Light may influence morning basking behaviour in captive gharials

A study on captive gharials suggests light could be a stronger cue than temperature for morning basking movements, particularly among young gharials.
Researchers call for studies on wild gharials to understand how light affects basking behaviour.
With a species recovery score of just 19%, habitat protection remains crucial for both wild gharials and captive-reared individuals reintroduced into rivers.

Like all crocodilians, Gangetic gharials regulate their body temperature using external heat sources. They warm their enormous bodies, which can grow to 15 to 20 feet in length, by basking on sandbanks, making it an important daily activity.

Both temperature and light are important environmental cues for basking. However, a recent study on captive Gangetic gharials (Gavialis gangeticus) finds that light is a stronger cue than temperature for their morning basking movements.

The researchers expected natural light to be a reliable indicator for gharials to determine the right time to bask on land, says Megha Khanduri, the study’s lead author and a postdoctoral researcher at the University of Helsinki, Finland.

However, artificial lighting at night and the resulting light pollution can disrupt the relationship between natural light and temperature in the environment. The study finds that if light is the more dominant cue for basking, this disruption could lead gharials to bask at the wrong time, when air temperatures are not warm enough, or choose suboptimal basking sites.

“Such behaviours could be potentially life-threatening as it can result in hypothermia,” says Khanduri. “If hatchlings leave the water earlier than the adults because of differences in sensitivity to light, it could also expose them to terrestrial predators in the early hours.”

Radiant cue for basking

The study was conducted on 382 captive gharials raised from eggs and housed in enclosures at the Deori EcoCentre in Madhya Pradesh. The eggs were collected from several nesting sites within the National Chambal Sanctuary between 2021 and 2023 and the gharials were divided into 12 groups based on age.

Observations were carried out between June 20 and 30, 2023, in three one-hour sessions each in the morning and evening. During each session, researchers counted the number of gharials basking in each group and recorded the air and water surface temperatures within the enclosures using a digital thermometer.

Light may influence morning basking behaviour in captive gharials — Captive gharials basking in the sun. Gharials regulate their body temperature by basking. A recent study on captive Gangetic gharials finds that light is a stronger cue than temperature. Representative image by Rameshng via Wikimedia Commons (CC BY-SA 3.0).

They also measured natural light intensity within the enclosures using an infrared thermometer. Artificial lights were switched off at night. The researchers measured the body temperature of up to 10 gharials per group by recording the temperature of the head region of individuals resting on land and in water.

“We found that the gharials, especially the youngest ones (less than a year old), were more sensitive to light than temperature in the morning. They responded to lower light intensity and would come out of the water much earlier in the morning compared to older gharials when the air was still colder than the water,” says Khanduri. However, she cautions that gharial behaviour and social interactions in captivity may differ from those in the wild.

Captive rearing is an important part of gharial conservation. Khanduri notes that enclosures are often equipped with heaters and basking lamps to regulate temperature. “Our findings, however, suggest that management of light conditions is also important. Individuals need access to direct sunlight, especially when heating devices are not in use.”

While acknowledging the study’s findings, Jailabdeen A, Project Director of the Gharial Ecology Project at the Madras Crocodile Bank Trust, raises methodological concerns. He points out that measuring temperature above the gharial’s head, rather than at its torso, may not provide an accurate estimate of body temperature, as the head heats up faster than the rest of the body.

He also questions the timing and duration of the observations. “Winter is the ideal time to observe basking behaviours in gharials, but the observations were taken in June when there is reduced need for basking as the water temperature remains warm,” he says. He adds that a 10-day observation period is too short to understand how basking patterns change across seasons.

While experts acknowledge the study, some raise methodological concerns; measuring temperature above the gharial’s head, rather than at its torso, may not provide an accurate estimate. Image by Pratiksha Baruah via Wikimedia Commons (CC BY-SA 4.0).

Light pollution, an emerging threat

With a species recovery score of just 19% (0% indicates extinction and 100% full recovery), gharials remain critically depleted as per IUCN indicating that the species survives only in small areas of its historical range and is highly dependent on conservation efforts. Protecting their habitats is a crucial part of ensuring their recovery.

Jailabdeen says anthropogenic disturbances along riverbanks, particularly illegal sand mining, remain a major concern, alongside changes to river dynamics caused by dams and barrages. Light pollution poses another potential threat.

Khanduri and her team investigated changes in artificial lighting at night across India’s major river basins between 2012 and 2020. Although gharials prefer dark habitats, the study found a three-fold increase in radiance along the Chambal and some other river systems that support gharial populations.

“We mostly focused on populations in the National Chambal Sanctuary and Katarniaghat Wildlife Sanctuary, which are relatively dark, protected areas. Therefore, we did not get a lot of information from sites that are really bright, such as the Upper Ganga river and non-protected rivers,” Khanduri notes.

A 2022 study examining changes in riverine and riparian habitats in Hastinapur Wildlife Sanctuary along the Ganga, including habitat suitability for gharials, found that the area under a dark night sky declined from 92% to 67% between 2015 and 2021.

Habitat protection remains crucial

The latest study found no link between light or temperature and gharials’ evening basking movements. Khanduri says further research into their nocturnal ecology is needed to understand how they respond to artificial light and temperature, as has been done for sea turtles, and to inform conservation measures.

Gharials and muggers rest along the Chambal river. While disturbances such as illegal sand mining and construction of dams along riverbanks remain major concerns, light pollution poses another potential threat too. Image by Mike Prince via Wikimedia Commons (CC BY 2.0).

Jailabdeen notes that captive and wild gharials, including juveniles, differ in behaviour. He calls for ecological studies in natural habitats to establish how light affects basking and whether light pollution poses a threat to wild gharials.

“Light pollution and other anthropogenic disruptions are surely a threat to any wildlife. However, gharials are mostly found within protected areas and away from populated areas which are mostly dark places, except areas around river bridges. As they prefer darker habitats, gharials in the wild actively avoid these lighted zones. They are currently protected from light pollution concerns,” he says.

Jailabdeen emphasises, however, that other threats require more urgent attention. Rampant sand mining, damming and river interlinking alter and destroy gharial habitats, threatening the survival of both wild gharials and captive-reared individuals reintroduced into rivers to bolster wild populations.

Banner image: A wild gharial basks on a sand bank downstream of the Bijnor barrage on the Ganges river. Image by Megha Khanduri.