Pollution, human-wildlife conflict might cause high stress among mugger crocodiles, finds study

New research in India reveals that mugger crocodiles in Vadodara, with high human-mugger conflict, exhibit chronic stress levels which could be harmful for the species. The study compared the stress levels of muggers in Vadodara with those in the more rural Charotar region, about 45 kilometres away, where there is high tolerance for the presence of muggers. The study demonstrated that physiological responses in muggers varied across habitats, and such variation could be because of ecological factors in the immediate, local environment.

Vadodara, with its polluted Vishwamitri river and an urban-rural mix, contrasts sharply with the largely rural Charotar region, where muggers inhabit cleaner ponds and are revered as sacred animals. This contrast provided an ideal setting to study the physiological stress responses of muggers in varying environmental conditions.

The study, by researchers from Ahmedabad University and Madras Crocodile Bank Trust, highlights the impact of human activities and environmental conditions on wildlife, emphasising the need for tailored conservation strategies. Physiological stress can be detrimental to wildlife. Previous studies have shown that chronic stress can impact the reproductive and immune systems in species. Monitoring the stress provides an understanding of whether the species is at risk under a given condition and such research can then better inform conservation practices.

“In India, many habitats suffer from threats such as pollution, habitat fragmentation, urbanisation, and increasing human-animal conflict. These challenges heavily impact the health and well-being of wildlife, making the environment highly unsustainable. Thus, assessing and implementing effective ecological and management strategies is crucial, especially for species like the mugger crocodile that live in close proximity to human populations,” says Ratna Ghosal, associate professor, biological and life sciences division, Ahmedabad University, and one of the authors of the study.

The Vadodara region is also a hotspot for human-wildlife conflicts due to its large population of muggers. Credit: Tathagata Bhowmik via Mongabay.

Cause of conflict

The mugger crocodile (Crocodylus palustris), a freshwater species found in diverse habitats across India, is classified as vulnerable on the IUCN Red List and is protected by Schedule I of the Wildlife Protection Act of India, 1972. As apex predators in their aquatic ecosystems, muggers are vital indicators of habitat health. They prey on fish, amphibians, birds, and small mammals, thus helping maintain the food chain, control diseases, and contribute to nutrient cycling.

However, mugger crocodiles are also responsible for the third-highest number of attacks on humans globally, after saltwater and Nile crocodiles, according to a study from 2008 to 2013. In India, such conflicts have surged five-fold over the past two decades, rising from 57 incidents between 2001 and 2010, to 338 between 2011 and 2020.

The Vadodara region, particularly around the Vishwamitri river, is a hotspot for these conflicts, due to its large population of muggers.

“Muggers serve as a model for studying the impacts of climate change. Despite their ecological importance, they are often viewed negatively due to their potential threat to livestock and humans. This perception undermines their critical role in the ecosystem and highlights the need for conservation efforts,” shares Ghosal.

“Human activities such as bathing, fishing, and boating frequently bring people into close contact with crocodiles. Additionally, muggers need open basking areas, which often conflict with human use of the land. Attacks often intensify during the breeding season when the crocodiles aggressively defend their nests and hatchlings. This kind of human-crocodile conflict is common worldwide. But, at the same time, some habitats such as Charotar show peaceful coexistence with significant populations of large reptiles,” explains Tanuj Singh, a wildlife biologist, who has been studying muggers in Gujarat for a decade, not associated with this study.

In the largely rural Charotar region, muggers inhabit cleaner ponds and are revered as sacred animals. Image by Tathagata Bhowmik.

A study of contrasts

Researchers collected scat samples to measure faecal glucocorticoid metabolites, or fGCM, during both the breeding (107 samples) and non-breeding (22 samples) seasons from all the zones. Measuring the fGCM is a popular non-invasive method to measure stress in animals. In this study, the captive muggers were used to validate the fGCM assay.

The results showed an 11-fold increase in fGCM levels from pre-capture to post-capture. The Vadodara muggers (in urban and rural areas) had significantly higher fGCM levels than those in Charotar, both during breeding and non-breeding seasons. This indicates sustained high stress levels, potentially due to pollution and human-wildlife conflict. The ongoing study also aims to determine whether these high stress levels are adaptive or harmful to their health.

“Contrary to expectations, there was no significant difference in stress levels between breeding and non-breeding seasons. Additionally, captive muggers showed similar stress levels to those in Charotar, suggesting that long-term captivity might not be as stressful as previously thought, possibly due to regular monitoring and adaptation to enclosure conditions,” says Ghosal.

More research

This study demonstrated that the mugger’s physiological responses, as assessed by fGCM levels, vary by habitat, likely due to different ecological conditions. High fGCM levels in Vadodara seen throughout the year suggest chronic stress. Since this research is possibly the first non-invasive monitoring of fGCM levels in free-ranging crocodiles, it highlights the need for further studies to determine if elevated fGCM in Vadodara is a sign of chronic stress or an adaptive trait.

“Stress hormones or glucocorticoids (GCs) are released in the body to mobilize energy to combat challenging conditions and are thus adaptive in nature,” the study states.

“Vadodara’s polluted and conflict-ridden environment leads to higher stress in muggers compared to the more serene Charotar region. While high stress often indicates health problems, it may also be an adaptation to tough conditions,” Ghosal explains.

Mugger crocodiles are vital for maintaining healthy aquatic ecosystems in India. Understanding their stress responses and ecological adaptations can help create effective conservation strategies, ensuring their survival and well-being.

“Conservation efforts should be customised to local conditions rather than applying a one-size-fits-all approach. Future research should explore how muggers adapt to various habitats. Public education about the ecological importance of muggers and promoting safe practices can also reduce conflicts and enhance conservation efforts,” concludes Singh.

This article was first published on Mongabay.

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