Behavioral Ecology and Sociobiology

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.

Many animals communicate using sequences of signals, but identifying recurrent, non-random signal combinations remains methodologically challenging. Collocation analyses are increasingly popular approaches for detecting which signals animals combine at rates greater than expected by chance. However, existing methods for animal collocation analysis face several limitations that reduce their statistical rigour: they lack uncertainty estimates, fail to control for non-independence in sampled data, and do not account for inflated family-wise error rates when identifying attraction among many different signal types. These limitations restrict the broader applicability of animal collocation analysis, including preventing robust comparisons of signal combination strength between cohorts (e.g. populations, sexes or age classes). We adapt a novel form of Multiple Distinctive Collocation Analysis using Pearson residuals (MDCA-Pr) that addresses these statistical limitations, and validate its use in animal communication research in three ways: first, using numerous simulated datasets of different sizes and levels of signal recombination; second, using simulated data to evaluate the performance of MDCA-Pr in intercohort comparisons, and third, by demonstrating how MDCA-Pr can be applied to compare the vocal sequences produced by male and female captive-living common marmosets (Callithrix jacchus). MDCA-Pr shows high sensitivity, including at small sample sizes, and generally low false-positive rates, which we further reduce by applying additional criteria for identifying attraction between signals. During intercohort comparisons, MDCA-Pr is conservative, with low false-positive rates, and statistical power increases with sample size. MDCA-Pr is a robust method for evaluating signal attraction in animal communication and enables accurate intercohort comparison of animal signal combinations. Significance StatementBy assessing the performance of MDCA-Pr on simulated animal-like data, we demonstrate that this method reliably detects signal combinations within and across animal cohorts, while overcoming statistical limitations of previous collocation analyses. We present an analytical pipeline for applying MDCA-Pr to animal signal data, including for intercohort comparisons, enabling identification and comparison of combinatorial strategies across entire signal repertoires. We illustrate this approach by comparing call combination strategies of male and female common marmosets when presented with food under experimental conditions, finding similar combinatorial strategies between sexes. MDCA-Pr therefore permits rigorous characterization of animal signal combinatoriality and opens avenues for investigating how demographic, social, and group-level factors influence combinatorial patterns.

Sentinel behaviour occurs when individuals use raised positions to scan for predators while the rest of the group forages. Here, we investigated whether a colonial cooperatively breeding species that forages in large groups, the sociable weaver, Philetairus socius, displays sentinel behaviour. This behaviour has been reported in species with similar ecology, behaviour and foraging habits, (e.g. ground foraging in open habitats where aerial predators are common) and, hence, we expected that it could occur in sociable weavers. On the other hand, sentinel behaviour appears to be less common in species that live in very large groups. We used an experimental set-up consisting of an artificial feeding station and perches to assess occurrence of sentinel related behaviours: (i) perching events > 30s on an elevated position, (ii) head-movements and (iii) alarm calling. Birds were seldom observed perching while others fed, and those that did, perched for periods that were too short to be considered as sentinel behaviour (less than 5s on average). Our results suggest that this behaviour is uncommon or even absent in sociable weavers. We discuss whether other factors such as foraging in very large groups, or interspecific foraging associations might make sentinel behaviour less important in this species.

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.

Group-living animals often coordinate their behavior using "contact calls". Identifying the function of these calls requires testing whether they are intended for any group member or targeted to specific preferred associates. If contact calling is used to coordinate with preferred associates, then higher rates of contact calling are expected between group members with a history of more frequent affiliation and cooperation. To test this hypothesis, we constructed a contact-calling network using synchronized recordings of vocal interactions between all 28 possible pairs of 8 female common vampire bats with well-sampled histories of social grooming and regurgitated food sharing. Bayesian multilevel models show that pairwise rates of contact calling were clearly predicted by social grooming and cooperative allofeeding rates in ways not explained by kinship. These findings show that common vampire bats use contact calls to coordinate with specific same-sex associates, unlike other studied bat species where individuals produce contact calls at similar rates towards different group members. We also found that, compared to white-winged vampire bats, common vampire bats are ten times less likely to rapidly respond to a contact call; this suggests yet-to-be-discovered differences in social behavior between vampire bat species. Finally, we discuss implications for the vocal grooming hypothesis.

Non-avian reptiles have been assumed to be non-social for many years, yet recent studies show diverse social behaviours in squamates, crocodilians, and turtles. Here, we investigate social structure within feeding aggregations of three freshwater turtle species caught in baited traps in a coastal marsh over 12 years. In 488 instances in which traps contained turtles, 45% contained multiple individuals, and these aggregations were strongly positively assorted by species. midland painted turtles (Chrysemys picta) and Blandings turtles (Emydoidea blandingii) were captured with conspecifics more often than expected in a non-social null model. Snapping turtles (Chelydra serpentina), the largest species in this study, were caught with conspecifics at rates consistent with the non-social null model, and were avoided by heterospecifics. This suggests that species differences play a role in how feeding aggregations are structured, with painted and Blandings turtles driving positive species assortment while avoiding snapping turtles around food sources. We did not detect significant intraspecific sex assortment in any of the three species, nor did turtle body size strongly affect the probability of aggregating with conspecifics at the perceived food source in the traps. Our study illustrates how long-term monitoring data can be used to investigate social structure in wild populations, an approach that may be valuable for species of conservation concern. Significance StatementReptile sociality has been historically overlooked, but recent work has revealed intriguing social behaviours in non-avian reptiles. We investigated associations among three species of freshwater turtle, captured in baited traps over 12 years of trapping. Turtles in these feeding aggregations grouped with their own species more often than expected by chance. This result was driven by the two smaller-bodied species (midland painted turtle and Blandings turtle), which were more likely to be caught with conspecifics than with other species. In contrast, the largest species (snapping turtle) showed no evidence of attraction to other snapping turtles, and midland painted turtles and Blandings turtles showed avoidance of the larger snapping turtles. Our analyses illustrate how long-term monitoring data can be used to uncover previously unrecognized social behaviour in turtles and other species in which behaviour is difficult to observe directly.

Ultrasonic vocalizations (USVs) are widely used in rodent social communication, yet the functional significance of male-male vocal interactions in mice remains unclear. Here, we investigated whether USVs produced during specific social behaviors influence the behavior of conspecifics. Using playback experiments, we compared responses to vocalizations recorded during chasing and being chased in male-male interactions. We found that USVs emitted by chased intruders consistently elicited approach behavior in receiver mice, whereas those emitted by chasing individuals did not. Acoustic analyses revealed that these vocalizations differed in syllable composition, with intruder calls containing a higher proportion of upward frequency-modulated syllables and exhibiting higher mean frequencies. In addition, the temporal organization of syllables appeared to contribute to the behavioral response. Together, these results suggest that male mice respond selectively to certain USV patterns associated with specific social contexts, indicating that acoustic features and temporal structure may jointly influence social approach behavior in mice. HighlightsO_LIBehavioral context (chased vs. chasing) shapes the composition of USV syllable types C_LIO_LIMale mice selectively approach USVs from chased intruders, but not chasing residents C_LIO_LIThe approach response exhibits high temporal synchrony across individual receivers C_LIO_LITemporal organization of syllables modulates approach behavior based on acoustic features C_LI

Capturing and handling wild animals is essential for ecological and evolutionary research, yet their effects on physiology, behaviour, and reproductive success remain poorly understood. We investigated short- and longer-term consequences of a capture-handling-restraint protocol in wild great tits (Parus major) over three breeding seasons. To assess short-term responses, we measured circulating corticosterone, a metabolic hormone that responds to unpredictable challenges, and automatically recorded provisioning behaviour. We also explored whether environmental and individual traits were related to provisioning latency (i.e., time to resume provisioning after capture). To evaluate longer-term effects, we monitored provisioning in the days following capture and related it to reproductive success (fledgling number and body condition). We predicted that longer handling would increase stress-induced corticosterone and provisioning latency, that these variables would be positively correlated, and that higher corticosterone and longer latencies would be associated with lower reproductive success. After capture, great tits showed elevated corticosterone and delayed provisioning. Contrary to our predictions, handling duration was negatively associated with stress-induced corticosterone in males (but not females) and did not affect provisioning latency. Provisioning latency was unrelated to corticosterone, environmental, or individual variables. Following capture, parents resumed provisioning, and short-term responses had little influence on reproductive success. We show that parental behaviour and physiology are affected by capture restraint protocols on the short term, but offspring condition and survival are not. However, these results should be interpreted cautiously, as our study lacks an uncaptured control group. Our findings highlight that evaluating welfare impacts requires rigorous study design incorporating both immediate and longer-term behavioural and fitness effects.

When novel nutrient-rich food sources become available to species sharing the same natural habitat, interspecies competition may arise, yielding insights into the ecological and social dynamics of the observed species. Here, we investigated food consumption patterns, and consequent social interactions, by two sympatric species of mice in response to a novel nutrient-rich food source. By deploying, in the mices natural habitat, baited video-monitored chambers, we collected, over a 5-month period, 1805 observations of food visiting by Apodemus agrarius and Apodemus flavicollis. We also documented interspecific encounters, with 86.7% of the cases showing agonism. In these interspecies agonistic encounters, A. flavicollis was always the initiator of agonism, attacking within 2 sec in 92.3% of the cases, and being dominant over A. agrarius in 84.6%. Analysis of food visiting behavior revealed that, initially, both species preferred nocturnality. However, after the interspecies fights, A. agrarius switched its temporal preference to diurnality, leading to temporal niche segregation between the two species and a significant reduction of interspecies encounters. Moreover, A. agrarius demonstrated hour-specific avoidance of A. flavicollis, visiting significantly less in hours with A. flavicollis compared to hours without. Through temporal niche switching, A. agrarius managed to access the food source safely, without fights. In contrast, A. flavicollis remained consistently nocturnal across the entire study. Notably, our study presents the first 24h foraging actogram for free-living rodents. Moreover, while rodent interspecific competition is a well-known phenomenon, most of what we know about it comes from indirect observations. Direct observations of rodent interspecific interactions in nature are rare. Our work is the first direct (video-monitored) observation of temporal switch-inducing interspecies interactions in nature. As free-living rodents are currently considered a major model system for the study of interspecific competition, these results may offer precious insights for a better understanding of social dynamics, especially in asymmetric relationships. Furthermore, our findings highlight the significance of considering temporal dynamics in studies of interspecific interactions.

The ectoparasitic mite Varroa destructor is the most significant parasite of the Western honey bee (Apis mellifera) and a major driver of colony losses worldwide. Although extensive research has focused on behavioral and physiological mechanisms of host resistance, comparatively little attention has been paid to potential phenotypic responses of the parasite itself. Here we investigated body size variation in Varroa destructor associated with varroa-resistant and non-resistant honey bee colonies across four European countries. We quantified the dorsal shield area of adult female mites from multiple colonies differing in the honey bee colonies resistance status, using standardized digital image analysis. Across geographically distant non-resistant populations, mite body size was remarkably consistent, with a median dorsal shield area of 1.47 mm2. In contrast, mites originating from varroa-resistant colonies were consistently smaller, with a median dorsal shield area of 1.37 mm2, representing an approximately 6.8% reduction in body size. This pattern was reproducible across different geographical areas, honey bee genetic backgrounds and beekeeping practices. The striking stability of mite body size in non-resistant populations contrasted with the consistent reduction observed in mites associated with resistant hosts, suggesting a host associated shift in parasite phenotype. Because body size in arthropods integrates developmental conditions, nutritional availability and resource allocation, the observed pattern may reflect altered developmental environments and selective pressures imposed by resistant hosts. Our results show a consistent morphological shift in this globally important parasite associated with resistant hosts and suggest that dorsal shield size in Varroa could serve as a new selection marker for varroa-resistant honey bee colonies.

In nature, the most common drivers of lethal aggression are predation and territorial defense. In northeastern Poland, the yellow-necked mouse (Apodemus flavicollis) coexists with several rodent species, including the bank vole (Clethrionomys glareolus). Compared to voles, A. flavicollis is larger, physically stronger, more aggressive, and dominant in the social ecosystem. However, no visually documented instance of a lethal attack by this species has been reported up to date. Here, we present the first recorded case of a fatal attack by a yellow-necked mouse following an encounter with a bank vole. A near-lethal attack is also reported. Importantly, these attacks were not predatory, as no consumption occurred. The attacks appeared instead to be related to interspecies competition, i.e., to competitive interactions between two species that live in the same habitat and use the same type of resources. Notably, while the aggressiveness of yellow-necked mice towards bank voles was known, it was unknown that it could take such extreme forms. Since, in rodents, most competition-related agonistic interactions are aimed at distancing the competitor, the physical destruction of the competitor appears as a surprisingly extreme way of addressing the game of interspecies competition through definitive removal of the opponent. Our observations highlight the need for further research on interspecific aggression among small mammals. They also emphasize the importance of field-based methods, such as camera trapping and continuous video monitoring, which allow for direct observation of animal behavior in natural settings and can reveal rare or previously overlooked interactions.

Parasitism is one of the key, structural, interspecific interactions in ecology. One remarkable parasitic strategy that has been documented in multiple systems is the behavioral manipulation of hosts to increase parasite fitness. While not yet documented in humans, we propose that a ubiquitous zoonotic parasite - Toxoplasma gondii - may change human behavior to favor the parasite by increasing the fitness of the parasites definitive host - cats. Specifically, we assess the possibility that human behavioral changes resulting from chronic, latent T. gondii infection lead to measurable changes in attitudes, actions and dopaminergic responses towards cats that function to increase domestic cat fitness. We assessed the potential role of humans in the T. gondii lifecycle by identifying and testing behavioral changes in humans that benefit the parasite; specifically, human affection for cats. We assessed T. gondii infection status in 68 participants using T. gondii serum antibody testing, and assessed their attitudes towards cats in three ways: i) surveys, ii) participant behavior in the presence of domestic cats, and iii) participant oxytocin levels before and after interactions with cats to assess dopaminergic changes. Only 2 of 68 participants were positive for T. gondii antibodies, limiting statistical power. However, our results indicated that T. gondii-positive participants both reported a greater affection for cats in surveys, and spent more time engaged with cats during behavioral trials than T. gondii-negative participants (87% of study time engaging with cats vs 75%). Oxytocin results were inconclusive.

Physical activity offers myriad benefits to health and well-being, in humans and other animals as well. In rodents, voluntary wheel running can attenuate the effects of both physical and social stressors on rodent social behavior. Whether wheel running affects rodent social behaviors per se remains less well understood. We conducted the current study to test whether home cage access to running wheels impacts the social behaviors of adult, group-housed C57BL/6J female mice during same-sex interactions with novel females. Group-housed females were either given continuous home cage running wheel access or a standard paper hut starting at weaning, and as adults, social behaviors were measured during interactions with novel females. In two cohorts, we found that 5 weeks of running wheel access during adolescence reduced the time that subject females spent investigating a novel female and also tended to reduce total ultrasonic vocalizations produced during interactions. These effects were not reversed by a 2-week period of running wheel removal but were recapitulated in a different cohort by 2 weeks of running wheel access in adulthood. Unexpectedly, we found that these effects on female social behavior were not due to wheel running per se, because females raised from weaning with immobile running wheels also showed low rates of social behaviors during same-sex interactions in adulthood. Overall, we find that the presence of a running wheel in the home cage has an enduring inhibitory influence on female social behavior during same-sex interactions, a finding that has implications for the design of studies that include same-sex interactions between female mice.

We assessed whether pigs provide consolation, referring to targeted affiliation that attenuates a partners stress, under experimental conditions that manipulated exposure to stressed partners. Using a within-subject design, 74 pigs were tested in three contexts: a helping task in which group members could observe and help a trapped focal pig to return to the group, a direct-reunion, in which group members were naive to the experience of a separated focal pig until reunion, and an undisturbed control. We measured affiliative and non-affiliative interactions, anxiety behaviours and changes in salivary cortisol. Only the helping context satisfied most consolation criteria: there were selective increases in unidirectional affiliative contacts from the observer to the focal pig, non-affiliative interactions remained at baseline, and focal pigs showed fewer anxiety behaviours. In contrast, direct-reunions triggered increases in affiliative and non-affiliative interactions and higher anxiety. Cortisol increased during both direct-reunions and helping, but its level was not linked to affiliation. Results add to growing evidence for consolation behaviour in pigs and suggest best practices for reintegrating pigs into groups. Graded reintroductions that allow observers to assess the emotional state of targets may promote social buffering, whereas abrupt regrouping may trigger more generalized arousal or personal distress.

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.

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.

The ability of animals to innovate - solve novel problems - can shape their ecology and evolution. Here we investigate how individual traits and environmental complexity relate to successful solving of a novel problem. We presented foraging bumble bees (Bombus impatiens) with artificial flowers of not-previously-encountered shapes and recorded the bees latency to access nectar. We measured individual foraging traits across multiple trips with simple flowers that did not require innovation, and bees were foraging either in a simple or complex environment (cluttered flight arena). Bees in complex environments took longer to find and were less likely to land on novel flowers, indicating that environmental complexity may take up cognitive resources and make search more difficult. However, we did not find an effect of environmental treatment on the ability or time to access reward in novel flowers once bees had landed on them. In contrast, behavioral traits significantly predicted how quickly bees solved novel flowers. In particular, overall foraging tempo as well as routine formation, i.e. how much bees followed a fixed route on known flowers, predicted innovation - faster bees innovated faster, and bees with more repetitive foraging sequences were slower to solve the novel tasks. Overall, while the degree of evolutionary novelty in tasks or solutions is always hard to evaluate, our findings demonstrate that environment and individual traits may affect innovation in different ways. Individuals in simple environments may be more likely to detect, and individuals that are generally faster and have a lower tendency to develop fixed routines may be more likely to solve, novel tasks.

Object-based tasks are widely used in rodent behavioral research, yet object selection remains largely unsystematic. We present a paired-object validation protocol in which objects differ along one researcher-defined feature, allowing assessment of whether that feature is salient to the animal. Using six object pairs varying in height, color, shape, or aperture presence, we tested two wild-caught mice species with contrasting ecologies. Wood mice (Sylvaemus uralensis) and striped field mice (Apodemus agrarius) showed equal preference for both objects in most pairs, indicating that color, apertures, and apex shape differences are not salient under the tested conditions and can be used interchangeably in object recognition tasks. Height, however, produced ecology-predicted responses: arboreal wood mice avoided the shortest object while open-habitat striped field mice did not. These results demonstrate that the protocol successfully detects feature salience when present and that ecological background predicts which features matter. Summary StatementA systematic paired-object protocol reveals that most researcher-defined features (color, holes, shape) do not affect rodent exploration, but height preferences emerge in ecology-predicted patterns, demonstrating that feature salience is species-specific.

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.

Reproductive Character Displacement (RCD) often occurs when species with mating-related polymorphism come into secondary contact, leading to divergence in reproductive traits. Ischnura elegans and Ischnura graellsii have formed two independent hybrid zones in Spain where reinforcement has strengthened a mechanical barrier, and RCD has shaped mating-related structures, although reinforcement is asymmetric only in gynochrome females. This study examines the link between asymmetric reinforcement and asymmetric RCD. Using geometric morphometrics, we analyze prothorax shape and size in both female morphs and males, and male caudal appendages, to assess morphological divergence, determine whether gynochrome females show stronger divergence, and test for morphological covariation between male traits involved in the tandem position. Our results reveal consistent patterns of size and shape variation across species and zones: in I. elegans, androchromes are larger and resemble males in size, with clear shape differentiation between female morphs that diminishes in hybrid zones. In contrast, I. graellsii shows less consistent size differences between males and morphs, and weaker shape differentiation. Our results confirm RCD in prothorax shape in I. elegans females from both hybrid zones, but reveal that RCD in prothorax size is asymmetric, occurring only in gynochrome females from the NC hybrid zone. We also detected RCD in the prothorax shape of I. elegans males from the NC hybrid zone, extending previous evidence of RCD in male caudal appendages, while morphological covariation between male cerci and the prothorax was limited to size in I. elegans. Together, these findings illustrate how hybridization may generate morph-specific patterns of reproductive divergence.