperatively breeding red wolf

For cooperatively breeding groups composed of close relatives, whether and how a group avoids inbreeding are questions of key evolutionary and conservation importance. A number of strategies for inbreeding avoidance may be employed by cooperative breeders, including extrapair reproduction, reproductive suppression, and juvenile dispersal. However, population-wide information on the prevalence of different strategies is difficult to obtain. We investigated the prevalence of inbreeding and potential mechanisms for inbreeding avoidance in a reintroduced population of the red wolf. Using long-term data on individuals of known pedigree, we determined that inbreeding among first-degree relatives was rare. Potential mechanisms for inbreeding avoidance included low levels of philopatric reproduction in spite of delayed dispersal, and reproductive suppression prior to dispersal. Inbreeding avoidance among siblings may have been further facilitated by independent dispersal trajectories, as many young wolves spent time alone or in small nonbreeding packs composed of unrelated individuals. The dominant pattern of breeding-pair formation involved the union of 2 unrelated individuals in a new home range. Replacement of 1 or both members of an existing breeding pair involved new immigrants to a pack or, in a small number of cases, ascendance of either resident offspring or adopted pack members to vacant breeding positions. Extrapair reproduction was rare, suggesting that it was not a major mechanism for outbreeding. We conclude that there are several prevalent behavioral strategies within the red wolf population that may work together to minimize inbreeding and any associated fitness costs, helping make cooperative breeding an evolutionarily viable strategy. Key wor ds: Canis r ufus, competition, lone wolves, monogamy, nonbreeding packs, pair formation. [Behav Ecol]

How dispersal influences parent–offspring conflict over investment

Models that investigate family conflicts over parental investment have generally ignored population structure above the level of the family. Here, we extend conventional models to assess the impact of spatial population structure and limited dispersal on the intensity and resolution of parent‐offspring conflict. Although one might naively predict that limited dispersal should reduce parent‐offspring conflict, due to increased local relatedness, we show that this is not necessarily the case. Parent‐offspring conflict may actually become more intense when dispersal is limited because local competition among related young can reduce the cost of any decline in maternal productivity, favoring greater selfishness on the part of the offspring. We also investigate the impact of limited dispersal on the resolution of parent‐offspring conflict when the young can solicit extra resources by means of costly begging. Our model predicts that begging will be more intense when dispersal is limited but that the impact of dispersal on size then depends on the effect of parental supply on offspring demand. When greater supply leads to reduced demand, we find that parental investment should be higher in more viscous populations. By contrast, when greater supply leads to greater demand, parental investment should be lower in more viscous populations. Key words: begging, kin selection, local competition, model, offspring demand. [Behav Ecol]

What is a subgroup? How socioecological factors influence interindividual distance

Fission–fusion dynamics derive from spatial adjustments that animals make depending on resource distribution, resulting in splitting and merging of subgroups. New frameworks propose to classify social systems depending on their degree of fission– fusion dynamics, but little has been done to quantify such dynamics. Operationally defining subgroup is a building block in such quantification. We aimed to define subgroup using interindividual distances (IIDs), while examining the relative contribution of social and ecological factors on the distribution of such distances. We employed a modeling approach using location data collected with a handheld GPS unit from single individuals belonging to a long-term study community of wild spider monkeys and determined the minimum distance at which 2 individuals can be considered in different subgroups. Our results strongly support the crucial role of both social and ecological factors in influencing the way individuals position themselves with respect to other group members. Our socioecological model explained the observed interactivity and interseasonal variation in IIDs in a biologically relevant manner. The critical IID to define subgroup fell within the expected range from field observations. This modeling approach can contribute to the understanding of the factors influencing the evolution of social systems. Key words: fission–fusion, modeling approach, social system, socioecology, spatial distance, subgroup. [Behav Ecol]

Male display areas in exploded leks: the importance of food resources for male mating success

The distinction between true lekking and resource-defense mating systems is difficult in exploded-lekking species, where resources in male territories may influence female choice. Here, we propose the use of path analysis as a way to establish the relative importance of phenotypic characteristics versus food resources on mating success and hence decide whether a population is closer to one or the other extreme of an exploded lek-resource-defense continuum. Using great bustards Otis tarda as model species, we first performed logistic regression analyses of potential predictors of display sites in three leks. Males selected sites with high visibility, close to resources, and exposed to females. Second, we examined through path analysis how the mating success of radiotagged males was affected by resources, females and competitors, compared with age and weight, the major phenotypic correlates of mating success. The direct effects of age and weight were much higher than their indirect effects via resources or females. This suggests that female choice based on male characters is the most important sexual selection mechanism during mating and supports defining the great bustard mating system as a true exploded lek. However, half of the males showed a variable degree of resource defense toward intruders, indicating that both mating tactics, exploded lekking and resource defense, occur together in the same population. We suggest that individual variability in the use of resource defense might exist in many other exploded-lekking species, increasing the difficulties found in fitting their mating behavior to accepted definitions of alternative mating tactics. [Behav Ecol]

Environmental and social factors affecting the excretion of intestinal parasite eggs in graylag geese

In group living vertebrates, patterns of parasite infection vary within and between populations because of environmental and social factors. In the present study, we investigated patterns of parasite product excretion in graylag geese, focusing on environmental (season and temperature), individual (sex and age), and social factors (pair-bond status, reproductive state, and rearing condition). From March 2008 to May 2010, we collected 3574 samples from 171 individuals and analyzed them for nematode eggs and coccidian oocysts. Fecal samples that were infected with nematode eggs were also positive for coccidian oocyst to a higher rate. Fecal samples of goose-raised geese were positive for nematode eggs more often than those of hand-raised geese. In parental birds, the number of goslings fledged had a positive influence on the percentage of samples positive for nematode eggs. The number of samples infected with coccidian oocysts decreased with age. In parent birds, the number of samples found to be positive for coccidian oocysts decreased during the parental season, and female bird samples were positive more often than those of males. In graylag goslings, the number of fecal samples with coccidian oocysts was influenced by the goslings’ age, with maximum excretion at 13 weeks. This is the first evidence to demonstrate an effect of a set of interacting environmental (e.g., season), individual (e.g., sex, age), and social (e.g., rearing condition, number of goslings fledged) factors on rate of parasite excretion in a socially complex bird. Our results suggest that socially stressful periods may have implications for individual health in graylag geese. Key wor ds: Anser anser , graylag geese, intestinal parasite excretion, social and environmental factors, social stress. [Behav Ecol]

Male courtship pheromones as indicators of genetic quality in an arctiid moth (Utetheisa ornatrix)

One of the fundamental issues regarding sexual selection is whether females select males based on signals that represent direct phenotypic or indirect genetic benefits. In Utetheisa ornatrix (Lepidoptera: Arctiidae), females choose males based on a courtship pheromone, hydroxydanaidal (HD), derived from defensive pyrrolizidine alkaloids (PAs). At mating, virgin males transfer a spermatophore whose contents are proportional to the HD titer and body size; as a result, females receive both phenotypic benefits (more nutrients and PAs) and genotypic benefits (genes for larger body size inherited by the offspring). Previous data from field-collected individuals, however, indicated that the HD signal of nonvirgin males may not correlate with the spermatophore contents. Using chemical analyses, we determined that the HD signal does not change based on mating history, thereby supporting the importance of HD in advertising a male’s genetic quality. Thus, male HD represents original body size and PA levels, and females, by choosing males based on this pheromone, are providing their offspring with genes to sequester chemicals that confer survival and reproductive advantages. We discuss the implications regarding the relative importance of direct and indirect selection in maintaining female preferences. Key words: Arctiidae, genetic benefits, mate choice, pheromone, pyrrolizidine alkaloid, sexual selection. [Behav Ecol]

Mass Fluctuations Suggest Different Functions of Bimodal Foraging Trips in a Central-place Forager

The function of a foraging trip (self-feeding or provisioning) and the spatial distribution of food resources play an essential role in foraging decisions of central-place foragers. Theory predicts that if foraging patches optimal for self-feeding and provisioning are spatially separated, a bimodal foraging strategy should often be favored. A bimodal foraging strategy involves the alternation of short feeding trips (ST) to collect food for offspring with long trips (LT) for self-feeding, as previously described in some procellariiform seabirds. Using an automated passage recording and weighing system, we tested whether the bimodal trip pattern observed in little auks (Alle alle), a small Arctic alcid, reflects a corresponding functional difference of bimodal foraging trips. We found that the body mass trajectory of little auks was closely related to their bimodal trip pattern. Little auks weighed significantly more returning from a LT and lost an equivalent amount of mass during subsequent ST. The decision to start a LT was not directly related to body mass but seemed to depend on the individual strategy of a bird. During LT, birds likely utilized distant foraging areas suitable for self-feeding while relying on near-shore areas during ST. These results support predictions of recent foraging models and indicate that the function of long and short trips of little auks is similar to procellariiforms independent of large differences in the temporal and spatial scale of feeding trips among these taxa. We suggest that bimodal foraging might be more widespread among central-place foragers than previously thought. Key words: Alle alle , bimodal foraging, body mass, foraging success, foraging trip duration, resource geometry. [Behav Ecol]

Fungus-gardening ants prefer native fungal species: do ants control their crops?

Fundamental features of mutualisms are solutions to conflict. The mutualism between fungus-gardening ants and their fungi is characterized by pronounced evolutionary ant-fungus fidelity. Ant-fungus fidelity is thought to be controlled by the fungus, which secretes somatic incompatibility compounds that suppress incompatible fungal strains that may invade gardens. Accordingly, the ants are thought to perceive incompatibilities and then reject invading cultivars. During cultivar-switch experiments, we discovered that if switched colonies of the ant Trachymyrmex septentrionalis are successful in retaining a minute fragment of their original garden, the ants will gradually co-cultivate the novel and the original fungus strains in a chimeric (polycultural) garden; this chimeric garden will revert after 1–2 weeks completely to a monoculture of the original fungal strain. Experimental examination of worker preference suggests that symbiont rejection behavior seems to be an innate response toward foreign fungi rather than mediated solely by incompatibility compounds secreted by the fungus. These observations suggest that the fungal strain cultivated by a colony imprints the attending ants, and this imprinting modifies ant behavior so that the ants prefer their original cultivar even when experimentally forced to grow a foreign cultivar for several weeks. Fungal monocultures and ant-fungus fidelities in this symbiosis therefore seem to be reinforced by factors intrinsic to both the ants and the fungi. Key words: Attini, chimera, cultivar switch, somatic compatibility, symbiosis. [Behav Ecol]

The energetics of lactation and the return to fecundity in wild chimpanzees

Parental provisioning of offspring is an intensive energetic investment that is expected to compromise future offspring production. This trade-off is particularly salient for mammals in which mothers bear the exclusive burden of lactation and draw from their own energy reserves to provision offspring. The degree to which lactation impacts future reproductive ability should vary not only by the absolute cost of milk production but also by the ability of individual mothers to afford it. Few studies have been able to quantify the costs of lactation or how they affect reproductive rates. Here, we examine the metabolic load of lactation in chimpanzees, a species with intensive parental investment and extremely slow reproductive rates. We used urinary C-peptide of insulin to trace changes in energetic condition in 17 wild, unprovisioned chimpanzee mothers in Kibale National Park, Uganda. C-peptide levels of nursing mothers were depressed for six months postpartum, thereafter showing a net increase through the second year. These changes are consistent with milestones of infant physical and behavioral development. Mothers inhabiting lower quality foraging areas experienced a higher metabolic load of lactation, as indicated by lower C-peptide profiles than mothers in food-rich areas. Ovarian activity was closely correlated with energetic condition. Cycling resumed only after a sustained period of energy gain, suggesting that the slow reproductive pattern in wild chimpanzees results not only from the direct expense of milk production but also from the long period that mothers require to recover their physical condition in a food-limited environment. Key wor ds: C-peptide of insulin, energy balance, interbirth inter vals, lactation, reproduction, weaning. [Behav Ecol]

Intersexual conflict during mate guarding in an androdioecious crustacean

Traditional interpretations of mating behaviors assume cooperation between the sexes. The field of sexual conflict provides a contrasting view: The sexes are commonly in conflict because they diverge in benefits/costs of reproduction. Precopulatory mate guarding, wherein males attempt to monopolize mates by physically pairing before fertilization, is one expected case of conflict. Initial guarding models focused mainly on male reproductive success, considering mate guarding as a pure male mating strategy. Intersexual conflict models instead consider costs and benefits for both sexes. We provide new insights exploiting a promising study system for intersexual conflict: the androdioecious crustacean Eulimnadia texana. Androdioecy (coexistence of males and hermaphrodites) provides unique opportunities to test sexual conflict hypotheses because hermaphrodites can facultatively self-fertilize. The ability to self-fertilize is expected to decrease the benefits of mating with males, whereas it is less likely to affect guarding costs. This asymmetry can thus intensify conflicts, which can be easily detected, if present. We show that hermaphrodites suffer higher costs than males during a guarding event and thus prefer shorter guarding times, whereas males, seeking to avoid loss of mating opportunities, prefer longer guarding times. Moreover, larger male size (relative to hermaphrodite size) confers greater control over hermaphrodites, leading to increased guarding durations. As sexual conflict theory predicts, guarded mates are not ‘‘passive’’ interactors but struggle to maximize their own fitness. The result of the conflict is a compromised guarding time that differs from the optimal strategy of either single sex. Key words: androdioecy, Eulimnadia texana, intersexual conflicts, mate guarding. [Behav Ecol]

Repeated probing of hosts: an important component of superparasitism

Parasitoids that encounter a previously parasitized host inspect it externally and internally, sometimes eventually laying additional eggs (superparasitism). The fitness effects of increased clutch sizes generated through superparasitism are widely studied, whereas the consequences of multiple host probings during the inspection received less attention. To address this issue, we offered a host to 1–5 females of the encyrtid wasp Copidosoma koehleri consecutively, or presented it 1–5 times to a single female. We noted whether the hosts died before pupation of either host or wasp, produced parasitoid pupae, or developed into moth pupae. Additional hosts were dissected after varying numbers of probings to determine their parasitoid egg loads. Host rejection rates prior to ovipositor insertion did not differ between treatments. Host rejections after ovipositor insertion, characterized by brief (<10 s) probing durations, were more common in the single- than in the multiple-female treatment. This could reflect avoidance of self-superparasitism, or increased selectivity by host-experienced females. Egg number per host increased with the number of prolonged probings in both treatments. Some hosts that received 1–2 probings (brief or prolonged) yielded moth pupae, while no hosts with five probings survived to pupation. Hosts probed three times (corresponding to <1 and 2.2 eggs in the single- and multiple-female treatments, respectively) produced the largest proportion of parasitoid pupae. The parasitoids’ success is thus strongly affected by the number of host probings. Overcoming host defenses through repeated probings is a previously overlooked potential benefit of superparasitism. Key wor ds: conspecific parasitism, Copidosoma, host handling, host inspection, host mutilation, parasitoid, self-superparasitism. [Behav Ecol]

The challenges of mixing associational learning theory with information-based decision-making theory

Behavioral ecologists frequently incorporate associational learning (AL) concepts into studies of choice behavior. Within behavioral ecology, AL is often considered a mechanism for information gathering. AL also provides alternative explanations of behavioral phenomena up to the level of motivational organization over the lifetime. AL assumes that all inputs to the learning system interact through a multistep process with feedbacks to control behavior and that cues are characterized by contingencies, whereas behavioral ecology assumes that learning inputs independently control responses, are in conflict, and convey information. Integrating the 2 perspectives is not straightforward and can lead to conflicting predictions or loss of predictive power. I examine 2 sets of case studies. First, I look at parallel research programs on mating in quail. Second, I consider how AL concepts have been integrated into foraging studies of nectarivores. The papers on quail mating demonstrate that to a large degree, the 2 approaches explain similar behaviors in compatible ways. The nectarivore papers show how the theories diverge, with AL predicting challenging results. Future studies should examine how much individuals select between sources of information and how much they respond to combinations of and interactions between cues within the process described by AL, using experimental designs that allow explicit cross-paradigm comparisons through the use of identical measurements of response. Key words: associational learning, Coturnix japonica, foraging, information, mate choice, nectarivore. [Behav Ecol]

Wait or fight? Ownership asymmetry affects contest behaviors in a parasitoid wasp

Contests for indivisible resources are often settled on the basis of the asymmetry that exists between contestants in their ownership status: Owners (i.e., individuals already possessing a resource) usually retain the resource when facing intruder competitors. This owner advantage is predicted to be more pronounced as the asymmetry in ownership status accentuates: when the residency period increases, the probability of owners keeping their resource should increase and contestant aggressiveness should be reduced. In the present study, we tested this prediction in a parasitoid wasp, Eupelmus vuilleti, where females fight for hosts, larvae of Callosobruchus maculatus, on which to lay their eggs. To do so, we experimentally varied the arrival times of 2 conspecific females on a host, generating different asymmetries in ownership status between them. We further explored the strategies adopted by intruder females according to the stage reached by the owners in their host exploitation sequence. When the residency period was short, both females fought and contests were won by the female valuing more greatly the host, rather than by the owner per se. The loser left the host vicinity and the winner exploited the host alone. In contrast, when the residency period was long (the owners were ready to parasitize the host), the intruders did not leave but instead waited for the departure of the resident female in order to superparasitize the host. This “waiting strategy” seems particularly adaptive in this species, in which females tend to prefer already parasitized hosts to healthy hosts. Key words: contest resolution, Eupelmus vuilleti, female egg load, ownership status, resource value. [Behav Ecol]

Nest value mediates reproductive decision making within termite societies

The influences of costs and benefits on social evolution remain poorly understood, despite their importance for explaining social behavior, and continue to fuel high-profile debates. Here, we investigate the effect of a major ecological factor, wood resource—that is, natal nest value—on reproductive decision making in the termite Cryptotermes secundus. Workers continually assess the available wood resources of the colony and adjust their developmental decisions, including investment into competition, accordingly. In particular, workers are more likely to stay and fight for inheritance within resource-rich nests. Comparable responses are seen in social wasps, which evolved sociality independently from the termites, demonstrating convergent evolution. Our results, therefore, indicate a general evolutionary link between resource wealth and societal conflict. Key wor ds: compe tition, costs and benefits, decision making, evolutionar y feedback, social evolution, termite. [Behav Ecol]

Spontaneous formation of multiple routes in individual desert ants (Cataglyphis velox)

Desert ants make use of various navigational techniques, including path integration and visual route following, to forage efficiently in their extremely hostile environment. Species-specific differences in navigation have been demonstrated, although it remains unknown if these divergences are caused by environmental adaptation. In this work, we report on the navigational strategies of the European ant Cataglyphis velox, which inhabits a visually cluttered environment similar to the Australian honey ant Melophorus bagoti, although it is more closely related to other North African Cataglyphis species. We show that C. velox learn visually guided routes, and these are individual to each forager. Routes can be recalled in the absence of global path integration information or when placed in conflict with this information. Individual C. velox foragers are also shown to learn multiple routes through their habitat. These routes are learned rapidly, stored in long-term memory, and recalled for guidance as appropriate. Desert ants have previously been shown to learn multiple routes in an experimental manipulation, but this is the first report of such behavior emerging spontaneously. Learning multiple paths through the habitat over successive journeys provides a mechanism by which ants could memorize a series of interlaced courses, and thus perform complex navigation, without necessarily having a map of the environment. Key words: Cataglyphis velox, desert ant, foraging, learning, navigation, route, visual navigation. [Behav Ecol]

Coping with social stress: heart rate responses to agonistic interactions in king penguins

In colonial breeders, agonistic interactions between conspecifics are frequent and may have significant physiological implications. Physiological responses (e.g., increased heart rate) to such social stressors may be determined by the potential costs of agonistic interactions, such as personal injury or risk of breeding failure, and by the motivation of the individuals concerned. The latter may vary according to individuals’ reproductive status or willingness to engage in agonistic interactions. In this study, we investigated heart rate responses to aggressive interactions in a breeding colony of king penguins Aptenodytes patagonicus. From heart rate (HR) and behavior recorded in 20 adults at various stages of the breeding season, we investigated how king penguins reacted to aggressive neighbors. A total of 589 agonistic interactions, 223 in which birds were actors and 366 in which birds remained bystanders (i.e., witnesses that were not involved in interactions), were characterized. We found that HR increased during agonistic interactions, both in actors and bystanders. The intensity (threat displays or physical attacks), duration, and rate of aggressive events (number of threats/blows per unit time) of an interaction significantly influenced the HR response in actors. For bystanders, however, only the duration of interactions seemed to matter. Our results also suggest a role for individual motivation, as initiators of agonistic interactions displayed higher HR increases than responders, and as increases were not constant throughout the reproductive season. We conclude that individual risk assessment and motivation modulate physiological responses to social stressors in group-living animals. Key wor ds: aggressive behavior , colonial breeding, context assessment, heart rate modulation, motivation, stress. [Behav Ecol]