Influence of perineal swellings on behavior and stress reaction in levonorgestrel implanted Macaca sylvanus females.

Objectives: Contraceptives are common tools in controlling the reproductive physiology in primates. Levonorgestrel, a synthetic gestagen, can usually enable contraception and does not suppress the extension of perineal swellings.

Setting: This study investigated the effects on intra-sexual behavior and fecal cortisol excretion rates in different swollen Barbary macaque females (N =24) during the non-reproductive phase of the year. The study was conducted on a semi-free population at Affenberg Salem (Germany).

Results: The results showed that females with reduced swellings received more aggressive encounters than females with enlarged perinea; whereas, at the same time the latter individuals have increased their active grooming interactions paralleled with decreased fecal cortisol excretion rates. No correlations were found between the perineal swelling size and the dominance rank, age, or implantation time of the females.

Main Findings: This report shows that the rate of perineal swelling expression can modulate intra-sexual behavior and stress reactivity in levonorgestrel implanted Barbary macaque females.

Conclusion: We conclude that large perineal swellings represent an influencing key factor in Barbary macaque females' social interactions, irrespective of whether these signals are shown naturally or induced biochemically.