Birth seasonality and calf mortality in a large population of Asian elephants

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.

Joint estimation of survival and breeding probability in female dolphins and calves with uncertainty in state assignment

“…However, another process could have biased our estimation of YOY survival: very early offspring mortality. Of first‐year young that do not survive, most seem to die during their first months of life (Mumby et al, for elephants, Henderson, Dawson, Currey, Lusseau, & Schneider, for bottlenose dolphins). In this case, early mortality could have led to the overestimation of YOY survival probability and an underestimation of the proportion of breeding females, as we could have missed some newborns if they died before our surveys.…”

Seasonality of reproduction in Asian elephantsElephas maximusand African elephantsLoxodonta africana: underlying photoperiodic cueing?

“…Most conceptions occurred in the long‐day period during the dry season (Mar , Mumby et al. ). The conception peak was in February, which was the end of the working season, and the breeding period lasted until shortly before the resumption of work in June (Table ).…”

“…) and number of conceptions by month of timber elephants in Myanmar calculated with the mean gestation length (black bars; Mumby et al. , 41% of conceptions took place from Feb to May, N = 2350, average conceptions calculated for each month). Feb‐May = rest time, Jun = start of working period and monsoon (Mar ).…”

A Welfare State for Elephants? A Case Study of Compassionate Stewardship

“…Elephants typically give birth to one calf, less than one percent of births involving twins. However, one and often both calves usually die within weeks or months of birth (Mumby et al 2013a). Intervention here will be needed to ensure a favourable outcome.…”