The effect of group composition on maternal reproductive success in cooperatively breeding golden lion tamarins (Leontopithecus rosalia) and Goeldi’s monkeys (Callimico goeldii) in European zoos.

In cooperatively breeding callitrichids, non-maternal investment in infants is thought to increase maternal fitness. This increase in maternal fitness may occur through an increase in offspring survival or maternal reproductive output and is likely to be affected by the group composition since the levels of investment undertaken by non-maternal group members vary with age and sex. To investigate the effects of group composition on female reproductive success in a captive environment, I used historical records of golden lion tamarins (Leontopithecus rosalia) and Goeldi’s monkeys (Callimico goeldii) in European zoos. In this large, multi-group data set, I found that the number of adult males and non-mother adult females present at birth positively affects offspring survival in golden lion tamarins, depending on the litter size. However, this association might not be caused by helpers’ investment levels as most deaths occurred neonatally, and the number of adult males present did not significantly increase offspring survival measured after 1 day. I found no evidence for an association between group composition and inter-birth interval or litter size in golden lion tamarins. In Goeldi’s monkeys, offspring survival and inter-birth intervals did not vary with the number of adult males or adult females. The number of non-littermate young present in Goeldi’s monkey groups was positively associated with offspring survival and negatively with inter-birth interval, which I suggest is related to local environmental factors, or female condition, rather than the helping behaviours of young individuals.