Intraspecific facial bite marks in tyrannosaurids provide insight into sexual maturity and evolution of bird-like intersexual display

Intraspecific aggression, or agonism, is a widespread intrasexual selective behavior important to understanding animal behavioral ecology and reproductive systems. Such behavior can be studied either by direct observation or inferred from wound/scar frequency in extant species but is difficult to document in extinct taxa, limiting understanding of its evolution. Among extant archosaurs, crocodylians display extensive intrasexual aggression, whereas birds show extreme visual/vocal intersexual display. The evolutionary origin of this behavioral divergence, and pattern in non-avian dinosaurs, is unknown. Here we document the morphology, frequency, and ontogeny of intraspecific facial bite lesions (324 lesions) in a large sample of tyrannosaurids (202 specimens, 528 elements) to infer patterns of intraspecific aggression in non-avian theropods. Facial scars are consistent in position and orientation across tyrannosaurid species, suggesting bites were inflicted due to repeated/postured behavior. Facial scars are absent in young tyrannosaurids, first appear in immature animals (~50% adult skull length), are present in ~60% of the adult-sized specimens, and show aggressor:victim size isometry. The ontogenetic distribution of bite scars suggests agonistic behavior is associated with the onset of sexual maturity, and scar presence in approximately half the specimens may relate to a sexual pattern. Considered in a phylogenetic context, intraspecific bite marks are consistent and widely distributed in fossil and extant crocodyliforms and non-maniraptoriform theropods, suggesting a potential plesiomorphic behavior in archosaurs. Their absence in maniraptoriform theropods, including birds, may reflect a transition from boney cranial ornamentation and crocodylian-like intrasexual aggression to avian-like intersexual display with the evolution of pennaceous feathers.

Killing conspecific adults in mammals

Mammals kill both conspecific infants and adults. Whereas infanticide has been profusely studied, the killing of non-infants (adulticide) has seldom attracted the attention of researchers. Mammals kill conspecific adults by at least four, non-exclusive reasons: during intrasexual aggression for mating opportunities, to defend valuable resources, to protect their progeny and to prey upon conspecifics. In this study, we test which reason is most likely to explain male and female adulticide in mammals. For this, we recorded the presence of adulticide, the ecological and behavioural traits, and the phylogenetic relationship for more than 1000 species. Adulticide has been recorded in over 350 species from the most important Mammalian clades. Male adulticide was phylogenetically correlated with the presence of size dimorphism and intrasexually selected weapons. Female adulticide was phylogenetically associated with the occurrence of infanticide. These results indicate that the evolutionary pathways underlying the evolution of adulticide differ between sexes in mammals. Whereas males commit adulticide to increase breeding opportunities and to compete with other males for mating, females commit adulticide mainly to defend offspring from infanticidal conspecifics.

Sexual Dimorphism in Aggression: Sex-Specific Fighting Strategies Across Species

Aggressive behavior is thought to have evolved as a strategy for gaining access to resources such as territory, food, and potential mates. Across species, secondary sexual characteristics such as competitive aggression and territoriality are considered male-specific behaviors. However, although female–female aggression is often a behavior that is displayed almost exclusively to protect the offspring, multiple examples of female–female competitive aggression have been reported in both invertebrate and vertebrate species. Moreover, cases of intersexual aggression have been observed in a variety of species. Genetically tractable model systems such as mice, zebrafish, and fruit flies have proven extremely valuable for studying the underlying neuronal circuitry and the genetic architecture of aggressive behavior under laboratory conditions. However, most studies lack ethological or ecological perspectives and the behavioral patterns available are limited. The goal of this review is to discuss each of these forms of aggression, male intrasexual aggression, intersexual aggression and female intrasexual aggression in the context of the most common genetic animal models and discuss examples of these behaviors in other species.

Women’s Strategic Defenses Against Same-Sex Aggression: Evidence From Sartorial Behavior

Women’s intrasexual competition has received significant attention only in the last decades, with even less work investigating women’s defenses against such aggression. Yet, we should expect that women can (a) grasp which perceptually-salient cues evoke same-sex aggression and (b) strategically damp the display of (some of) those cues when aggression risk is greatest, thereby avoiding the potentially high costs of victimization. Women selectively aggress against women displaying cues of sexual permissiveness (e.g., revealing dress) and/or desirability (e.g., physical attractiveness). We find that (a) women (and men) anticipate greater intrasexual aggression toward women dressed revealingly versus modestly, especially if targets are attractive. Employing behavioral and self-report measures, we also find (b) women create outfits baring less skin, select more modest clothing, and intend to dress less revealingly to encounter other women, flexibly damping permissiveness cues depending on individual features (physical attractiveness) and situational features (being a newcomer) that amplify aggression risk.