Abstract
In seasonal environments, many species concentrate their reproduction in the time of year most likely to maximize offspring survival. Asian elephants (Elephas maximus) inhabit regions with seasonal climate, but females can still experience 16-week reproductive cycles throughout the year. Whether female elephants nevertheless concentrate births on periods with maximum offspring survival prospects remains unknown. We investigated the seasonal timing of births, and effects of birth month on short- and long-term mortality of Asian elephants, using a unique demographic data set of 2350 semicaptive, longitudinally monitored logging elephants from Myanmar experiencing seasonal variation in both workload and environmental conditions. Our results show variation in birth rate across the year, with 41% of births occurring between December and March. This corresponds to the cool, dry period and the beginning of the hot season, and to conceptions occurring during the resting, nonlogging period between February and June. Giving birth during the peak December to March period improves offspring survival, as the odds for survival between age 1 and 5 years are 44% higher for individuals born during the high birth rate period than those conceived during working months. Our results suggest that seasonal conditions, most likely maternal workload and/or climate, limit conception rate and calf survival in this population through effects on maternal stress, estrus cycles, or access to mates. This has implications for improving the birth rate and infant survival in captive populations by limiting workload of females of reproductive age. As working populations are currently unsustainable and supplemented through the capture of wild elephants, it is imperative to the conservation of Asian elephants to understand and alleviate the effects of seasonal conditions on vital rates in the working population in order to reduce the pressure for further capture from the wild.
Highlights
Seasonal conditions are known to influence postnatal mortality in mammals (Descamps et al 2008) through direct effects on infant survival (Mumby et al 2013) or indirectly through its effects on maternal body condition and nutrition quality and availability (Pettorelli et al 2006)
Ecology and Evolution published by John Wiley & Sons Ltd
We found clear evidence that female Asian elephants can cycle throughout the year, their reproductive rate fluctuates seasonally
Summary
Seasonal conditions are known to influence postnatal mortality in mammals (Descamps et al 2008) through direct effects on infant survival (Mumby et al 2013) or indirectly through its effects on maternal body condition and nutrition quality and availability (Pettorelli et al 2006). For this reason, in seasonal environments mothers may match their timing of conception and birth to the months or seasons with lower offspring mortality (Stearns 1992).
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