The role of size and aggression in intrasexual male competition in a social lizard species, Egernia whitii

A Smaller Habenula is Associated with Increasing Intensity of Sexual Selection

The habenula is a small structure in the brain that acts as a relay station for neural information, helping to modulate behaviour in response to variable and unpredictable stimuli. Broadly, it is evolutionarily conserved in structure and connectivity across vertebrates and is the most prominent bilaterally asymmetric structure in the brain. Nonetheless, comparative evolutionary studies of the habenula are virtually non-existent. Here, we examine the volumes of the medial and lateral habenular subregions, in both hemispheres, across a group of Australian agamid lizards in the genus Ctenophorus. In males, we found bilaterally asymmetrical selection on the lateral habenula to become smaller with increasing intensity of sexual selection, possibly as a mechanism to increase aggressive responses. In females, we found bilaterally symmetrical selection on both the medial and lateral subregions to become smaller with increasing sexual selection. This is consistent with sexual selection increasing motivation to reproduce and the habenula's well-characterized role in controlling and modifying responses to rewarding stimuli. However, as there are currently no studies addressing habenular function in reptiles, it is difficult to draw more precise conclusions. As has happened recently in biomedical neuroscience, it is time for the habenula to receive greater attention in evolutionary neuroscience.

What Determines Paternity in Wild Lizards? A Spatiotemporal Analysis of Behavior and Morphology

Mating behavior in animals can be understood as a sequence of events that begins with individuals encountering one another and ends with the production of offspring. Behavioral descriptions of animal interactions characterize early elements of this sequence, and genetic descriptions use offspring parentage to characterize the final outcome, with behavioral and physiological assessments of mates and mechanisms of copulation and fertilization comprising intermediate steps. However, behavioral and genetic descriptions of mating systems are often inconsistent with one another, complicating expectations for crucial aspects of mating biology, such as the presence of multiple mating. Here, we use behavioral and genetic data from a wild population of the lizard Anolis cristatellus to characterize female multiple mating and the potential for sexual selection through female mate choice in this species. We find that 48% of sampled females bore offspring sired by multiple males. Moreover, spatiotemporal proximity between males and females was associated with whether a male sired a female's offspring, and if yes, how many offspring he sired. Additionally, male body size, but not display behavior, was associated with reproductive outcomes for male-female pairs. While much remains to be learned about the mechanisms of mating and targets of sexual selection in A. cristatellus, it is clear that female multiple mating is a substantial component of this species' mating system in nature.

Asymmetric interspecific competition drives shifts in signalling traits in fan-throated lizards

Interspecific competition can occur when species are unable to distinguish between conspecific and heterospecific mates or competitors when they occur in sympatry. Selection in response to interspecific competition can lead to shifts in signalling traits-a process called agonistic character displacement. In two fan-throated lizard species-Sitana laticeps and Sarada darwini-females are morphologically indistinguishable and male agonistic signalling behaviour is similar. Consequently, in areas where these species overlap, males engage in interspecific aggressive interactions. To test whether interspecific male aggression between Si. laticeps and Sa. darwini results in agonistic character displacement, we quantified species recognition and signalling behaviour using staged encounter assays with both conspecifics and heterospecifics across sympatric and allopatric populations of both species. We found an asymmetric pattern, wherein males of Si. laticeps but not Sa. darwini showed differences in competitor recognition and agonistic signalling traits (morphology and behaviour) in sympatry compared with allopatry. This asymmetric shift in traits is probably due to differences in competitive abilities between species and can minimize competitive interactions in zones of sympatry. Overall, our results support agonistic character displacement, and highlight the role of asymmetric interspecific competition in driving shifts in social signals.

Ain’t going down without a fight: state-and environment-dependence of antipredator defensive aggressive personalities in Carpetan rock lizard

Abstract

Aggression is one of the most frequently studied behavioural traits across a wide range of taxa; however, most studies evaluate aggressive behaviour in a social context, in which aggressive interactions between conspecifics are motivated by resource control (offensive or social aggression). However, in an antipredator context, the primary role of aggression is defence (defensive or antipredator aggression). Although the neuroendocrinology of antipredator aggression is often studied in domesticated and laboratory animals, how environment and individual state affect this behavioural trait in the wild is largely unknown. Here, by conducting a manipulative experiment, we tested whether (i) consistent between-individual differences (i.e. animal personality) are present in antipredator aggression in adult male Carpetan rock lizards (Iberolacerta cyreni) and (ii) short-term environmental changes (presence vs. absence of predator cues) and differences in individual state (body length, head size, hind limb length) affect individual mean behaviour (i.e. behavioural type). We found moderate-high repeatability in antipredator aggression (willingness to bite a human), indicating the presence of animal personality in this behavioural trait. Lizards were on average more defensive in the presence of predator cues; furthermore, short-legged males showed higher antipredator aggression than long-legged males in the presence of predator cues, probably as an attempt to balance their decreased escape speed. Larger (~ older) males were more defensive than smaller ones, probably due to their increased fighting ability. We conclude that antipredator aggression is an important part of an individual’s behavioural repertoire and its expression is driven by both environmental situation and individual state.

Significance statement

Antipredator/defensive aggression is not the primary antipredatory response; however, when other ways of escape are not possible, actually hurting the predator could be the only way of survival. While this behaviour obviously has substantial effects on fitness, it is severely understudied compared to social/offensive aggression. In a manipulative experiment, we found that there are consistent between-individual differences in antipredator aggression (i.e. willingness to bite during handling) of adult male Carpetan rock lizards (Iberolacerta cyreni), supporting the presence of animal personality and suggesting that this behavioural trait might respond to natural selection. Furthermore, short-term environmental variation (i.e. presence vs. absence of predator cues) in interaction with individual state affected antipredator aggression of individuals, emphasising the ecological and evolutionary relevance of this behaviour.

Musculoskeletal mass and shape are correlated with competitive ability in male house mice (Mus musculus)

Intense physical competition between males for mating opportunities is widespread among mammals. In such agonistic encounters, males with combinations of morphological, physiological and behavioral characters that allow them to dominate an opponent have greater fitness. However, the specific physical traits associated with competitive ability are poorly understood. Larger body size is often correlated with fitness in mammals. Interestingly, fitness is maximized at intermediate body masses in male house mice (Mus musculus), a species with a polygynous mating system in which males compete physically for access to reproductive resources. Here, we used competition trials in semi-natural, mixed-sex population enclosures to directly measure competitive ability in male house mice based on control of a preferred nesting site. We tested the hypothesis that the musculoskeletal systems of male mice demonstrating high competitive ability are more specialized for competition by comparing the masses of 10 major muscle groups and eight bones as well as a set of 12 skeletal shape indices associated with anatomical specialization for fighting performance in a set of nine winners and 20 losers. Winning males possessed several traits hypothesized to enhance performance in male-male contests: relatively greater mass in several muscle groups and bones of the forelimb and hindlimb and larger scapular surface area. Unexpectedly, no measurements of the head and neck differed significantly between winners and losers. These results identify musculoskeletal traits associated with competitive ability in male house mice and suggest that our current understanding of mammalian fighting performance is incomplete and more nuanced than previously considered.

Comparative morphology and trophic ecology in a population of the polymorphic lizard Sceloporus minor (Squamata: Phrynosomatidae) from central Mexico

Polymorphism among individuals of the same population has generally been linked to alternative reproductive tactics, where different morphs can exhibit differences in their morphological, ecological, and behavioral attributes. These differences may result in a divergence in diet between morphs due to differential exploitation of habitat, morphological differences that influence prey selection, or differential energy expenditure that results in different nutritional needs. The present study analyzes the morphology (morphometry and body mass) and diet of red and yellow male morphs in a population (El Enzuelado) of the lizard Sceloporus minor from central Mexico. No differences between morphs were found for any of the morphometric variables analyzed (snout-vent length, tail length, jaw length, jaw width, head length, head width, head height, tibia length, femur length, forearm length and ventral patch length). In both morphs, allometric growth was observed in all body features analyzed, as well as in morphometric features of the head across seasons. Analysis of stomach contents showed that the diet of red males was composed of 12 categories of prey, while that of yellow males was composed of 10 categories; those categories of diet not shared between morphs (e.g., Isoptera, Psocoptera) were consumed by their respective morph in very low proportions. Categories of diet with the highest values of food importance for both groups were Coleoptera, Orthoptera, and leaves; a similar pattern was seen across seasons. This, in turn, is reflected in low niche breadth values for each morph and a very high niche overlap. There were no significant differences between morphs overall, or between morphs per season, in the weight and volume of stomach contents or in the number of prey items found in stomachs; however, differences in these variables across all males (independent of morph) were recorded between seasons. Likewise, no significant correlations were found between body size (snout-vent length) and the volume of stomach contents for either morph or between lizard mandibular dimensions and the volume of stomach contents for red morph males. For the yellow morph, prey volume unexpectedly decreased significantly with jaw size rather than increasing as expected. Overall, this study adds new information about the morphology and feeding of males in this species, and suggests that in this population, color morphs lack the morphological and ecological differences found in some other species of polymorphic lizard.

Experimental assessment of social interactions in two species of the genus Teratoscincus (Gekkota)

Social organization of many reptile species that are rare in the wild remains rather unexplored due to difficulties when setting experiments in the field. Behavioral analysis of standard social situations in laboratory conditions is considered an indirect method to reveal social behavior in the field. We studied two rare species of geckos, Teratoscincus scincus and Teratoscincus keyserlingii, inhabiting sand dunes of Uzbekistan and Eastern Iran. A series of experiments was carried out to quantify responses in social interactions among conspecific adults as well as reactions of these towards conspecific/heterospecific subadults and juveniles. We also assessed the effect of species and sex on recorded behavior. Finally, the reaction to threat stimuli simulating predator attack was analyzed. The species effect was recorded only in the response to a simulated predatory attack: T. scincus typically escaped whereas larger T. keyserlingii attacked the stimulus. In accordance with the sexual competition hypothesis, agonistic interactions were nearly exclusively confined to male-male encounters while females were tolerant to each other. Male-female encounters regularly resulted in mating attempts, which suggests that females are not strongly selective in choosing partners. Therefore, male aggression can be linked to mate guarding or territoriality. Adults' lack of interest in immature geckos may indicate generalized tolerance of adults towards young.

Examining the role of testosterone in mediating short-term aggressive responses to social stimuli in a lizard

Hormones have been suggested as a key proximate mechanism that organize and maintain consistent individual differences in behavioural traits such as aggression. The steroid hormone testosterone in particular has an important activational role in mediating short-term aggressive responses to social and environmental stimuli within many vertebrate systems. We conducted two complementary experiments designed to investigate the activational relationship between testosterone and aggression in male Egernia whitii, a social lizard species. First, we investigated whether a conspecific aggressive challenge induced a testosterone response and second, we artificially manipulated testosterone concentrations to examine whether this changed aggression levels. We found that at the mean level, plasma T concentration did not appear to be influenced by an aggression challenge. However, there was a slight indication that receiving a challenge may influence intra-individual consistency of plasma T concentrations, with individuals not receiving an aggression challenge maintaining consistency in their circulating testosterone concentrations, while those individuals that received a challenge did not. Manipulating circulating testosterone concentrations had no influence on either mean-level or individual-level aggression. Combined with our previous work, our study adds increasing evidence that the relationship between testosterone and aggression is not straightforward, and promotes the investigation of alternative hormonal pathways and differences in neuro-synthesis and neuroendocrine pathways to account for species variable testosterone - aggression links.