Season of birth affects juvenile survival of giraffe

For some giraffe calves, being born in the dry season brought better odds. In northern Tanzania, calves arrived throughout the year, but those births did not lead to equal chances of reaching their first birthday. The advantage lay with a season that might seem an unlikely choice for raising a young animal.

Derek Lee, Monica Bond and Douglas Bolger followed 449 individually identified calves in the Tarangire ecosystem between 2012 and 2014. Repeated photographs allowed them to recognise animals without capturing them. They compared estimated first-year survival among calves born during the dry season, the short rains and the long rains, accounting for the possibility that a living calf was missed during a survey.

Estimated survival to one year was 60% for dry-season births, compared with 51% for long-rains births; the short-rains estimate lay between them. The study established a seasonal association, not its cause. Mothers' stored energy, nutritious new leaves around the dry season's end and reduced hiding cover for predators were possible explanations. A further calculation found that delaying every birth to the favourable season would cost more births than the measured survival advantage compensated for. A better season for one calf therefore need not mean a better lifetime breeding schedule for its mother.