Timing and synchrony of ovulation in red deer constrained by short northern summers.

Iteroparous mothers often face a trade-off between further investments in current offspring at the expense of the start of the next reproductive cycle. In the strongly seasonal environments at northern latitudes, large herbivores are typically calving in early summer each year to get a long growth season and to hit peak protein levels of vegetation. Late-born offspring are more likely to die since they are smaller in autumn. Low female condition in autumn due to prolonged investment in current-year offspring may lower her ability to ovulate sufficiently early to get a good start for the calves the following spring. On the basis of autopsies of uteri from 10,073 red deer (Cervus elaphus), we show that ovulation was delayed as well as more synchronous with increasing population density. This suggests that ovulation beyond a certain date incurs some fitness costs. Ovulation occurs progressively earlier with increasing age up to around 13 yr of age, after which ovulation again occurs later. Low ovulation rates in young compared with prime-aged deer were correlated with late ovulation in the fall. Also, yearling groups with a low rate of ovulation (e.g., because of low weight) also ovulated later, and old senescent deer not calving the previous year ovulated less frequently and markedly later than those raising a calf. Our findings suggest, therefore, that mothers unable to ovulate before a certain date fail to do so altogether that year.