The evolution of exaggerated structures used as weapons in male-male contests often drives correlated changes in traits that compensate for their costs or enhance their benefits. Given their interdependence, weapons and compensatory traits are expected to evolve with some degree of genetic integration. The bulb mite Rhizoglyphus echinopus, a male-dimorphic species, offers an opportunity to examine this question. In this species, the third legs are dimorphic in width, functioning as a weapon, while the fourth legs are dimorphic in length, potentially serving as a compensatory trait. Fighter males, identified by their thicker third legs, also tend to have longer fourth legs than scramblers. Here, we first tested whether body size and the width of the third legs predict contest success, and whether the length of the fourth legs also contributes to fighting ability. We then evaluate phenotypic and genetic integration between these traits. Behavioral trials showed that variation in body size, third leg width, and fourth leg length did not explain contest outcomes. Moreover, although males with thicker third legs tended to have longer fourth legs, quantitative genetic analyses revealed that the traits are phenotypically, but not genetically, correlated. This suggests that their phenotypic match likely results from shared environmental thresholds rather than genetic integration. Overall, our findings emphasize the need to move beyond phenotypic correlations when evaluating the evolutionary role of compensatory traits, as such correlations alone may mislead interpretations of their adaptive significance.

A new species of the genus Dongmoa Roewer is described from Thanh Hoa Province, Vietnam, with its intricate external morphology examined using micro-CT scanning. Until now, Dongmoa included only two species, one from northern Vietnam and another from the Ryukyu Islands, Japan. Here, we provide a revised diagnosis of the genus, with detailed comparisons to the closely related genera Bonea and Erecanana. In addition, two species from Hainan Island, China, previously assigned to Bonea, are transferred to Dongmoa, increasing the total number of recognised species in the genus to five. This study represents the first modern systematic treatment of Dongmoa, substantially refining the traditional Roewerian framework and enabling a clearer delimitation of species distributed across Indochina and Japan. Moreover, the recognition of a previously overlooked but variable morphological structure, here called the insula lunata, highlights its potential relevance for taxonomy within Podoctidae, providing the basis for future phylogenetic studies.http://www.zoobank.org/urn:lsid:zoobank.org:pub:06CD60D3-A39F-406A-9450-AFC904E50D16

Female mimicry is a reproductive tactic typically employed by small males (minors) in species with female- or resource-defense polygyny. By mimicking females, minors may secure copulations without being detected by territorial males (majors). We used three complementary approaches to investigate the possibility of female mimicry in the male-dimorphic harvestman Serracutisoma proximum, a chemically-oriented arachnid with a resource-defense mating system. Minors rely primarily on sneak copulations, and if they exhibit female mimicry, we expect them to elicit behavioral responses from majors similar to those directed toward females. Indeed, a field experiment showed that majors displayed reduced aggression toward minors and even attempted copulation with some of them. To examine the mechanisms underlying of mistaken sex identification, we compared the cuticular chemical profiles of majors, minors, and females, and quantified the density of chemoreceptors on the sensory legs of majors and minors. Contrary to female mimicry predictions, the overall chemical profile of minors did not resemble that of females. However, some compounds that signal female reproductive status in spiders showed similar abundances in minors and females. Additionally, chemoreceptor density did not differ between majors and minors, suggesting that mistaken sex identification is also not due to sensory limitations in majors. In conclusion, while behavioral evidence is consistent with female mimicry, the underlying mechanisms remain unresolved. Regardless of the underlying mechanism, the ability of some minors to access and mate with females even in the presence of majors likely confers reproductive advantages, which could maintain alternative reproductive tactics within the population.

The exaggerated evolution hypothesis proposes that intrasexual competition accelerates the evolution of contest-related traits. This hypothesis is supported by a simulation model and empirical data from beetles and mammals using horns or antlers as true weapons. However, it has never been tested for threat devices, which are expected to be under directional selection. Using a clade of neotropical harvestmen where males employ their fourth leg pair as a threat device, we test whether intrasexual competition accelerates the evolutionary rates of these legs. We used a time-calibrated molecular phylogeny and morphometric data to estimate evolutionary rates of each leg pair in males and females of 23 species. We found that the fourth legs in males evolved at rates 5.5-44 times higher than the other legs and 6.5 times higher than the homologous legs in females. In conclusion, threat devices also show accelerated evolutionary rates, supporting the exaggerated evolution hypothesis.

Mate choice is shaped by both intrinsic and extrinsic factors, including signal conspicuity, receiver body condition, and environmental properties. Multimodal signals are generally more conspicuous than unimodal ones and are expected to elicit stronger responses. However, variation in body condition and background noise modulates receivers' responsiveness. We investigated how signal type (uni- or multimodal) interacts with body condition and background acoustic noise to influence female responses to male displays in the stream-breeding frog Crossodactylus schmidti. In a field experiment using a male-mimicking robot frog, females were exposed to acoustic (advertisement calls), visual (toe flags), and multimodal (advertisement calls + toe flags) stimuli, while their behavioral responses, body condition, and environmental acoustic noise were recorded. Females responded preferentially to multimodal stimuli over acoustic or visual ones. Those in poorer body condition were more responsive overall, reacting more readily to all stimulus types, indicating condition-dependent mate choice. High levels of acoustic noise generally reduced responses to visual signals, suggesting cross-modal sensory interference. However, when exposed to unimodal acoustic stimuli in noisier sites, females increased their visual signaling, possibly as compensatory behavior in response to auditory masking. These findings highlight the adaptive significance of multimodal signals in noisy environments and emphasize the role of female condition and background noise in shaping intersexual communication during mate choice. We used a robotic male frog to test how different types of signals influence female choice in a frog species. Females responded more strongly to signals combining sound and movement than to those with a single component. Females in good body condition were choosier and slower to respond. Acoustic noise also affected responses, sometimes making it harder for females to detect signals. These findings highlight the complex interplay of multiple factors in frog mate choice.

Exaggerated male traits used in intrasexual contests are often condition dependent and, in many taxa, intrasexual dimorphism in these traits arise in association with alternative reproductive tactics. Although it is frequently assumed that trait exaggeration and intrasexual dimorphism evolve in concert, this hypothesis has not yet been tested within a comparative framework. Here, we investigate whether the degree of exaggeration of contest-related traits is evolutionarily correlated with intrasexual dimorphism across species of Neotropical harvestmen. We focus on two subfamilies of Gonyleptidae—Goniosomatinae and Mitobatinae—in which males use extremely elongated legs as threat devices during ritualized contests. Using morphological data from individuals representing 46 species, we quantified leg exaggeration as relative leg length and estimated the degree of intrasexual dimorphism using bootstrap-supported mixture models. We then applied phylogenetic comparative analyses in a Bayesian framework to test for correlated evolution between these traits. In Goniosomatinae, we detected moderate positive phylogenetic correlation between leg exaggeration and intrasexual dimorphism. In contrast, no evidence of correlated evolution was found in Mitobatinae, where many species exhibit pronounced leg exaggeration without intrasexual dimorphism. Together, these patterns suggest that although trait exaggeration may be a necessary condition for the evolution of intrasexual dimorphism, it is not sufficient on its own. Instead, the macroevolutionary association between trait exaggeration and intrasexual dimorphism appears to depend on functional constraints, behavioral flexibility, and the genetic architecture underlying condition-dependent trait expression.

Citizen-science platforms have become powerful tools for documenting natural history, but their potential for reconstructing behavioural evolution remains largely unexplored. Here, we combine field and laboratory records with a systematic survey of iNaturalist observations to investigate the evolution of parental care in the superfamily Gonyleptoidea, the clade containing the most documented cases of care in harvestmen. We report 85 new records of parental behaviour (78 of maternal or paternal care and seven of no care), more than doubling the number of species with available data. These records, together with all published information, were mapped on to a newly assembled supertree of 165 species spanning nine families. Transition rates revealed that maternal care arises exclusively from ancestors without care, whereas paternal care evolves through two routes-from no care and from maternal care-suggesting distinct selective pressures underlying its repeated origins. Transitions from parental care back to no care are frequent in both sexes, suggesting that alternative egg-protection strategies (e.g. oviposition in cryptic sites and debris coverings) may compensate for the absence of parental attendance, while avoiding the costs of prolonged care. Our findings highlight the remarkable evolutionary lability of parental care in Gonyleptoidea and demonstrate that citizen-science data can substantially accelerate macroevolutionary research.

Male trimorphism occurs in few animal species and remains poorly understood, both in its proximate causes and ultimate adaptive significance. Here, we provide the first evidence of male trimorphism in a Neotropical harvestman. Using modern procedures for morph recognition, we show that Longiperna concolor, previously considered male-dimorphic, actually has three male morphs (alpha, beta and gamma). We also examine the allometric patterns of fourth-leg length, a trait used as a threat device in male-male contests. Our results show that alphas exhibit hypoallometry, betas exhibit hyperallometry and gammas exhibit isometry. Reanalysis of previously published behavioural data revealed that alphas are the primary harem holders and the most frequent contestants, but a subset of betas also holds harems and engages in territorial contests. In contrast, gammas were never observed defending harems or engaging in contests. We propose that a gene-by-environment interaction involving two developmental switchpoints is the most plausible mechanism underlying male trimorphism in this species. The morphs are associated with three mating tactics: alphas rely on resource defence, gammas on sneak copulations and betas likely adopt a flexible 'jack-of-all-trades' tactic. Future work should test whether exaggerated traits in hemimetabolous species favour male trimorphism and whether intermediate males show greater behavioural flexibility. (c) 2026 The Author(s). Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/ by/4.0/).

Four new species of Ibaloniinae harvestmen are described from the Solomon Islands, Melanesia. Based on the original diagnosis of Euibalonius Roewer, 1915, and description of E. maculatus (Roewer, 1915), this material appeared related to that genus. However, examination of syntype photographs of the type species E. maculatus confirmed the validity of Euibalonius, here revalidated from the synonymy of Ibalonius Karsch, 1880, while simultaneously revealing that the Solomon Islands specimens differ substantially from E. maculatus and cannot be accommodated in that genus. The new genus Kakamora gen. nov. is therefore described for the Solomon Islands material. A revised diagnosis of Euibalonius is provided, and a key to the known species of the genus Kakamora is presented. All newly described species bear a cheliceral comb, a structure only recently documented in Podoctidae, and which we also record here for two additional Ibaloniinae species, suggesting broader occurrence within the subfamily. In one of the new species, males were found carrying eggs firmly attached to their femora and tibiae, a behaviour here interpreted as paternal care. This represents a further case of egg carrying by male podoctids, offering a counterpoint to previous doubts about the meaning of this behaviour in the family.

Male weaponry evolution is often linked to male-male competition for resources or access to females, a pattern observed in well-studied taxa, such as beetles and mammals. Whether factors such as breeding site type influence weaponry evolution remains an open question. We explored this question using frogs of the subfamily Leptodactylinae, where males of species that spawn in exposed sites (water bodies) fight to hold oviposition sites or dislodge rivals during amplexus. Conversely, males of species that spawn in concealed sites (ground nests and crevices) rarely engage in physical contests. We hypothesized that male weaponry evolution would be associated with reproduction in exposed sites. Using two complementary phylogenetic comparative methods, we first found that both gains and losses of weapons occurred more frequently in species spawning in exposed sites. This unexpected finding suggests that the dynamics of weaponry evolution are more intricate than anticipated. Second, we found a macroevolutionary correlation between male weapons and breeding site type: the presence of weapons is associated with exposed sites, while their absence is associated with concealed sites. We explore how additional factors, such as population density, the energetic costs of producing and maintaining weapons, and female choice, may also influence this macroevolutionary pattern. Finally, we hope our findings stimulate further investigations into weaponry evolution in other clades of external fertilizers.

Females of many species that exhibit exclusive paternal care prefer parental over non-parental males. According to the essential male care model, once males fulfill the minimum care requirements for offspring survival, they should allocate their surplus energy to advertise their good body condition and increase their attractiveness. To test this prediction, we conducted a field experiment using the harvestman Iporangaia pustulosa, in which males provide exclusive parental care. Employing a full factorial design, we manipulated the body condition of parental males (well-fed and poorly fed) and the status of their sexually dimorphic glands (blocked and unblocked), which likely release sexual pheromones. Then, we recorded males’ probability of receiving new eggs and estimated their mean fitness. Parental males in good condition with unblocked glands had a higher probability of receiving new eggs compared to those in poor condition with blocked glands. Males in the other two experimental groups exhibited intermediate values. Parental males with unblocked glands had a mean fitness 1.38 times higher when in good condition. Additionally, parental males in good condition had a mean fitness 3.48 times higher when the glands were unblocked. These results suggest that the production of sexual pheromones in harvestmen is condition-dependent and emphasize the significance of sexual advertisement as an indicator of male quality. Overall, our findings support the predictive power of the essential male care model and shed light on the interplay between body condition, sexual advertisement, and mating success in species exhibiting exclusive paternal care. Why do females of species with male-only care prefer parental males? The good parent hypothesis predicts that if males vary in caregiving quality, females gain direct benefits by selecting the best caregivers. The essential male care model posits that if all males fulfill minimum care requirements, females gain indirect benefits by selecting mates that allocate surplus energy to advertise their good condition. A previous study with the harvestman Iporangaia pustulosa found that while females prefer males in good condition, their caregiving quality does not differ from that of males in poor condition, which refutes the good parent hypothesis. Here we demonstrate that female preference aligns with the essential male care model, as male fitness was influenced by both body condition and capacity to emit sexual advertisements. This study is the first examination of the essential male care model in invertebrates.

Sexual selection can drive divergence in male genital morphology between species, but its role in within-species divergence remains underexplored. Male-dimorphic species offer an opportunity to investigate this, as different morphs often employ distinct reproductive tactics and face different challenges in sperm competition and female stimulation during copulation. We tested the hypothesis that sexual selection promotes within-species genital divergence using the harvestman Poecilaemula lavarrei as a model. This arachnid has two male morphs: larger, territory-holding majors, and smaller, sneaker minors. We first described copulatory interactions, finding no differences in female acceptance or copulation duration between morphs. Morphological comparisons showed that major males have longer pars basalis and ventral plate area, although no differences in ventral plate width, shape, or relative area of the mat of microsetae were detected. Allometric analyses revealed similar hypoallometric patterns for pars basalis length and ventral plate width in both morphs. However, ventral plate length was isometric in minors and hypoallometric in majors, which is the first report of allometric differences in genital traits between male morphs of a species. Our results suggest that despite differing reproductive tactics, the form and intensity of sexual selection on genital morphology is similar between morphs, potentially constrained by stabilizing selection.

Biodiversity loss is a process that has accelerated in recent decades. In this scenario, the loss of habitat-forming species is of great concern since they facilitate the occurrence of other organisms. These habitat-forming organisms facilitate the occurrence of other species by providing new habitats or increasing the complexity of existing ones. Also, they can minimize adverse abiotic conditions and negative interactions and, thus, their presence allows for the development of an abundant and diverse associated fauna, such as amphipod species. Amphipods have an intimate relationship with macroalgae and can use macroalgae as shelter and food. Marine macroalgae have been under strong decline in abundance and diversity, mainly due to climate change. The loss of diversity of host macroalgae means the loss of the facilitation process that these organisms perform, potentially affecting associated amphipods. Thus, the aim of this study was to investigate how the loss of habitat-forming species affects the associated fauna. Specifically, the questions are (1) What is the effect of host macroalgal diversity on the amphipod assemblage? (2) What is the relative importance of richness and identity of host macroalgae for the amphipod assemblage? To answer these questions, a field experiment is conducted to manipulate the richness of host macroalgae (Dichotomaria marginata, Padina gymnospora, Sargassum filipendula) on a subtropical rocky shore (Ubatuba, SP, Brazil). Treatments ranged from monoculture to polycultures, combining additive and substitutive designs. The polyculture treatment harbored higher amphipod abundance and richness than monocultures and had a unique assemblage composition. These results were attributed to the richness, but not to the identity of host macroalgae. This pattern was observed in additive and substitutive experimental designs performed. Moreover, an overyielding of the polyculture relative to that expected from the individual contribution of the monocultures (Dt>0) and based on the yield of the best monoculture (Dmax>0) is observed, indicating a complementarity effect. These results show that more diverse substrates provide a greater variety of resources (e.g. habitat and food) than monospecific substrates, harboring a richer associated fauna. Therefore, the importance of maintaining the diversity of host macroalgae for the associated faunal community should be emphasized.

The family Podoctidae, which includes 131 species predominantly from Tropical Asia and Australasia, faces significant taxonomic challenges due to historically inadequate species descriptions. This study presents a detailed redescription of Bonea sarasinorum Roewer, 1914, the type species of the genus Bonea, based on specimens collected from its type locality in Sulawesi. The male of B. sarasinorum is described for the first time, enabling a reassessment of the genuss systematic placement. Our findings confirm that Bonea belongs to Podoctinae lato sensu (including Erecananinae) rather than Ibaloniinae. However, several morphological traits suggest that Bonea is not a monophyletic group, with B. sarasinorum sharing close affinities with Dongmoa, while other Bonea species may be more closely related to Baramia and Hoplodino. Morphological terminology for the Podoctidae is updated, and the male genitalia of B. sarasinorum are compared to other Podoctinae species, revealing unique traits such as a deeply cleft distal truncus and poorly developed follis. This study highlights the importance of acquiring new material and employing modern imaging techniques, such as microCT, to refine taxonomic understanding of this family.

In species exhibiting egg attendance, parents remain with their eggs, protecting them against harsh abiotic conditions, such as dehydration or drowning, and biotic conditions, such as predation, parasitism and diseases. This form of postoviposition parental care is widely observed in animals, including amphibians. Long -term egg attendance (spanning several days) is common among glass frogs, and removal experiments have demonstrated the critical role of the parent, whether male or female, in increasing egg survival. However, in a few glass frog species, females stay close to their eggs for less than 3 h after oviposition. Previous studies have found that maternal presence reduces dehydration and predation despite the short duration of this attendance behaviour. In the emerald glass frog, Espadarana prosoblepon , females remain close to their eggs for less than 1.5 h after oviposition. Given that the embryonic development period in this species spans an average of 25 days, our main question was whether remaining with the clutch for only 0.15% of this time is suf ficient to increase egg survival. To address this question, we conducted a female removal experiment in semicaptivity and found no evidence that female presence improved egg hydration after oviposition. In a maternal commitment behavioural assay, most females promptly abandoned their clutches when subjected to a gentle disturbance and did not return to them. Lastly, a female removal experiment under field conditions revealed that clutches with and without mothers experienced similar levels of mortality, primarily caused by invertebrate predators. Through a series of experimental assays, we demonstrate that the short -term female presence in E. prosoblepon did not increase egg survival. Therefore, we argue that the postoviposition behaviour observed in this species cannot be considered parental care behaviour. Our findings challenge the assumption that the proximity of parents and their eggs is an unequivocal indicator of parental care. (c) 2024 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Female mate choice is usually based on traits that signal male quality as a sexual partner. According to the “good parent” hypothesis, female mate choice may also consider male quality as a caregiver in species with male-only care. Because parental activities may be costly, males in good condition should be more attractive to females than those in poor condition. We experimentally manipulated the body condition of non-parental and parental males of the egg-tending harvestman Iporangaia pustulosa and then evaluated how it affected their mating success and ability to protect eggs under field conditions. For non-parental males, individuals in good condition had twice the probability of mating than those in poor condition. For parental males, individuals in good condition had two times more chances of mating and acquired five times more eggs than those in poor condition. Surprisingly, males’ body condition had no effect on the efficiency of egg protection. Although our results indicate that the male condition is a sexually selected trait, we found no support for the “good parent hypothesis” given that an increase in body condition does not improve the survival of the offspring under male care. Instead, these findings are congruent with predictions of the “essential male care” model, which suggests that, when the costs of parental care are low (as is the case of egg attendance), most males can provide the minimum necessary care for offspring survival. However, only males in good condition can allocate surplus energy to advertise their overall quality and attract more mates. Studies examining female mate choice based on condition-dependent traits that serve as reliable indicators of male caregiving quality are mostly limited to vertebrates. We present one of the first empirical examples demonstrating that male body condition influences male attractiveness in an arthropod species exhibiting male-only care. Our field-based results show that females prefer males, whether non-parental or parental, in good body condition over those in poor condition. However, we found no evidence that an increase in body condition improves the survival of the offspring under male care. We propose that males in good body condition are more attractive because the relatively low costs of egg attendance allow them to allocate their surplus energy into advertising their overall quality. Our findings are congruent with predictions of the “essential male care” model, which explores resource allocation between paternal care and sexual advertising.

The detection of male dimorphism has seen numerous statistical advances. Packard has recently criticized a widely used method, reanalyzing data from beetles and harvestmen using an alternative method. We disagree with Packard conclusions, probably due to different implicit definitions of male dimorphism. We consider that male dimorphism manifests in a distribution when it is significantly better described by a model with two values of central tendency (bimodality), rather than a model with only one (unimodality). Thus, while Packard suggests sigmoid allometries as alternatives to male dimorphism, we argue that such allometries are manifestations of mechanisms that generate bimodal distributions. Instead of focusing on this dichotomy, we propose an approach to test whether bimodality in a trait simply arises from its allometry by: (1) characterizing the trait static allometry, (2) simulating body size values based on original data parameters, and (3) generating new trait sizes using the static allometries. The percentage of simulations generating equal or greater bimodality than the data represents the likelihood that the bimodality can be explained by the allometry alone. Our method offers a null model linking sigmoid allometries and bimodal distributions, providing a test for mechanisms that accentuate trait bimodality beyond what the trait allometry generates.

Male weaponry evolution is often linked to resource or female defense polygyny. This pattern emerges from highly studied taxa, such as arthropods and mammals. However, whether factors such as breeding site type influence weaponry evolution remains an open question. We explore this question using frogs of the subfamily Leptodactylinae, where males of species spawning in exposed sites (water bodies) fight to hold oviposition sites or dislodge rivals during amplexus. Conversely, males of species spawning in concealed sites (ground nests and crevices) rarely engage in intense physical contests. We hypothesize that male weaponry evolution would be associated with reproduction in exposed sites. Using two complementary phylogenetic comparative methods, we first found a macroevolutionary correlation between male weapons and breeding site type. The presence of weapons is consistently associated with exposed sites, while their absence is linked to concealed sites. We explore how this macroevolutionary pattern can also be influenced by female mate choice. Second, we found that both gains and losses of weapons occurred more frequently in species spawning in exposed sites. This unexpected finding suggests that the dynamics of weapon evolution are more intricate than anticipated. We propose some explanations aspiring to stimulate further investigations with other external fertilizer species. ### Competing Interest Statement The authors have declared no competing interest.

Mesograzers often use macrophytes as both food and habitat and may have strong effects on primary producers. In this context, understanding the factors mediating their interaction with macrophytes, such as host food value, is an important step to predict the impacts of these herbivores on aquatic ecosystems. The amphipod Sunamphitoe pelagica is a mesograzer with distribution restricted to a few brown macroalgal hosts, however it is unclear if food value drives host use by this mesograzer. Herein, we investigated the distribution of S. pelagica and the food value of its hosts. For that, the abundance of S. pelagica on the macroalgae Sargassum filipendula, Padina gymnospora, and Dichotomaria marginata was evaluated seasonally in a subtidal rocky shore at Fortaleza Beach (23°32′S, 45°10′W), state of São Paulo, Brazil. Furthermore, we conducted laboratory experiments to test feeding preference and rate, as well as the performance (e.g. survival, growth, and reproductive potential) of S. pelagica using the same macroalgal hosts used in the field abundance investigation. Overall, the mesograzer was more abundant on the brown macroalgae Sargassum and Padina than on the red macroalga Dichotomaria. In both feeding preference and rate experiments, S. pelagica consumed more Sargassum and Padina than Dichotomaria. Moreover, the consumption of Padina and Sargassum resulted in high performance of this mesograzer, while juveniles raised on Dichotomaria had a low survival, perishing within few days of experiment. These results suggest the host use by S. pelagica is strongly driven by the food value of macroalgal hosts and are in accordance with the narrow host breadth of this mesograzer. Therefore, considering the vulnerability of large macroalgae to climate change and habitat alterations, the restricted host use by S. pelagica to few brown macroalgae may increase its risk of local extinction.