Fitness benefits of male dominance behaviours depend on the degree of individual inbreeding in a polyandrous lizard

Artificial light at night disrupts male dominance relationships and reproductive success in a model fish species

Environmental light is perceived and anticipated by organisms to synchronize their biological cycles. Therefore, artificial light at night (ALAN) disrupts both diurnal and seasonal biological rhythms. Reproduction is a complex physiological process involving integration of environmental signals by the brain, and release of endocrine signals by the pituitary that regulate gametogenesis and spawning. In addition, males from many species form a dominance hierarchy that, through a combination of aggressive and protective behavior, influences their reproductive success. In this study, we investigated the effect of ALAN and continuous daylight on the behavior and fitness of male fish within a dominance hierarchy using a model fish, the Japanese medaka. In normal light/dark cycles, male medaka establish a hierarchy with the dominant males being more aggressive and remaining closer to the female thus limiting the access of subordinate males to females during spawning. However, determination of the paternity of the progeny revealed that even though subordinate males spend less time with the females, they are, in normal light conditions, equally successful at producing progeny due to an efficient sneaking behavior. Continuous daylight completely inhibited the establishment of male hierarchy, whereas ALAN did not affect it. Nonetheless, when exposed to ALAN, subordinate males fertilize far fewer eggs. Furthermore, we found that when exposed to ALAN, subordinate males produced lower quality sperm than dominant males. Surprisingly, we found no differences in circulating sex steroid levels, pituitary gonadotropin levels, or gonadosomatic index between dominant and subordinate males, neither in control nor ALAN condition. This study is the first to report an effect of ALAN on sperm quality leading to a modification of male fertilization success in any vertebrate. While this work was performed in a model fish species, our results suggest that in urban areas ALAN may impact the genetic diversity of species displaying dominance behavior.

Adaptive significance of affiliative behaviour differs between sexes in a wild reptile population

In recent years, we have begun to appreciate that social behaviours might exhibit repeatable among-individual variation. Such behavioural traits may even covary and have critical evolutionary implications. Importantly, some social behaviours such as aggressiveness have been shown to provide fitness benefits, including higher reproductive success and survival. However, fitness consequences of affiliative behaviours, especially between or among sexes, can be more challenging to establish. Using a longitudinal behavioural dataset (2014-2021) collected on eastern water dragons (Intellagama lesueurii), we investigated whether various aspects of affiliative behaviour (i) were repeatable across years, (ii) covaried with each other at the among-individual level, and (iii) influenced individuals' fitness. In particular, we considered affiliative behaviours towards opposite-sex and same-sex conspecifics separately. We found that social traits were repeatable and covaried with each other similarly for both sexes. More notably, we found that male reproductive success was positively correlated with the number of female associates and the proportion of time spent with females, while females' reproductive success was not correlated with any of the measured social behaviour metrics. Overall, these findings suggest that selection may be acting differently on social behaviour of male and female eastern water dragons.

Selection in the city: Rapid and fine-scale evolution of urban eastern water dragons

Oceanic archipelagos have long been treated as a Petri dish for studies of evolutionary and ecological processes. Like archipelagos, cities exhibit similar patterns and processes, such as the rapid phenotypic divergence of a species between urban and nonurban environments. However, on a local scale, cities can be highly heterogenous, where geographically close populations can experience dramatically different environmental conditions. Nevertheless, we are yet to understand the evolutionary and ecological implications for populations spread across a heterogenous cityscape. To address this, we compared neutral genetic divergence to quantitative trait divergence within three native riparian and four city park populations of an iconic urban adapter, the eastern water dragon. We demonstrated that selection is likely acting to drive divergence of snout-vent length and jaw width across native riparian populations that are geographically isolated and across city park populations that are geographically close yet isolated by urbanization. City park populations as close as 0.9 km exhibited signs of selection-driven divergence to the same extent as native riparian populations isolated by up to 114.5 km. These findings suggest that local adaptation may be occurring over exceptionally small geographic and temporal scales within a single metropolis, demonstrating that city parks can act as archipelagos for the study of rapid evolution.