Behavioral Ecology

Interactions between cleaner fish Labroides dimidiatus and client fish, from which cleaners remove ectoparasites and mucus, represent a textbook example of mutualism involving sophisticated strategic decision-making. However, cleaners must also face intraspecific social challenges within a size-based hierarchy, where the largest females may eventually change sex and become males with higher reproductive rates. Following 540 individuals over 11 months, we found that, contrary to expectations, slow-growing females spent more time cleaning and cheated more frequently, without causing more negative client responses than fast-growing females did. Instead, variation in growth was best explained by social factors: fast-growing individuals experienced reduced social control, while slow growers spent more time in proximity to dominant individuals. As there was no evidence that spawning activity affected growth patterns, it appears that fast growth as a viable strategy for becoming a male largely depends on the lack of control by dominants.

Prolonged association between mothers and their offspring is common in ungulates, with the level of maternal investment likely to play a central role in shaping this trait. Here we examined patterns of association between mothers and offspring over time, the apparent benefits of association to offspring, and costs to mothers. We analyzed 40 years worth of census data from an individually-monitored, food-limited population of red deer (Cervus elaphus) on the Isle of Rum, Scotland. Starting from birth, female calves associated more frequently with their mothers than male calves in their first year. Calves also associated less with their mothers if the mother did not conceive a new calf. Association frequency decreased with mothers age and population density, and survival over the first year was not related to mother-calf association. Yearlings, now in their second year, were more often associated with their mothers if they were female, if there was no subsequent calf (or the subsequent calf died as a neonate), and if they were still being suckled. Increased association between mothers and yearlings was associated with increased survival to adulthood at 28 months, but suckling a yearling did not improve its probability of survival. For individuals that reached maturity, increased association in the yearling year was associated with slightly shorter adult life spans. The level of association between a calf and mother was not associated with the mothers immediate survival or fecundity. Our findings suggest that juveniles born to poor-condition mothers benefit from prolonged association through improved yearling survival.

Social interactions are ubiquitous in nature and have the potential to affect trait evolution, particularly in group-living animals such as cooperative breeders. Interactions among conspecific individuals can affect the amount of additive genetic variation for a trait when the phenotype of an individual is also affected by the genotype of its social partner(s) via indirect genetic effects. Thus, quantifying both direct and indirect genetic effects of social partners is critical for understanding and predicting evolutionary trajectories. While much is known about maternal indirect genetic effects, empirical estimates of indirect genetic effects from other social partners remain limited, particularly in wild populations. Here, we use animal models to assess the contribution of indirect genetic effects from all social partners in a family group (mothers, fathers, and helpers) on juvenile morphometric traits across ontogeny in the cooperatively-breeding Florida scrub-jay (Aphelocoma coerulescens). We found indirect genetic effects of helpers and fathers on nestling weight, but no indirect genetic effect of mothers. Across ontogeny, we found increasing additive genetic variation in both weight and tarsus length. Our study provides a comprehensive assessment of within-group indirect genetic effects in a cooperative breeder and highlights the importance of considering indirect genetic effects beyond maternal effects.

Many primates exhibit female philopatry and live in stable, female-bonded social groups. Permanent group fusions are rarely documented in these populations. We present a case study on a fusion of two social groups from a hybrid population of baboons (Papio cynocephalus x P. anubis) living in the Amboseli basin of Kenya. The fusion occurred following a period of increased human-induced mortality in one of the two social groups. After the fusion, females from the smaller group became the lowest ranking. We compared female behavior in the months following the fusion to the behavior of females in groups that had not fused and also compared pre- and post-fusion fitness outcomes. Following the fusion, the groups activity budget and patterns of agonistic interactions were typical for the study population. Females preferred familiar grooming partners for a short period following the fusion; however, after three months, patterns in female grooming were comparable to other groups, indicating rapid social integration. With the caveat that our sample size was limited, we observed no detectable fitness-related costs of group fusion in terms of birth rates or offspring survival, and adult female mortality was low following the fusion. These results demonstrate the flexibility of female baboons in navigating exposure to novel same-sex conspecifics despite a species-typic pattern of female philopatry. Based on this and previous examples of group fusions, we propose that group fusions may be most likely to occur when groups are too small to retain adult males, defend against predators, or compete with other groups.

Multimodal communication plays a central role in animal behavior, particularly when individuals must integrate information from different sensory channels to make rapid decisions. In aquatic environments, chemical and visual cues differ markedly in their spatial and temporal properties, such that chemical signals may be constrained by limited spatial resolution and temporal instability, potentially requiring visual information to reliably guide social decisions. In decapod crustaceans, both cue types are known to mediate reproduction, yet their relative contribution to mate-location behavior remains unclear. Here, we tested how visual and chemical cues from males influence mate-location behavior in females of the prawn Macrobrachium rosenbergii. Females were placed in a central arena and exposed to four stimulus configurations combining visual cues (a life-size photograph of a male or a control background) and chemical cues (water from an aquarium with or without a male). Attraction was quantified as the time spent in each half of the arena. Females showed no directional preference when exposed to chemical cues alone or when visual and chemical cues were spatially incongruent. In contrast, females spent significantly more time near male-associated stimuli only when visual and chemical cues were spatially congruent. These results indicate that mate-location behavior in this species depends on multimodal integration with a strong contextual dependence on visual information, which appears to gate the effectiveness of chemical cues. Spatially congruent multimodal signals are therefore necessary to guide orientation during mate search, suggesting that disruption of visual or chemical information in aquaculture systems may impair mating efficiency.

Within species, individuals of the same age can differ in size. Previously, parental genetics, nutrition, space, and social interactions have been suggested to explain different growth rates. However, direct effects of larger individuals on the physiology and growth of smaller individuals are poorly understood. In this study, we investigated how larger individuals of the marine worm Platynereis dumerilii can impact the growth of smaller conspecifics. Comparing growth distributions in communally and individually reared worms, we show that larger worms suppress the growth of smaller ones. Furthermore, we were able to demonstrate that this suppression is chemically mediated. The chemical cue does not originate from faeces but is water soluble, stable for several days and smaller than 3 kDa. Our findings highlight the importance of non-reproduction related chemical signalling, showing evidence that dominant individuals can chemically suppress the growth of their conspecifics. This study provides new insights into how hierarchy can be established and maintained in a population and is particularly relevant for the growing community studying this model species.

Conspecific sperm precedence via cryptic female choice is a post-ejaculatory selection process that reduces hybridization, and can be pronounced in sympatric species. In their native Europe, Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) exert conspecific sperm precedence under heterospecific sperm competition, which is at least partially enabled by female reproductive fluid. We examined post-ejaculatory selection of both species in Newfoundland, Canada, where Atlantic salmon evolved in absence of brown trout, but now experience hybridization threats due to anthropogenic introductions. Using split-ejaculate and split-clutch in-vitro fertilizations we evaluated whether allopatric evolution has relaxed this selection in Atlantic salmon, and found that they had no ability to bias paternity towards conspecific males, whereas naturalized brown trout retained a strong ability to do so. Female reproductive fluid influenced this, as when fluid associated with a species eggs was swapped, hybridization increased. In the artificial situation of no female reproductive fluid during sperm competition, paternity changed dramatically, but sperm swimming performance did not predict it. Our findings contribute to understanding the evolution of cryptic female choice and how the mechanisms of reproductive isolation can be reinforced through sympatry, while also highlighting a new potential conservation concern for North American Atlantic salmon.

Sex ratio is a key demographic parameter shaping population dynamics and evolutionary trajectories. In biocontrol agents, demographic bottlenecks during species introduction to a new habitat and subsequent mass rearing can elevate inbreeding, potentially biasing sex ratios through sex-specific mortality associated with inbreeding depression. Moreover, reproductive endosymbionts such as Wolbachia are known to manipulate host reproduction and further skew sex ratios. However, the relative contributions of these processes to sex-ratio variation remain poorly resolved. In this study, we evaluated the effects of cross-generational full-sibling inbreeding and Wolbachia infection on sex ratio and key life-history traits in the biocontrol beetle Zygogramma bicolorata using controlled laboratory crosses across three generations. Inbreeding did not significantly alter offspring sex ratio, which remained close to parity across generations, while pupal mortality increased in later generations, consistent with delayed expression of inbreeding depression. Adult body weight remained largely unaffected by inbreeding. Wolbachia infection was detected in a subset of females and was associated with a modest but significant increase in female-biased offspring production, although the effect was variable across lineages. Strain typing identified a single supergroup A Wolbachia, consistent with previous descriptions of the wBic strain from this species. These findings indicate that sex-ratio variation in introduced populations of Z. bicolorata is not driven by inbreeding alone but instead emerges from the interaction between demographic processes and symbiont-mediated effects, providing crucial insights for optimizing biocontrol programs where sex-ratio stability is essential for population establishment and persistence. SignificanceSex ratio is a key determinant of population growth and stability - the essential parameters determining success of biocontrol programs. Yet, the mechanisms shaping sex-ratio variation remain poorly resolved. Using controlled crosses in Zygogramma bicolorata, we show that short-term inbreeding does not directly alter sex allocation, despite inducing delayed fitness costs through increased pupal mortality. In contrast, Wolbachia infection contributes to female-biased offspring production, although with variable outcome across lineages. These findings demonstrate that sex-ratio variation in Z. bicolorata arises from the interaction of demographic processes and symbiont effects, rather than a single mechanism, with important implications for predicting the establishment, persistence, and efficacy of mass-reared biocontrol populations.

Birdsong mediates territory acquisition and mate choice. In agonistic interactions, local songs generally elicit stronger responses than songs from more distant populations. However, the molecular mechanisms associated with differential responses to local vs. foreign songs are poorly understood. We addressed this knowledge gap by combining behavioral assays in the field with blood transcriptomic analysis, using a within-subjects design to ask whether male song sparrows (Melospiza melodia) show differential gene expression when exposed to playback of local and foreign songs. Transcriptomic profiles reflected the difference in behavioral response to local vs. foreign songs, with individuals exposed to local songs showing greater expression of genes associated with song perception and production, anti-inflammatory responses and energy metabolism. Our study suggests that changes in expression of key molecular pathways correlate with behavioral responses to geographic song variation, providing insight into the potential mechanisms regulating signal recognition and response to social challenges. HighlightsO_LIGene expression in sparrow blood was measured after simulated territorial intrusion. C_LIO_LIStronger response to local songs was associated with differential gene expression. C_LIO_LISong-associated genes (FOXP2, NRXN1) had higher expression when birds heard local songs. C_LIO_LIGene expression in the blood contains potential biomarkers of song recognition. C_LI

Animal weapons are ecologically important traits that mediate contests over limiting resources and can strongly influence survival and reproduction. Weapon traits often exhibit substantial intraspecific morphological diversity, raising questions about the ecological drivers of this variation. Acrorhagi are weapons produced by sea anemones that are used in intraspecific territorial encounters. Although acrorhagial morphology varies widely within species, patterns of intraspecific variation remain poorly characterized, and the extent to which such variation reflects differences in local intraspecific competition is unclear. Here, we conduct morphometric analyses to characterize within-population variation and allometry in acrorhagial traits of the solitary anemone Anthopleura sola. We show that these traits covary with habitats differing in conspecific density. The number of acrorhagi scaled positively with body size, and individuals occupying a high-density habitat tended to possess more acrorhagi than did similar sized individuals from a low-density habitat. In addition, anemones from high-density habitats exhibited longer acrorhagial cnidae, a pattern that was not explained by differences in body size or acrorhagial density. Together, these results suggest that competitive context influences weapon-related traits at multiple levels of biological organization, potentially via phenotypic plasticity or selective processes. More broadly, our findings highlight how fine-scale ecological variation may contribute to the maintenance of trait diversity within and across species.

Dormancy is a widespread adaptive strategy that allows organisms to survive in temporally varying habitats by suspending development and reproduction. Although environmental variability is expected to shape dormancy strategies, it remains unclear how differences in environmental variability and predictability influence both the production of dormant embryos and the termination of dormancy. We addressed these questions by comparing D. pulex and D. obtusa, two closely related species that inhabit environments differing in variability and predictability. We hypothesized that D. obtusa, which inhabits ephemeral environments, would exhibit a greater propensity for sexual reproduction and dormancy and would require stronger cues to break dormancy than D. pulex, which occurs in more permanent, predictable habitats. Consistent with our hypothesis, D. obtusa lineages produced significantly more males and ephippia than D. pulex when reared under identical laboratory conditions, indicating greater investment in sexual reproduction and dormancy. Contrary to our hypothesis, we found no difference in responsiveness to cues between the two species. Across species, embryos broke dormancy and hatched most readily after experiencing changes in cold and light, even if not experienced at the same time. In contrast, desiccation reduced the propensity to break dormancy. Together, these results indicate that species occupying more ephemeral environments invest more heavily in the production of dormant offspring, but that the environmental cues regulating dormancy termination appear broadly similar between species. This pattern suggests that while investment in dormancy may evolve in response to environmental variability, the mechanisms controlling dormancy termination are more conserved.

The evolution of alternative male reproductive strategies represents an intriguing evolutionary phenomenon. Divergent strategies are persistently at risk of local extinction or invasion, depending on the suites of traits expressed within and between morphs; hence, understanding the correlational selection that aligns reproductive strategies with behaviour, morphology and physiology is key to understanding the origin and maintenance of genetic polymorphisms. In the polychromatic painted dragon, Ctenophorus pictus, yellow, orange and red morphs are well characterised, but the blue morph has been historically absent from studied populations. Here we document the local distribution, morphology and male-contest interactions in a population where blue males are relatively common. We find that blue males express head colouration after a reaching a threshold body size, and that small blue males can reside in close proximity to other males; patterns consistent with a novel size-dependent conditional tactic within the suite of genetic strategies seen in this species. Condition-dependent, positively allometric throat bibs were non-randomly distributed among male morphs, implicating variation in correlational selection and the genetic architecture of the polymorphism. We were unable to definitively assign a morph that was superior in male competition but found that within morphs, male size was the determinant of competitive success, whilst between morphs it was not. Furthermore, contests between morphs were resolved with less aggression than contests within morphs, supporting the idea that badges resolve conflict, and that the invasion of new colour morphs may be facilitated by negative frequency dependent benefits to novel colour variants. These findings highlight the divergent phenotypic, genetic and selective environments that lead to the diversity of colour morphs.

To attract mating partners, female mammals communicate their reproductive status through one or multiple sensory modalities, providing redundant or complementary information. Chimpanzees (Pan troglodytes) are an excellent model for studying multimodal communication. Exaggerated sexual swellings of females serve as a visual proxy for ovulation but increased male mating interest during maximum swelling suggests that olfactory cues may pinpoint fertility more accurately than the swelling alone. Here, we combined gas chromatography-mass spectrometry, hormonal analyses, and bioassays to examine (1) whether chemical composition of female anogenital odours changes during the fertile period, and (2) whether males are able to detect these changes. Our results suggest that, in addition to prominent olfactory changes associated with swelling stages, chemical cues provide complementary information regarding the timing of the fertile window. These changes, however, are minor compared to those related to swelling stages. Male behavioural responsiveness in bioassays was too low to draw conclusions regarding their ability to detect these subtle shifts when presented with a chemical cue only. Overall, our findings support the existence of a multimodal fertility cue in chimpanzees, wherein visual signals are complemented by subtle olfactory changes indicating the timing of the fertile period.

Social decisions often require animals to integrate information across multiple attributes of potential partners. Using biological motion stimuli, point-displays generated from tracked live shoals, we tested how adult zebrafish (Danio rerio) weigh shoal size and movement speed during social preference, and whether these preferences are susceptible to contextual manipulation by an asymmetrically placed alternative. In Experiment 1, we established a multi-attribute indifference point by presenting males and females with dichotomous contrasts in which shoal size and movement speed were traded off. Both sexes showed no preference when a larger, slower shoal (4 fish at 0.75x speed) was pitted against a smaller, faster shoal (2 fish at 1.25x speed), but preferred the smaller, faster shoal when the speed difference was greater (4 fish at 0.5x versus 2 fish at 1.25x), indicating that zebrafish are sensitive to graded differences in movement speed relative to numerical cues. In Experiment 2, unidimensional tests confirmed that both sexes preferred larger shoals when speed was held constant but revealed sex-based differences in speed sensitivity: males preferred faster-moving shoals at both shoal sizes tested, whereas females showed no significant speed preference. Male shoal size preferences were stronger at higher movement speeds, suggesting that speed modulates the strength of size preference. In Experiment 3, we tested the asymmetric dominance effect in males, the only sex sensitive to both dimensions, using the indifferent contrast from Experiment 1 as the primary options and four decoy shoals asymmetrically placed along either the size or speed dimension, under counterbalanced presentation orders. No decoy shifted male preference significantly from chance under any condition. These results indicate that zebrafish weigh social cues in a sex-specific manner, and that asymmetric decoy options do not induce preference biases in males when shoals vary along the dimensions of movement speed and size.

The evolution of eye coloration in mammals and its potential ecological significance remain understudied. Evidence from anthropoid primates suggests that photoprotective demands are crucial determinants of pigmentation in the peri-iridal tissues, which encompass the conjunctiva and portions of the sclera peripheral to the iris. However, it is unclear to what extent these findings can be generalized. Here, we quantify peri-iridal brightness in a photographic sample of 62 terrestrial non-primate mammal species (n = 930). Phylogenetically-controlled analyses revealed significant effects of eye size as well as ecology on ocular pigmentation. Peri-iridal brightness exhibits a notable phylogenetic signal, correlates negatively with eye size and hence exposure to UV light, and is more pronounced in nocturnal species. Significant interspecific effects of latitude on peri-iridal brightness were not recovered, but tentative evidence for non-negligible impacts of this variable at the intraspecific level were found. Overall, these results align with and help to contextualize findings on primates and suggest that photoprotective demands importantly shape ocular appearance across the mammalian radiation. Furthermore, they have implications for hypotheses tying eye pigmentation chiefly to gaze signaling and provide a broad evolutionary framework for the emergence of human eye appearance.

Locating, tracking, and intercepting objects is a fundamental behavior for many organisms. For instance, predators must track and capture erratically moving prey for their survival. Using the echolocating bat as a model species, we investigate how short-term changes in target motion predictability affect longer-term motor plans when tracking a prey item. We used a paradigm where prey motion is under experimental control, and then applied computational methods to characterize how target motion predictability influences short- and long-term behavioral control. We find that target motion predictability during the tracking phase of insect capture influences both short-term changes in sonar call control, as well as longer-term behavioral control for transitioning between hunting phases. For changes in immediate behavioral control, bats produce more bursts of calls at a higher rate when tracking unpredictable moving prey, an indication that the bat is collecting more information about the targets motion for unpredictable than predictable trials. In terms of longer-term behavioral control, target motion unpredictability delays the transition from tracking to capture phase behaviors. We suggest that the bat does this to collect more information about target motion to time the transition from tracking to capture behaviors for hunting success. Additionally, we find the effects of target motion unpredictability are first seen as changes in the vocal motor plan and then the auditory motor plan (ear motion), hinting at a sequencing of motor changes that warrant further investigation. SummaryWhen presented with a more challenging hunting task, bats will increase their production of bursts of calls at a higher rate and delay their transition into capture behaviors.

In many tropical areas, seasonal rainfall leads to distinct dry and wet seasons. Many butterflies developing under wet season conditions develop into adults with large ventral eyespots on the wing margins, whereas those developing under dry season conditions have smaller or no eyespots. In greener, wet season habitats, larger eyespots can divert predator attacks toward the wing margins, while reduced eyespot size improves camouflage in the dry leaf litter-dominated habitat during the dry season. However, the dry season is also characterised by higher desiccation stress than the wet season. We hypothesised that larvae developing under dry season conditions develop into adults with higher desiccation tolerance than those reared under wet season conditions. We tested this by rearing larvae of the butterfly Mycalesis mineus under simulated dry and wet season conditions and assaying the desiccation tolerance of the resulting adults. Butterflies reared in dry conditions survived longer under desiccation stress, lost lesser water during pupal-adult metamorphosis, and were heavier than those reared in wet conditions. We also tested the correlation between eyespot size and desiccation tolerance. A negative correlation between the traits would be expected if similar developmental pathways regulate them. Consistent with this expectation, individuals with smaller eyespots had higher desiccation tolerance. Our results demonstrate plasticity in desiccation tolerance, and suggest that predator avoidance and desiccation tolerance traits may share similar developmental pathways.

A central challenge in characterizing species niches is ensuring that foraging data accurately capture both the resources used and their relative importance, and the role of resource abundance in shaping foraging patterns. Most studies infer diet breadth and resource-use patterns from observational records, yet such data can mask resource-specific decisions when animals forage with different goals. Here, we test this experimentally using individually identifiable bees in controlled resource communities to quantify foraging decisions between nectar (for sustenance) and pollen (for offspring provisioning). Combining observations, pollen DNA metabarcoding, and pollen microscopy, we show that observed visitation patterns misrepresent the floral resources most important for offspring provisioning, which ultimately determines offspring survival and population persistence. We further show that interaction patterns are structured from processes beyond resource abundance. Our results demonstrate that commonly used observational approaches can mischaracterize diet breadth, potentially challenging conclusions about species generalization.

Within species, male testosterone is often linked to mating competition and paternal care, suggesting that sex differences in endogenous testosterone values across mammals may covary with broader reproductive strategies. Using a structured literature search, we compiled 63 studies, spanning 31 non-human species and 9 human populations, reporting endogenous, non-experimentally manipulated testosterone values for both adult males and females within the same population and context. From these studies, we calculated male-to-female testosterone ratios, and analysed these data using Bayesian phylogenetic multilevel models. We tested whether testosterone dimorphism was associated with paternal care and sexual size dimorphism while accounting for sampling matrix, assay method, breeding context, and wild versus captive setting. Across non-human mammals, neither paternal care nor sexual size dimorphism (indexing competition) showed a clear association with testosterone ratios, and the same pattern emerged in the primate-only subset. By contrast, sampling matrix was consistently associated with testosterone dimorphism across all analyses, with lower male-to-female ratios in non-blood than in blood-based measures. In primates, testosterone ratios were also lower in captive than in wild populations, although this pattern was not clearly supported in the broader non-human dataset. In the human-only analysis, testosterone ratios did not clearly differ between industrialized and small-scale societies, whereas the matrix effect remained evident. Overall, our results suggest that sampling matrix is a major source of variation even for ratio-based measures, highlighting the need for caution when inferring between-species endocrine differences from studies using different substrates. More broadly, directly comparable, non-experimentally manipulated testosterone data for both sexes remain rare across mammals, limiting comparative inference.

The fitness of immigrants and their descendants determines the effectiveness of gene flow. Genetic incompatibilities or outbreeding depression can limit the spread of novel alleles, while highly fit immigrant lineages can hasten introgression. These fitness effects of gene flow can also differ between generations as immigrant and resident haplotypes recombine. Understanding the genetic factors that shape immigrant fitness over multiple generations is increasingly important as habitat fragmentation threatens populations by reducing genetic variation and leading to increased levels of inbreeding. Few studies have measured the multigenerational fitness of immigrant lineages, especially within populations that had histories of high gene flow. We used 33 years of life history and pedigree data on a population of Florida scrub-jays (Aphelocoma coerulescens) with historically high immigration to quantify the fitness of immigrants and their descendants. We compared the fitness of immigrants and residents as well as their resulting descendants (F1, F2, etc.) to determine the composite genetic effects responsible for fitness differences. We found evidence of additive benefits of immigrant ancestry and heterosis driven by non-additive effects that persists for multiple generations. These results are promising for conservation efforts aiming to increase connectivity and illustrate the complex dynamics that determine the rates of introgression in natural populations.