For Asian elephant calves, big sisters are better than big brothers

Elephants form close-knit matrilineal family groups traditionally contain genetically related siblings. Interactions between siblings in these groups, as in most mammalian families, may come with costs and benefits. The costs usually relate to competition between siblings for limited parental resources, while the benefits include sharing food, increased protection, and enhanced growth and survival rates through the assistance of sibling helpers. 

In elephant families, sibling relationships are complex and have not been studied intensively, partly due to the fact that elephants live for many decades and are not easy to follow in their natural surroundings. In a study published in the Journal of Animal Ecology, researchers from Finland, the UK and Myanmar report that Asian elephant calves in semi-captive conditions benefit from having older siblings, especially when those siblings are female. 

The study was based on data collected from a herd of government-owned timber elephants in Myanmar. These elephants work during the day as riding, transport and draft animals but are left unsupervised in the forests during the night. There they select their own forage and can interact and mate “Because the elephants live in their natural habitats, there are many similarities to wild elephants, such as natural foraging and no assistance in breeding,” said study co-author Dr. Mirkka Lahdenperä researcher at the University of Turku. “While there are differences – in the wild, family groups are probably bigger – there are more similarities than differences and we could assume that some of the associations found in our study would also hold true for wild elephants. But of course, these should be studied.”

The elephants belong to the Myanmar Timber Enterprise, which keeps detailed records of all aspects of their life histories. The records include data on the reproduction, body mass and longevity of 2,344 Asian elephant calves born between 1945 and 2018. This information was used by the researchers to assess the impacts that older siblings of both sexes have on the body mass, reproductive rate and survival of subsequent calves. 

The findings confirm that, compared with having no older sibling, the presence of an older sibling of either sex increases a calf’s long-term survival rate. In addition, sibling effects were observed for calf body mass (which was higher) and for female calf age at first reproduction (which was earlier) in calves with older siblings. Interestingly, older sisters had a more pronounced effect on calf life history than older brothers. 

Female elephants raised with older sisters survived for longer and reproduced for the first time an average of two years earlier, than did those who were raised with older brothers. Male elephants raised with older sisters, however, had higher body weights but lower survival rates than those raised with older brothers. The authors explain this unexpected effect as the result of a ‘live-fast, die young’ strategy that proposes that young male calves with increased body mass may be involved in more conflict later in life and that this may come with survival costs.

“Our research confirms that sibling relationships shape individual lives, particularly in social species, such as the elephants, where cooperative behaviors are essential to the development, survival and reproductive potential of individuals,” explained study lead author Dr. Vérane Berger.

Since the study made use of correlational analysis, the influence of other, external factors, such as the quality of maternal care, maternal body condition and elephant workload, cannot be evaluated. The researchers hope to collect more data in future so that sibling effects on calf life history can be separated from maternal effects. 

“By collecting more information on the body mass of mothers at birth, we hope to disentangle maternal effects from sibling effects,” said Dr Berger. “More data will also let us explore the effects of the environment on sibling relationships and go into more detail on the effects siblings have on specific aspects of a younger calf’s health, such as immunity, muscular function and hormonal variations.”

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By Alison Bosman, Earth.com Staff Writer