- Russell’s vipers tracked in rural Karnataka were found to spend a large amount of time in agricultural plantations and are most active during May and late autumn.
- People often came within 50 metres of radio-tracked Russell’s vipers. However, snakebites may not be linked to the spatial activity of the vipers or how frequently people come close to the snakes, the findings suggest.
- Vipers are not aggressive, contrary to popular notions, and usually bite in self defence.
Native to the Indian subcontinent, Russell’s vipers are highly venomous snakes and the major cause of mortalities and permanent disabilities each year in South Asia. Now, a new study, which tracked their movement patterns in rural Karnataka finds that the snakes spend a large amount of time in agricultural plantations and are most active during late autumn and May. An earlier study by the author finds that people frequently came close to the vipers that the researchers were radio-tracking.
“Snake ecology has largely been overlooked in the context of snakebites but it seems quite logical to study the organism that is in conflict with humans in order to better understand the factors linked with peaks in snakebites,” says Xavier Glaudas, who was an honorary research associate for Bangor University in Wales, during the study, which was funded by the National Geographic Society.
Karthikeyan Vasudevan, a senior principal scientist at CSIR-Centre for Cellular & Molecular Biology (CCMB), who was not associated with the study agrees, emphasising the need to understand the biology of snakes for effective mitigation of conflicts with humans. “Our knowledge of the natural history of medically important snakes is virtually nonexistent,” he says, adding that “financial support for natural history research has declined over the last few decades and has impacted our ability to address these problems.”
According to a study, 1.2 million snakebite deaths were estimated to have occurred in India from 2000 to 2019, most of which occurred at home in rural areas. In 2019, the World Health Organization (WHO) outlined a global strategy for the control and prevention of snakebites. Their goal is to halve the number of deaths and disabilities from snakebites by 2030. However, the strategy fails to detail the need to study the snakes that are responsible for the envenoming, Vasudevan points out.
To understand the movement patterns and habitat use of Russell’s vipers, 18 snakes (7 males and 11 females) were implanted with radio transmitters by local veterinarians. Upon release, Glaudas tracked their movement from late October 2018 to early August 2019 near the villages of Rathnapuri and Chowdikatte. The highly modified landscape consisted mostly of agricultural plantations and three artificial lakes with canals.
The snakes were tracked on 2,478 occasions although they were not seen on all occasions as they were hidden under thick vegetation, piles of palm fronds or in burrows. Russell’s vipers spent a large amount of time (33%) in plantations followed by scrub habitats and water canals. This explains why people working in agricultural fields are at high risk of snake bites.
In terms of movement, the snakes were least active during March, a very dry and hot month, while their activity increased in late autumn and May. It is the mating season during late autumn, which may explain the rise in movement and May marks the end of the dry season.
Basking was more likely when the temperatures were lower. Russell’s vipers basked more frequently in January and February, which had the lowest monthly minimum temperatures. Basking was lowest in April when the average temperatures were the highest.
The vipers were observed in ambush (hunting) foraging postures in the daytime throughout the year with decreased foraging at colder temperatures. Trails of scent molecules left by prey such as rats or mice are distinguished by the vipers. “The vipers pick up on it using their highly sensitive chemoreceptive organ, and will set themselves in ambush along this trail, patiently waiting for the prey to pass by and strike it to envenomate them,” explains Glaudas, who is now an honorary research associate at the University of the Witwatersrand in South Africa.
“In the case of Russell’s vipers, the typical ambush posture is when the chin rests on the ground with a bend in the neck and the body loosely coiled.” The odds of observing a snake in ambush were higher with increased minimum temperatures indicating that the snakes decreased foraging at colder temperatures.
Close encounters with humans
Based on the movement of the snakes over a period of seven months, a prior study by Glaudas found that people came within 50 metres of a radio-equipped viper in about 17 percent of the 2,066 locations. In 5 percent of the cases, a person was observed within 10 metres of a snake, which is “literally just a few steps away” Glaudas notes.
Close interactions between people and the vipers were observed in January and July compared with March when people were least frequently observed close to snakes. Overall, the encounter frequency between snakes and people was relatively similar during the study period. No bites occurred during the study period.
Snakebite envenoming is caused by toxins in the bite of a venomous snake and could be life-threatening. In India, it peaks during the monsoon season from July to September.
These findings suggest that snakebite envenoming may not be linked to how frequently people and snakes cross paths and the spatial activity of the snakes. If snakebites were related to how frequently people come close to snakes, then one would expect snakebite incidence to be relatively similar throughout the year except in March when interactions were less frequent, explains Glaudas.
Vipers are not so aggressive after all
During the study, Glaudas had thousands of close encounters with Russell’s vipers but he was never bitten because they are generally hidden in the vegetation, he points out. “The snakes generally remained motionless, and in some rare cases, fled deeper in the vegetation and/or retracted their head to be more concealed.”
Contrary to popular notions, vipers are not aggressive, Glaudas says, clarifying that an unprovoked attack constitutes aggressive behaviour. “Snakes bite in defence, because they are scared of us, and one way to think about it is that they fight for their life when they are confronted with a large moving organism. A Russell’s viper weighs on average 0.9 kg, and we are much bigger.”
Some vipers can put on impressive defensive displays. When they feel threatened, Russell’s vipers can make a loud hissing sound similar to that of a pressure cooker, by rapidly exhaling air from the lungs, explains Glaudas. The sound warns of a potential threat trying to approach the viper’s presence and its ability to defend itself.
Having studied vipers for around 20 years on three continents, Glaudas says “they are generally mild-mannered snakes” and “they generally do not try to bite.” However, if one steps on them, harasses them, or tries to pick them up, the odds of getting bitten would be higher, he warns.
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Glaudas, X. (2021). Natural History of a Highly Medically Important Snake, Russell’s Viper (Daboia russelii), in a Human-Dominated Indian Rural Landscape. Journal of Herpetology, 55(2), 151-159.
Glaudas, X. (2021). Proximity between humans and a highly medically significant snake, Russell’s viper, in a tropical rural community. Ecological Applications, 31(4), e02330.
Banner image: Russel’s viper. Photo by Xavier Glaudas.