Subscribe
Pollution and conflict with humans lead to increased levels of stress hormones among mugger crocodiles (Crocodylus palustris), a new study has found.
Mugger crocodiles in the Gujarati city of Vadodara were more likely to display such levels than their counterparts in the rural Charotar region in Kheda and Anand districts, according to the team of researchers.
These findings have implications for mugger crocodile conservation in India. The species, also known as marsh crocodiles, is listed as ‘vulnerable’ according to the International Union for Conservation of Nature Red List and are provided legal protection under Schedule I of the Wildlife (Protection) Act, 1972.
Among all crocodilian species, muggers rank third in terms of human–crocodile conflict, after the saltwater (Crocodylus porosus) and Nile (Crocodylus niloticus) crocodiles.
“India saw a fivefold increase in human–mugger conflict (HMC) incidences over the last two decades, from 57 incidents in 2001–2010 to 338 reported incidents in 2011–2020,” the researchers noted.
The body, whether human or non-human, releases ‘stress hormones’ or glucocorticoids (GCs) in response to challenging conditions. These chemicals mobilise energy to help the body combat such conditions.
This is thus an adaptive mechanism. However, this may be harmful for an organism in the long-term as it can lead to reproductive failure and poor immune function.
The stress hormones are thus an important indicator of whether an organism is facing stress.
“Physiological responses and the overall well-being of free-ranging species have primarily been monitored via gluco-corticoid metabolite (GCM) measurements,” the reserachers wrote.
While GCM can be measured from animal hair, fur, urine, saliva and faeces, the faecal GSM (fGCM) has become popular due to the ease of collection.
For their research, the team zeroed in on two locations — Vadodara and Charotar, both located in central Gujarat. They had reasons to select these two places.
Both are Gujarat’s own examples of ‘Croc country’. However, the similarities end there.
Vadodara is Gujarat’s third largest city, with a population of over 2 million. The Vishwamitri, a seasonal river, originates in the hills of Pavagadh in the neighbouring district of Panchmahal and flows through Vadodara, before ending in the Gulf of Khambhat.
All rivers of Vadodara city and district — Dev, Surya and Dhadhar, besides Vishwamitri — have large crocodile populations. But the Vishwamitri is special.
“Vadodara is a mix of urban and rural zones, supporting a large number of migrant workers (Desai, 2017), and is characterized by the presence of several chemical industries that lie mostly within the urban areas. Most rivers within Vadodara support a significant population of muggers, but these riverine habitats within the urban zones greatly suffer from sewage pollution from neighbouring industries. Further, most of the locals in the rural areas of Vadodara heavily depend on the rivers for daily chores, including the collection of potable water,” the paper notes.
Consequently, 23 (14 fatal and 9 non-fatal) of the 55 reported incidents of HMC in 2019-2023 from the entire state of Gujarat, took place in Vadodara.
In contrast, Charotar is a largely rural region located between the Mahi and Meshwo rivers, located largely in Kheda district. It mostly has agricultural farms and no industries. Moreover, the local residents have practiced peaceful coexistence with mugger crocodiles, which mostly inhabit ponds in the area.
Charotar had only two reported incidents (one fatal and one non-fatal) of HMC in 2019-2023.
The researchers first did a study on a captive population of muggers in an enclosure housing 20 females and two males at the Madras Crocodile Bank Trust (MCBT) in Chennai.
These muggers were captured from the water and then examined by a veterinarian for 15 minutes. The capture and restrain process “can induce stress in most large vertebrates, activating the hypothalamic–pituitary–adrenal axis and leading to the secretion of glucocorticoids, indicative of stress,” the study notes.
The muggers had been under observation before capture and their scats had been collected. A similar procedure was carried out after capture.
The researchers followed this with capture of wild individuals from various locations across Vadodara and Charotar, which they grouped into three zones:
Zone A, with five sampling sites, was a rural, predominantly agricultural region within Charotar with no industries and with local people having a high tolerance towards coexisting muggers living in ponds or lakes and facing low or no incidents of HMC.
Zone B, with four sampling sites, was an urban belt within Vadodara with high HMC and had several chemical industries disposing sewage in the Vishwamitri river.
Zone C, with four sampling sites, was a rural part within Vadodara having mainly agricultural lands and a low number of industries with high dependency of local people on the Vishwamitri.
The scats of the wild muggers were treated with an assay that had been validated for use with the scats of the captive muggers in Chennai.
“Our study revealed that the captive (pre-capture) mugger crocodiles at MCBT had similar levels of fGCM as the free-ranging muggers in Charotar (Zone A). Contrastingly, fGCM levels of muggers in Vadodara (zones B and C) were four times higher compared to both the captive (pre-capture) and Charotar populations,” the paper read.
The high fGCM levels in muggers of Vadodara were recorded during both breeding and non-breeding seasons.
“Thus, it can be speculated that muggers in Charotar (a free-ranging natural condition) are facing less challenging situations compared to muggers in Vadodara and are possibly experiencing physiological conditions similar to those of the captive muggers at MCBT. In contrast, Vadodara muggers have significantly high fGCM levels, which could either represent a compromised, maladaptive condition or an adaptive physiological response towards a highly challenging environment,” the researchers concluded.
Monitoring the stress physiology of free-ranging mugger crocodiles (Crocodylus palustris) across diverse habitats within Central Gujarat, India has been published in the Conservation Physiology journal.