The Uda Walawe Elephant Research Project is now approaching eight years, a unique study of Asian elephants. Because elephants are such long-lived animals, it takes a long time to understand them – particularly for important variables like who reproduces and how often. Studies of wild African elephants have been conducted at multiple sites over 10 years or more (in some cases as many as 40!), but this has not been the case for the Asian species in the wild. There have been long-term records of Asian elephants populations in captivity from places like Myanmar, where they have long been used in timber camps, and it’s interesting to see how the two compare.
This past December we published a comprehensive paper analyzing six years of data on the wild Asian elephants of Uda Walawe National Park, Sri Lanka. This paper is one of a kind as it has been difficult for researchers to successfully monitor wild Asian elephants due to the difficulty of their habitats, and the logistical challenges of conducting steady research over longer time periods.
In terms of reproduction, we found that the median age at first reproduction was approximately 13 years and the 50th percentile inter-birth interval (IBI, the time between consecutive calves) for females, was 6 years. This means that roughly half the females we learned to recognize had given birth twice within the span of the study. Clearly, even a six years study was quite short relative to the reproductive life of an elephant! Like African elephants, it appears that Asian elephants in this population continue to reproduce until at least the age of 60 – astonishing given that elephants have developmental trajectories that are quite similar to that of humans!
Counting births by time of year and season, the annual spike in births occurs towards the beginning of the dry season, which is also true for captive populations recorded in Myanmar and India – oddly in contrast to the expectation that the best time to have calves would be during the monsoons. Similar to the elephants in timber camps, there was some indication that lack of milk (due to weakness/age of the mother or separation from her) during the first two to three years of life could weaken or stunt calves, perhaps even leading to their deaths. At ages greater than four however, this might not be such a problem. Overall though, the data shows that the population is productive at Uda Walawe National Park, though we could not accurately determine exactly how many animals tended to die each ear. Injury and mortality observations did show however that males were as much as three times more likely than females to get injured or killed through human activity, most likely because males tend to crop raid and engage in more risky behavior.
It is quite remarkable that the elephants continue to persist at such densities in Uda Walawe, given that Sri Lanka contains one of the highest densities of people per area. There is a possibility that the population at Uda Walawe National park is exhibiting a delayed population response to gradual changes in birth and mortality rates (this may be called “demographic inertia” – reflecting past habitat conditions rather than present ones). Only time and continued research can show whether or not this is true. We hope these data will be useful for understanding the needs of elephant populations, and informative for those studying elephants elsewhere in Asia. Such data could be used to predict population trends over longer time scales such as hundreds of years – though one has to be careful, since elephants everywhere probably do not exhibit identical patterns. But such understanding is ultimately required if we want to ensure the survival of elephants and their habitats into the future.
To encourage more individual-based research on Asian ellies, we’ve relased the EARS database for recording IDs. Read more about it here.
The paper is available open-access (free) at:
de Silva S, Webber CE, Weerathunga US, Pushpakumara TV, Weerakoon DK, & Wittemyer G (2013). Demographic variables for wild asian elephants using longitudinal observations. PloS one, 8 (12) PMID: 24376581
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