Print PageLife history theory predicts a tradeoff between current reproductive effort and future survival and reproduction. In ungulates, few studies have detected strong reproductive costs in terms of future reproduction, and fewer still have observed survival costs. However, reproductive trade-offs have rarely (if ever) been studied in a tropical migratory ungulate species, largely due to the scarcity of long-term, individually marked tropical populations. Because tropical ungulates maintain relatively low body fat levels, we suggest that they may be less buffered to reproductive costs than temperate species. Using computer-assisted photographic capture-recapture method, I examine four hypotheses related to reproductive costs in adult female wildebeest in the Tarangire Ecosystem, Northern Tanzania: 1) that reproduction in any given year lowers the probability of reproducing the following year, 2) that reproduction incurs survival costs, 3) that survival costs are largely incurred during the dry season (June-October) when food is limited and calves are still nursing, and 4) that survival of breeding females varies with wet season rainfall amount. I develop a robust-design multi-state capture-recapture model that allows for misclassification of breeding state. This framework provides a robust statistical approach for overcoming biases caused by uncertainty in breeding states of females, a common problem in analyses of reproduction-survival trade-offs.
Results/Conclusions
We identified 3130 females over three breeding cycles (three dry seasons and two wet seasons), yielding 395 observed recaptures. Despite abnormally low wet season rainfall totals during both years (coinciding with the months when lactation demands are highest for breeders), we found no evidence that breeding in one year reduces the probability of breeding the following year. Overall, breeders had slightly higher apparent survival than non-breeders (0.624±0.040 vs. 0.548±0.053, respectively). However, the best model was time-varying across all sampling intervals, and breeders had lower survival than non-breeder in two of the four intervals (one dry season and one wet season interval; all intervals standardized to a 7-month length), indicating that potential reproductive trade-offs in survival are inconsistent over time. The probability of an adult female being a breeder during the wet season (roughly two months post-birth pulse) ranged from 0.47-0.83. The lack of both strong future reproductive and survival costs of reproduction is consistent with most previous studies of Northern temperate ungulates. We discuss our results in light of recent population declines in the Tarangire-Manyara wildebeest, as well as future avenues for comparison between temperate and tropical ungulates.
