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.

Data were collected from January 2006 through March 2020 via field observations of wild, individually-recognized blue monkeys living in multiple groups in the Kakamega Forest, Kenya. Specific study location: 0°19 N, 34°52 E. The study groups were part of a long-term project that began in 1979. During the 2006-2020 period, each group was monitored on a near daily basis, and the appearance of newborn infants was noted on each observation day. Further details about the data collection are available in the associated publication.

The dataset (Cords_Gometz_2021_Dataset) represents 484 births, one per row. For each birth, the data set includes information on the following variables, listed from left to right.

1. kidDOB: the date of the birth (YYYY-MM-DD);
2. momID: a unique identity code for each mother (note that some mothers have multiple births in this data set)
3. group: the identity (name) of the group in which the birth occurred. A particular mother could give birth in more than one group because some groups fissioned during the study period, and daughter groups received new names;
4. daytimebirth: expresses whether the birth occurred during the daytime (daylight hours), coded 0/1 (no/yes)
5. dies365: expresses whether the infant died within 365 days of its birth, coded 0/1 (no/yes)
6. kidsex: F for female, M for male. Blanks represent missing data (infant not sexed).
7. kidbirthmonth: the month of the birth (1-12 representing January-December)
8. devpeak: binary variable expressing the degree to which the birth month deviated from the population peak of births in January-February; coded 0 if birth month was 0-4 months away from January-December peak (i.e. birth in September through June), or 1 if birth month was 5 months away from peak (births in July-August).
9. Nfems_in_hier: count of the number of adult females in the group, and in the group’s dominance hierarchy, during the year of the birth
10. st_Nfemsinhier: Z-score of previous variable
11. momrank: mother’s dominance rank, expressed on a scale from 0 (lowest) to 1 (highest). Ranks were assigned for each group and year based on the outcome of decided dyadic agonistic encounters among the group’s large juvenile and adult females. These are females that turned 5 years old in the year of the birth, or who had turned 5 years old previously. Ranks were computed via the I&SI method, implemented in Domicalc (Schmid and deVries 2013, Animal Behaviour 86: 1097-1105). Publication provides further details.
12. st_momrank: Z-score of the previous variable
13. momageatbirth: mother’s age at the time of the birth (in years)
14. st_momageatbirth: Z-score of the previous variable

Cells that are blank represent missing values.