Flying fox bats for vigilance while roosting

Keeping a watch around for predators takes up 7% of the much-maligned mammal’s roosting time

August 17, 2023 10:12 am | Updated 10:12 am IST - GUWAHATI

Flying fox and one of its colonies in southern Assam’s Barak Valley.

Flying fox and one of its colonies in southern Assam’s Barak Valley. | Photo Credit: SPECIAL ARRANGEMENT

India’s largest species of bats, named after a canine fabled to be sly, spend 7% of their day-roosting time being environmentally vigilant, a new study has said.

The nectar and fruit-eating flying fox (Pteropus giganteus) is generally considered a vermin as they raid orchards. It had a similar official status under Schedule V of the Wildlife (Protection) Act of 1972 until it was put on the Schedule II list entailing a higher degree of protection.

As a keystone species causing seed dispersals of many plants in tropical systems, the flying fox has fascinated zoologists over the years. This fascination took three scientists to southern Assam’s Barak Valley for 38 days in 2019.

A keystone species is one that has a disproportionately large effect on its natural environment relative to its abundance, impacting many other organisms in an ecosystem and helping to determine the types and numbers of other species in an ecological community.

Flying fox in southern Assam’s Barak Valley.

Flying fox in southern Assam’s Barak Valley.

The trio’s study, focussing on the environmental and social vigilance of one of two subspecies of the Indian flying fox, found space in Behavioral Ecology and Sociobiology, a peer-reviewed international journal.

The scientists are Pratik Das of the Centre for Ecological Sciences in the Bengaluru-based Indian Institute of Science, and W. Surendra Prakash Goyal and Salvador Lyngdoh of Dehradun’s Wildlife Institute of India.

The study describes vigilance behaviour as an individual scanning its surroundings for both competition and predation risk. Vigilance is social if an individual looks directly at another close-by individual approaching or likely to fight while vigilance of the environmental kind is gazing elsewhere, primarily watching for any signs of danger.

“The Indian flying fox roost gregariously and externally in tree canopies. In such conditions, hierarchy and competition for preferred roosting positions may result in the social structuring of animal aggregation. Vigilance is a manifestation of competition in canopy roosting bats, which can vary temporally, and according to the spatial position,” the study said.

Most bats forage at night and spend more than half of their lives roosting during the day in camps or colonies. Being external roosters, the flying fox is exposed to predators and disturbances apart from environmental indicators such as heat and light.

The scientists chose to study three of 21 verified flying fox colonies based on the age of the colony, permanency, and population.

Flying fox in southern Assam’s Barak Valley.

Flying fox in southern Assam’s Barak Valley.

“The flying mammal showed varying levels of vigilance according to roosting architecture. Across all positions, on average, they spent 7% of their roosting time being vigilant. Peripheral bats on a roosting tree showed a significant increase in environmental vigilance compared to the core, thus confirming the edge effect hypothesis,” the study said.

The flying foxes studied spent 82% of their roosting time sleeping but remain alert to their surroundings. Dependence on auditory perception gives the added advantage of not sacrificing sleep entirely, the study said.

The predators found in the area studied included jungle cats, golden jackals, common palm civets (found to use the roosting trees at night), crested serpent eagles, black kites, spotted owlets, collared scops owls, and barn owls.

“The biggest threat to the flying fox is from humans. Hunting for meat and medicine and probable threats like the felling of roost trees have contributed to the dramatic decrease in the population of the species,” Mr. Lyngdoh said.

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