Physiological suppression eases in Damaraland mole-rat societies when ecological constraints on dispersal are relaxed

Abstract

In many vertebrate societies, subordinate females exhibit down-regulated reproductive physiologies relative to those of dominants, a condition commonly termed physiological suppression. Research into the causes of physiological suppression has focused principally on the role of the subordinate's social environment (typically the presence of the dominant female and/or an absence of unrelated males within the group), while few studies have considered the additional role that the physical environment may play. Here we present new evidence from wild Damaraland mole-rats, Fukomys damarensis, revealing that physiological suppression among subordinate females eases markedly during the annual rains (a time when ecological constraints on dispersal are relaxed), despite the continued presence of the dominant female and in groups that contain no new immigrant males. Subordinate females showed substantially higher pituitary sensitivities to GnRH challenge during the wet period than the dry, a contrast that cannot be attributed to between-female differences (as it holds for paired within-female comparisons), associated changes in body mass (as our analyses control for this), or concomitant reductions in physiological stress (as their urinary cortisol concentrations were actually higher in the wet period). We suggest that our findings reflect selection for the maintenance of reproductive readiness among subordinate females during high rainfall periods, given the increased likelihood of encountering dispersal and/or mating opportunities with extra-group males when ecological constraints on dispersal are relaxed. These findings reveal new complexity in the processes that regulate physiological suppression, suggesting a key role in some species for changes in the physical as well as social environment.