December 1, 2021
A study of elephants in Myanmar in Southeast Asia has found that having older siblings increases calves’ long-term survival. And the young animals seemed to benefit more from having older sisters than older brothers. The results were published in the Journal of Animal Ecology.
“Sibling relationships in animals have traditionally been investigated in the context of negative effects looking at competition effects—for instance in wolves, or in humans,” study co-first author Sophie Reichert of the University of Turku in Finland recently told Treehugger.
“However,” she says, “sibling interaction can also result in beneficial effects, through cooperation effects (to share food or provide protection). For instance, in highly social and cooperative breeders, such cooperative behaviors from helpers—which are often offspring born in previous years—have positive effects on juveniles growth, reproduction, and survival.”
The researchers were fascinated by the relationships between, and the impact of older and younger siblings for several reasons.
“We were particularly interested to study these siblings’ effects in Asian elephants, because associations between siblings may be particularly complex in social species with high cognitive capabilities, but have been little-studied to-date,” Reichert says.
“During one field trip to Myanmar, we noticed how youngs were interacting, which gave us the idea to use our long-term demographic database to investigate sibling costs and benefits on the life trajectories of younger offspring.”
However, it is difficult for researchers to study the long-term effects of having siblings in animals that live long lives. There are challenges to conducting field studies that follow animals for their entire lives.
Researchers overcame that obstacle in this study by following a semi-captive group of Asian elephants in Myanmar. The animals are owned by the government and have thorough history records.
The elephants are used during the daytime for riding, transportation, and as draft animals. At night, they roam in the forest and can interact with wild and tame elephants. Calves are raised by their mothers until they are about fivw years old, when they are trained to work. A government agency regulates the daily and annual workload of elephants.1
Because the elephants spend so much time in their natural habitats with natural foraging and mating behaviors, there are many similarities to wild elephants, the researchers say.
For the study, researchers analyzed data on 2,344 calves born between 1945 and 2018. They looked for the presence of older siblings and studied the effect on the animals’ body mass, reproduction, sex, and the survival of the next calf.
They found that, for female elephants, those raised with older sisters had better long-term survival rates and reproduced about two years earlier on average, compared to elephants with older brothers. Generally, elephants that reproduce earlier have more offspring during their lifetimes.
They found that male elephants that were raised with older sisters had lower survival rates but higher body weight, compared to elephants that had older brothers. The positive increase early on in body weight could end up costing the elephants in survival later on in life.1
“Older siblings are pivotal for the lives of the subsequent calves. Their effects depend on their sex, their presence during weaning and the sex of the subsequent calf,” study co-first author Vérane Berger of the University of Turku tells Treehugger. “We showed that elder sisters improved females’ survival and are associated with earlier age at first reproduction. Moreover, the presence of elder sisters increased males’ body mass.”
The researchers expected some of the outcomes but were surprised by others.
“As expected, we showed that elder sisters have a beneficial effect on the subsequent calf and especially in females,” Berger says. “While, we expected a negative effect of elder brothers, we actually did not detect it.”
The findings are key because it shows that it’s important to include the effects of having siblings when analyzing survival, body, condition, or reproduction, the researchers point out.4
Berger adds, “Our results also show in elephants that family members should be kept together which could be interesting in the field of zoo conservation.”
Research contact: @Treehugger