Why do captive pied tamarins give birth during the day?

Diurnal primates typically give birth at night, when it is presumed that they are safer at a very vulnerable time, and this is reflected in an overwhelmingly nocturnal pattern of delivery in most species of Callitrichidae. However, over half (51.1%) of 88 births to pied tamarins (Saguinus bicolor) at Durrell Wildlife Park occurred during the day (0800-1700), almost always in the afternoon. Nearly three quarters of breeding females (17/23) had at least one diurnal birth, including females from all generations in captivity from wild-caught to fifth captive-born generation, and from all six matrilines represented at Durrell. The proportion of diurnal births has remained relatively stable over time despite management changes. We used generalized linear mixed modeling to investigate several factors that we hypothesized could affect time of birth: maternal experience, season, female rearing history, and whether or not the group was on public display. We fitted all possible models to the data, but none explained more than 7.5% of the variation. Daytime delivery had few statistically significant detrimental effects, although infant survival was somewhat lower and parental rejection increased in diurnal births. Pied tamarins do not seem to fit any of the hypotheses previously put forward to explain exceptions to the typical primate circadian pattern of delivery. Zoo Biol. 35:487-494, 2016.