Changes in steroid hormone levels based on a plastic behavioral role in two biparental cichlids, Julidochromis transcriptus and Julidochromis marlieri: A test of the Ancestral Modulation Hypothesis

Cross-sexual transfer describes the situation when one sex takes on the phenotypic values of the other sex, either plastically or over evolutionary time. The underlying regulatory mechanisms of this process have been generally assumed to be related to sex-biased hormonal regulation, but explicit empirical tests have not been conducted. More recently, the Ancestral Modulation Hypothesis (AMH) has been proposed as a framework to understand the hormonal regulation that underlies cross-sexual transfer. We leverage the behavioral changes in two species of biparental cichlid, Julidochromis transcriptus and Julidochromis marlieri, to test hormonal changes when cross-sexual transfer occurs and provide an empirical test of the AMH. One species, J. transcriptus, typically forms male-larger pairs, which is generally considered to be the ancestral condition; yet females take on male behaviors when they are the larger individual in the pair. The other species, J. marlieri, typically forms female-larger pairs, a cross-sexual transfer, but can likewise shift behaviors when paired atypically. We established pairs by giving males and females of both species both larger and smaller partners and measured behavioral and hormonal changes. We found J. transcriptus females increased 11-ketotestorerone (11-KT) when becoming the larger partner and 11-KT and estradiol (E2) correlated with behaviors associated with the larger partner. We did not find any changes in hormone levels based on size or sex in J. marlieri, though behaviors did correlate with testosterone (T). These results provide qualified support for the tested prediction from the AMH framework that cross-sexual transfer can occur by coopting hormonal signaling by adjusting circulating hormones.