Borbála Kocsis

Social hierarchies develop through repeated agonistic interactions among group members and can be shaped by both agonistic encounters and non-agonistic relationships, such as patterns of association; in turn, they can influence access to resources, individual fitness, and group stability. While sex and age are well-known determinants of dominance, the role of early-life rearing history remains poorly understood, particularly in long-lived birds. Here, we investigated how sex, age, and rearing method shape dominance structure and its temporal dynamics in African penguins (Spheniscus demersus) housed at the Budapest Zoo, Hungary. We conducted 98 days of behavioural observations during feeding events and quantified aggressive interactions using social network analysis. Dominance was measured with David’s Score (DS), a widely used dominance index, calculated across 14 consecutive 7-day observation windows. We fitted a longitudinal linear mixed model, with individual identity as a random effect to account for repeated observations. Males consistently occupied higher dominance ranks than females, and dominance showed a significant non-linear relationship with age, declining steeply in younger individuals and levelling off in older birds due to a significant positive quadratic effect of age. Rearing history did not predict overall dominance level; however, both mix-reared and hand-reared individuals showed significantly more positive dominance trajectories over time compared to parent-reared conspecifics. Sex did not significantly predict the rate of change in dominance rank. Importantly, we discuss how the positive dominance trajectories of mix- and hand-reared birds may reflect a highly competitive approach to food acquisition in feeding contexts, and we caution against interpreting dominance success during feeding as a general indicator of social integration or colony-wide competence. These findings demonstrate that intrinsic traits and developmental history jointly structure social hierarchies in a threatened seabird species, and underscore the need to incorporate early-life history into the management and conservation of captive populations.

The adult sex ratio (ASR; the proportion of males in the adult population) is a key demographic trait that varies widely across species and is linked to mating systems and sexually selected traits. Previous research on wild tetrapods showed that ASR is associated with genotypic sex determination system: species with male-heterogamety (XY sex chromosome system) tend to have more female-biased ASRs than species with female-heterogamety (ZW system). If genotypic mechanisms of sex determination are involved, these may also shape ASRs in controlled environments. Here, we test the latter proposition using demographic data from 1,192 zoo-housed reptile, bird, and mammal species. Across amniotes, ASRs in captive populations follow patterns similar to those observed in the wild: XY species tend to have more female-biased ASRs than ZW species. Captive mammals and birds showed similar ASR biases as found in wild populations, although in reptiles, the clade with diverse sex determination systems, the ASR did not differ significantly between XY and ZW species. Instead, reptiles with temperature-dependent sex determination (TSD) differed from reptiles of both genotypic sex determination groups. Together these results support that non-ecological processes linked to sex determination systems contribute to ASR patterns both in captive and wild animals.