How female elephants compete for patchy food availability

 thehindu.com  10/12/2019 14:24:10 

Nakshatra and her group moved slowly down the grasslands in Kabini. They could see members of Victorias group having a feast. In general, they would have avoided them, but Nandini rushed excitedly forward seeing a fresh green grass patch. She was stopped rather abruptly by someone who was aggressively pushing at her back with a strong trunk. It was Sunetra from Victorias clan. Nandini, feeling Sunetras strength, could only move back and join her group. Sunetra relaxed, having done her bit to retain the lush grass in the patch for their own use.

Recorded by researchers from Bengaluru, this is an example of a contest competition between individuals belonging to different clans of Asian elephant. Female Asian elephants usually move about in clans which are further subdivided into smaller groups of about four or five individuals. There is usually low interaction among individuals from different clans unless there is a competition for food or some other resource

Personal touch

T.N.C. Vidyas group from JawaharlalNehru Centre for Advanced Scientific Research have been studying elephant behaviour and ecology for over a decade. They identify individual elephants by name using markers like ear shape, scars, pigmentation, etc. In this work, the group studied female Asian elephant clans in the grassland areas at Kabini, during the dry seasons of years 2015 and 2016. They videographed and counted three types of dominance (agonistic) interactions: (a) between individuals within a clan, (b) between individuals belonging to different clans and (c) between several members of two different clans.

The team found that between-clan individual-level agonism is more frequent than within-clan agonism. This implies that the costs of between-clan contest are greater than within-clan contest in this small grassland habitat which becomes concentrated with several clans in the dry season, says Hansraj Gautam who is the first author of a preprint on the work, published in the preprint server BiorXiv. By costs, he refers to loss of feeding opportunity and increased stress from agonistic interactions. These results suggest that the competition regime involves strong between-clan contest, which conforms to the grassland habitat being a small resource-rich habitat patch, he explains in an email to The Hindu. The grassland habitat has more than three times the grass biomass than in the adjacent forests.

Group size matters

The researchers found that as the group size increased, the number of within-clan agonistic encounters just scaled up. This is because higher number of females means more competition, says Dr Gautam. Hence, having a larger group also has costs, which are related to within-group competition.

However, a previous study from the same lab found that larger groups tended to win against smaller groups in the grassland habitat. A strong force of between-clan contest might be keeping groups at sizes larger than the levels at which within-group competition would be minimised. This gives a way to test the results of the study: We would like to quantitatively test this prediction in the future by comparing the group size observed in the two habitats, says Dr Gautam.

The study also shows how animal behaviour may be affected by inadvertently providing extra resources, says Dr. T.N.C. Vidya. She refers to the construction of the Beechanahalli dam over the River Kabini in the 1970s, which has led to the formation of the Kabini backwaters and the high grass availability that draws elephants and other herbivores to the area.

This, we find, gives rise to high aggression between elephant clans, she says. Because of this being a high quality large patch amidst forests that have lower grass availability, it actually creates more aggression between clans. It is one of the effects that anthropogenic changes like this can have, she says..

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