Capture from the wild has long-term costs on reproductive success in Asian elephants

Abstract

Capturing wild animals is common for conservation, economic or research purposes. Understanding how capture itself affects lifetime fitness measures is often difficult because wild and captive populations live in very different environments and there is a need for long-term life-history data. Here, we show how wild capture influences reproduction in 2685 female Asian elephants (Elephas maximus) used in the timber industry in Myanmar. Wild-caught females demonstrated a consistent reduction in breeding success relative to captive-born females, with significantly lower lifetime reproduction probabilities, lower breeding probabilities at peak reproductive ages and a later age of first reproduction. Furthermore, these negative effects lasted for over a decade, and there was a significant influence on the next generation: wild-caught females had calves with reduced survival to age 5. Our results suggest that wild capture has long-term consequences for reproduction, which is important not only for elephants, but also for other species in captivity.

Keywords: birth origin; confinement; fertility; human contact; long-term stress; wild-born.

Figure 1.

Wild-caught females had a lower probability to reproduce during their lifetime than captive-born females. Box plots indicate the median and interquartile range; violin plots indicate the density of raw (mean) data at different trait values. The mean lifetime reproduction probability averaged for each birth cohort (decade), lifespan (grouped in to 20-year bins) and censoring group (dead or censored) between captive-born (n = 912) and wild-caught (n = 766) females are shown. (Online version in colour.)

Figure 2.

Wild-caught females had reduced age-specific reproduction probabilities compared with (a) captive-born females and (b) females captured at different ages had different age-specific reproduction. Age-specific patterns of reproduction from the best threshold regression model (age groups: 5–12, 13–19, 20–44, 45–64) for all females (1323 captive-born females and 1362 wild-caught females) are shown. (a) Points are the raw mean annual predicted birth rates at each age for all females, with the size of the points denoting the square root of the sample size at each age (range = 11–1323 time-event data points). Lines are the mean predicted values for an extended data set (observed females but extended to span all ages) of the observed females in the 1960 birth cohort, which were most similar to raw mean birth rates. (b) Mean predicted values from an extended data set of observed females in the 1960 birth cohort for captive-born females (grey), and example predicted values from wild-caught females captured at the ages of 5, 13, 20 and 30 (coloured lines).

Figure 3.

Age-specific reproduction for wild-caught females was lowest immediately after capture, and it took approximately 12 years before wild-caught females reached the mean annual reproductive rate of captive-born females (1362 wild-caught females and 1323 captive-born females—dashed line). The mean annual birth rates for wild-caught females with the number of years since capture across all ages are shown. Points are raw mean annual birth rates, with the size indicating the sample size (range = 856–1338). Lines are the mean fitted values from the best-fit model including all females. The dashed line indicates the raw mean annual birth rate for captive-born females across all ages. (Online version in colour.)

Figure 4.

Wild-caught females had a later age at first reproduction than captive-born females. Box plots indicate the median and interquartile range; violin plots indicate the density of raw age at first reproduction data for captive-born (n = 560) and wild-caught females (n = 283). (Online version in colour.)

Figure 5.

Calves of wild-caught mothers had an increased mortality to age 5 compared with the calves of captive-born mothers, the effect being strongest during the first years after the mother's capture from the wild but lasting on average 16 years (n = 10 192 observations, 2423 calves, 1030 mothers). The red points are raw mean mortalities for calves from wild-caught mothers, with the size indicating the sample size (range = 1–226). The red line shows predicted values for calves born to wild-caught mothers as a function of the years since the mother's capture. The grey dot indicates the raw mean mortality of calves from captive-born mothers (n = 5411). The black line shows the predicted value for calves born to captive-born mothers. The predicted values are mean values from birth to age 5. One raw mortality value for calves of wild-caught mothers is not displayed (mortality of 0.28 1 year after capture).