A Smaller Habenula is Associated with Increasing Intensity of Sexual Selection

Natural parasites in conjunction with behavioral and color traits explain male agonistic behaviors in a lizard

Male competition conforms to a cost-benefit model, because while aggression may increase reproductive prospects, it can also increase the risk of injury. We hypothesize that an additional cost in aggressive males would be an increase in parasite load associated with a high energy investment into sexual competition. Some of these infections, in turn, may downmodulate the level of host aggression via energetic trade-offs. We staged dyadic male contests in the lab to investigate the relationships of multiple parasites with the agonistic behavior of lizard hosts, Sceloporus occidentalis. We also included both color and behavioral traits from opponents in the analyses because (1) color patches of lizards may serve as intraspecific signals used by conspecifics to assess the quality of opponents, and (2) contests between male lizards fit classical models of escalated aggression, where lizards increase aggression displays in response to an opponent's behavior. The results conform to our hypothesis because male lizards displayed more pushups when they had more ticks. Moreover, some parasites may modulate the levels of aggression because lizards infected by hematic coccidians performed fewer pushups. Interestingly, lizards also displayed fewer pushups when both the chroma and size of the opponent's blue patch were greater. The results thus also supported the role of the blue patch of S. occidentalis as a sexual armament, because it contributed to the deterrence of aggression from opponent lizards. We revealed that natural parasitic infections in lizard hosts can contribute to their agonistic behavior. We encourage future studies to account for parasites in behavioral tests with lizards.

Nuclei and tracts in the epithalamus of crocodiles

The epithalamus, an area of the dorsal diencephalon found in all vertebrates, consists of the habenula, the subhabenular nuclei, and associated tracts. The habenula is itself divisible into two parts-a medial and a lateral nucleus differing in their inputs, outputs, and cellular morphology. The medial component is related to the limbic system and serotonergic raphe, while the lateral nucleus is more interconnected with the basal ganglia and midbrain dopamine systems. These findings, which come from experiments mainly done on mammals, serve as a basis for comparison with other vertebrates. However, similar studies in other amniotes, such as reptiles, are few. To fill this gap in knowledge, two species of crocodiles were examined utilizing a variety of histological methods in various planes of section. The following results were obtained. First, the habenula was divided into medial and lateral parts based on its cytoarchitecture. Neurons in the medial habenula were small, were closely packed, and had a limited dendritic arbor characterized by unusual distal dendritic appendages, whereas neurons in the lateral habenula were larger, were more loosely packed, and had longer dendritic processes that were commonly beaded. Second, the stria medullaris, the major input to the habenula, was identified by its immunoreactivity to parvalbumin. Third, the fasciculus retroflexus (habenulointerpeduncular tract), the primary output of the habenula, was visualized by staining with acetylcholinesterase. Fourth, nuclei associated with the habenula, the subhabenular nuclei, have been identified and characterized. These features provide a means to recognize the major nuclei and tracts in the epithalamus in crocodiles and are likely applicable to other reptiles.

Sex differences in the human brain: a roadmap for more careful analysis and interpretation of a biological reality

The presence, magnitude, and significance of sex differences in the human brain are hotly debated topics in the scientific community and popular media. This debate is largely fueled by studies containing strong, opposing conclusions: either little to no evidence exists for sex differences in human neuroanatomy, or there are small-to-moderate differences in the size of certain brain regions that are highly reproducible across cohorts (even after controlling for sex differences in average brain size). Our Commentary uses the specific comparison between two recent large-scale studies that adopt these opposing views-namely the review by Eliot and colleagues (2021) and the direct analysis of ~ 40k brains by Williams and colleagues (2021)-in an effort to clarify this controversy and provide a framework for conducting this research. First, we review observations that motivate research on sex differences in human neuroanatomy, including potential causes (evolutionary, genetic, and environmental) and effects (epidemiological and clinical evidence for sex-biased brain disorders). We also summarize methodological and empirical support for using structural MRI to investigate such patterns. Next, we outline how researchers focused on sex differences can better specify their study design (e.g., how sex was defined, if and how brain size was adjusted for) and results (by e.g., distinguishing sexual dimorphisms from sex differences). We then compare the different approaches available for studying sex differences across a large number of individuals: direct analysis, meta-analysis, and review. We stress that reviews do not account for methodological differences across studies, and that this variation explains many of the apparent inconsistencies reported throughout recent reviews (including the work by Eliot and colleagues). For instance, we show that amygdala volume is consistently reported as male-biased in studies with sufficient sample sizes and appropriate methods for brain size correction. In fact, comparing the results from multiple large direct analyses highlights small, highly reproducible sex differences in the volume of many brain regions (controlling for brain size). Finally, we describe best practices for the presentation and interpretation of these findings. Care in interpretation is important for all domains of science, but especially so for research on sex differences in the human brain, given the existence of broad societal gender-biases and a history of biological data being used justify sexist ideas. As such, we urge researchers to discuss their results from simultaneously scientific and anti-sexist viewpoints.