A tale of two hierarchies: Hormonal and behavioral factors underlying sex differences in social dominance in cooperative breeding callitrichids

Fecal steroids of breeding and non-breeding free-ranging black-tufted marmoset females

The relationships between members of the groups include behaviors related to affiliation, dispute for dominant positions, parental care, and facing disputes for food and territory. All these activities are under hormone modulation and those of a steroidal nature are heavily involved. Despite this, only few data are available on steroid hormones in free-ranging marmosets of the Callithrix genus, which limits the understanding of the physiological functioning and modulation of the socio-sexual behavior by steroid hormones of this taxon. In this study, we characterized fecal concentrations of progesterone, estrogens, and glucocorticoids of six breeding and non-breeding females from two groups of free-ranging Callithrix penicillata (É. Geoffroy, 1812). The concentration of progesterone was significantly higher in females which gave birth, compared to non-breeding females. The levels of fecal estrogens and glucocorticoids did not differ between breeding and non-breeding females. The data are in agreement with the few studies on steroid values of wild and captive marmosets. This study shows the concentrations of progesterone and glucocorticoids in free-ranging C. penicillata for the first time, and it is the only study reporting the concentration of fecal estrogens in wild marmosets. Overall, the high levels of progesterone associated with pregnancy in free-ranging C. penicillata as well as levels of estrogens and glucocorticoids close to those reported for other species, suggest a conserved pattern of hormonal secretion between Callithrix species that have been studied in captivity.

The best of both worlds: no apparent trade-off between immunity and reproduction in two group-living African mole-rat species

Co-operatively breeding mammals often exhibit a female reproductive skew and suppression of the subordinate non-breeding group members. According to evolutionary theory and the immunity-fertility axis, an inverse relationship between reproductive investment and survival (through immunocompetence) is expected. As such, this study investigated if a trade-off between immunocompetence and reproduction arises in two co-operatively breeding African mole-rat species, namely the Damaraland mole-rat (Fukomys damarensis) and common mole-rat (Cryptomys hottentotus hottentotus), which possess female reproductive division of labour. This study also attempted to investigate the relationship between the immune and endocrine systems in Damaraland mole-rats. There was no trade-off between reproduction and immunocompetence in co-operatively breeding African mole-rat species, and in the case of the Damaraland mole-rats, breeding females (BFs) possessed increased immunocompetence compared with non-breeding females (NBFs). Furthermore, the increased levels of progesterone possessed by Damaraland mole-rat BFs compared with NBFs appear to be correlated to increased immunocompetence. In comparison, BF and NBF common mole-rats possess similar immunocompetence. The species-specific differences in the immunity-fertility axis may be due to variations in the strengths of reproductive suppression in each species. This article is part of the theme issue 'Evolutionary ecology of inequality'.