Species that cross strong environmental gradients are expected to face divergent selective pressures that can act on sexually-selected traits. In the present study, we examine the role of hypoxia and carotenoid availability in driving divergence in two sexually-selected traits, male colour and reproductive behaviour, in the African cichlid Pseudocrenilabrus multicolor victoriae. Low-dissolved oxygen (DO) (hypoxic) environments are expected to be energetically challenging; given that male nuptial colour expression and courtship displays can be costly, we expected fish in low-DO versus high-DO environments to differ in these traits. First, a field survey was used to describe natural variation in male nuptial colour patterns and diet across habitats divergent in DO. Next, using wild-caught fish from a low-DO and high-DO habitat, we tested for differences in reproductive behaviour. Finally, a laboratory rearing experiment was used to quantify the interaction of DO and diet (low-versus high-carotenoid availability) on the expression of male colour during development. In energetically challenging low-DO environments, fish were more red and, in high-DO environments, fish were typically brighter and more yellow. The frequency of reproductive displays in fish of low-DO origin was 75% lower, although this had no consequence for brooding frequency (i.e. both populations produced the same number of broods on average). Our laboratory rearing study showed carotenoid availability to be important in colour production with no direct influence of DO on colour. Additionally, weak patterns of diet variation across wild populations suggest that other factors in combination with diet are contributing to colour divergence.

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The early social environment an animal experiences may have pervasive effects on its behaviour. The social decision‐making network (SDMN), consisting of interconnected brain nuclei from the forebrain and midbrain, is involved in the regulation of behaviours during social interactions. In species with advanced sociality such as cooperative breeders, offspring are exposed to a large number and a great diversity of social interactions every day of their early life. This diverse social environment may have life‐long consequences on the development of several neurophysiological systems within the SDMN, although these effects are largely unknown. We studied these life‐long effects in a cooperatively breeding fish, Neolamprologus pulcher, focusing on the expression of genes involved in the monoaminergic and stress response systems in the SDMN. N. pulcher fry were raised until an age of 2 months either with their parents, subordinate helpers and same‐clutch siblings (+F), or with same‐clutch siblings only (−F). Analysis of the expression of glucocorticoid receptor, mineralocorticoid receptor, corticotropin releasing factor, dopamine receptors 1 and 2, serotonin transporter and DNA methyltransferase 1 genes showed that early social experiences altered the neurogenomic profile of the preoptic area. Moreover, the dopamine receptor 1 gene was up‐regulated in the preoptic area of −F fish compared to +F fish. −F fish also showed up‐regulation of GR1 expression in the dorsal medial telencephalon (functional equivalent to the basolateral amygdala), and in the dorsolateral telencephalon (functional equivalent to the hippocampus). Our results suggest that early social environment has life‐long effects on the development of several neurophysiological systems within the SDMN.

Prolactin is often referred to as the “parental hormone” but there are examples in which prolactin and parental behavior are disconnected. One intriguing example is in avian obligate brood parasites; species exhibiting high circulating prolactin but no parental care. To understand this disconnect, we examined transcriptional and behavioral responses to prolactin in brown‐headed (Molothrus ater) and bronzed (M aeneus) brood parasitic cowbirds. We first examine prolactin‐dependent regulation of transcriptome wide gene expression in the preoptic area (POA), a brain region associated with parental care across vertebrates. We next examined prolactin‐dependent abundance of seven parental care‐related candidate genes in hypothalamic regions that are prolactin‐responsive in other avian species. We found no evidence of prolactin sensitivity in cowbirds in either case. To understand this prolactin insensitivity, we compared prolactin receptor transcript abundance between parasitic and nonparasitic species and between prolactin treated and untreated cowbirds. We observed significantly lower prolactin receptor transcript abundance in brown‐headed but not bronzed cowbird POA compared with a nonparasite and no prolactin‐dependent changes in either parasitic species. Finally, estrogen‐primed female brown‐headed cowbirds with or without prolactin treatment exhibited significantly greater avoidance of nestling begging stimuli compared with untreated birds. Taken together, our results suggest that modified prolactin receptor distributions in the POA and surrounding hypothalamic regions disconnect prolactin from parental care in brood parasitic cowbirds.

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