Data from: The birth hour of mammals: insights from intra-specific variation in wild blue monkeys

While most mammals show birth hour peaks at times of the 24-hour cycle
when they are less active, there are exceptions to this general pattern.
Such exceptions have been little explored, but may clarify evolutionary
reasons for the diel timing of births. We investigated intraspecific
variation in birth hour in wild blue monkeys (Cercopithecus mitis
stuhlmanni), a diurnal primate, to identify factors that differentiated
daytime vs. nighttime births. Behavioral and life history data from 14
groups over 14 years revealed that 4% of 484 births occurred during the
day. Probability of daytime birth varied with mother’s age,
peaking at 15.7 years. Births whose annual timing deviated most from the
population’s peak birth months were five times more likely to occur during
daytime than those that deviated less. There was no evidence that mother’s
rank or infant sex influenced birth hour, and mixed evidence that daytime
births were more probable in larger groups. Survivorship did not differ
significantly for infants born during the day vs. night. Prime-aged
mothers may be able to handle the consequences of an unusual birth hour
more successfully than mothers with less experience or those weakened by
age. Daytime birth may be more advantageous in the off-season because
nights are colder at that time of year. These findings are consistent with
hypotheses relating birth hour to the risk of losing social protection in
group-living animals, but are not consistent with those emphasizing risk
of conspecific harassment. Patterns of within-species variation can help
in evaluating evolutionary hypotheses for non-random birth hour.