Is being with girls stressful? Social environment influences hormone levels in male giraffes

Abstract

Giraffes (Giraffa camelopardalis giraffa) live in a fission-fusion social system, characterized by the splitting and reunion of subgroups within a larger social network. Adult bulls show a roaming tactic to search for fertile females, whereas younger bulls are often seen in all male groups. In this study we combined behavioural observations with hormonal data. Therefore, a giraffe population of about 80 individuals (11 adult males, 28 adult females, and 32 juvenile / sub-adult animals) was monitored six days a week from dawn to dusk for a period of 12 months (Nov 2014–Oct 2015). Giraffes were individually identified by their unique pelage pattern and adult bulls assigned to age classes based on their appearance (A, B, and C), with class A bulls being the oldest and tallest. A total of 790 faecal samples were collected and analysed for faecal androgen (fAM) and glucocorticoid metabolite (fGCM) concentrations. Class A bulls show significantly higher fAM levels compared to younger bulls, with lowest fAM levels found for the juvenile / sub-adult group. Longer periods of increased fAM levels in the oldest bulls mainly occur during summer and are associated with an observed birthing peak. Within class A, fAM levels of bulls in all male groups are significantly lower than when associated with females, however, these levels are still significantly higher compared to respective fAM levels from youngest males. No significant differences in overall fGCM levels could be found between the different age classes. However, within class A the bulls show a trend for higher fGCM levels when sexually active compared to when associated with an all-male group. In contrast, juvenile / sub-adult bulls show highest fGM levels when they are in all-male groups, which is possibly linked to intrasexual competition as younger bulls still need to establish their position within the hierarchy.