A multidisciplinary study on mugger crocodiles (crocodylus palustris), including reproductive behaviour, endocrine correlates, and biometric identification

Abstract

Reproduction is one of the key survival strategies that a species has within a given environment (1). Understanding reproductive behaviour is not only limited towards assessing courtship and mating sequences but also includes studying several other behaviours, for example, mate choice, mate guarding, aggressive interactions, territorial defence, and parental care (2). In addition to studying behaviours, investigating the underlying proximate (physiological) mechanisms that govern such reproductive behaviours (3) promotes a holistic understanding about the biology of the species. Furthermore, assessment of reproductive behaviour and its physiological correlates contributes towards interpreting the adaptation processes prevalent at a population level within a given environment (4–6). Till date, research on reproductive behaviour and physiology has predominantly focussed on mammalian and avian taxa and have provided detailed work on different species. However, there are significant gaps in understanding reptilian physiology and behaviour in the context of reproduction and needs further investigation and exploration in this domain (7–9). My dissertation project focuses on studying reproductive behaviours and its endocrine correlates in mugger crocodiles (Crocodylus palustris), a large freshwater reptile. Muggers are distributed across diverse habitats, from agricultural landscapes, semi arid forests to urban gutters, and occur in approximately 50 small pockets within India and the subcontinent (10). According to the IUCN’s red list, muggers have a vulnerable status and have been locally extinct in Myanmar and Bangladesh for over five decades (11). The mugger population in India was threatened due to skin trade and poaching less than four decades ago. Later, towards the 1980s, a conservation breeding program started for muggers at the Madras Crocodile Bank Trust (MCBT), Chennai, India with an aim to rescue the species from extinction (12). Despite undergoing such bottlenecks in their populations, no studies have systematically conducted research work to understand the reproductive behaviour and underlying endocrine physiology of mugger crocodiles. In my dissertation, I have attempted to characterise and define reproductive behaviours of the mugger crocodiles and have generated baseline data on behaviour and endocrine physiology for a focal breeding population of captive muggers at MCBT. I have defined several behaviours in the context of courtship and mating, for example, ‘jaw clap’ in males and ‘snout lift’ in females. Apart from these, I have also defined aggressive behaviours, a class of behaviour known to play a significant role in terms of nest and territory defence in the context of reproduction (13–16). Such kind of detailed behavioural characterisation will possibly contribute towards the field of animal behaviour, where there is a serious gap in terms of understanding the behavioural biology of the large reptilian species. Moreover, it will also be interesting to compare and contrast the generated behavioural profiles of captive muggers with that of the free-ranging ones to assess variations that may exist under different environmental and ecological conditions. One of the bottlenecks that I realised while studying the reproductive behaviour and physiology of captive mugger crocodiles is with regard to the sampling methods. Compared to random cross-sectional sampling, focal animal sampling (17) provides a comprehensive understanding of reproductive behaviour, enabling detailed observation and repeat sampling of individuals to trace the reproductive phases over time. Although focal individual sampling is a tedious method and is logistically difficult to conduct compared to random cross-sectional sampling, it remains one of the most rigorous approaches to studying animal behaviour and physiology. Thus, I have developed a biometric system based on deep learning methods to non-invasively sample focal individuals over time to identify mugger crocodiles using their dorsal scute patterns. Overall, my dissertation majorly has three pieces of data on reproductive behaviour, endocrine correlates, and biometric identification, which provides several novel findings in terms of biology of the mugger crocodiles and showcases promising methodologies for potential ecological applications.