Group composition affects seasonal birth timing in captive Japanese macaques

We analyzed birth dates recorded during an 18-year period in a group of Jap anese macaques housed in the Rome zoo to assess the influence of environmen tal physiological and social factors on birth seasonality. Birth timing dif fered significantly among years. Birth timing was affected by reproductive condition of females-ones that had given birth in the previous year deliver ed significantly later than those that had not-but not by their age or domi nance rank. We conducted further analyses separately on females that had or had not given birth in the previous year. In both subgroups of females mea n birth date was not influenced either by environmental temperature and rai nfall during the previous mating season or by group size. On the contrary, among females that had not given birth in the previous year, socionomic sex ratio-ratio of sexually mature males to sexually mature females-is positiv ely correlated with both mean birth date and date of the first birth but no t with date of the last birth. Contrarily, among females that had given bir th in the previous year, there is no significant relationship between these variables. We hypothesize that the effects of socionomic sex ratio on birt h timing might depend on competition among males for access to fertile fema les. When the number of males per female was higher, mutual disruption of c onsort pairs may have led to a delay in the onset of mating.