Total length and head length relationship in Mugger Crocodiles Crocodylus palustris (Reptilia: Crocodilia: Crocodylidae) in Iran

 This study quantifies the morphometric relationship between total length (TL) and head length (HL) for a sample of 53 Mugger crocodiles (43–280 cm TL) in Iran. Across the size range of crocodiles sampled, the relationship was best described by a linear regression (TL= 15.52 + 6.283HL). The average TL/HL ratio was 7.034, which is similar to that reported for other crocodilian species and confirms its usefulness for estimating TL based on HL during population surveys.

A small population of around 500 Mugger crocodiles Crocodylus palustris in southeastern Iran occupies the western-most extreme of the species' global range (de Silva & Lenin 2010). This population is scattered across the region and individuals often come into close contact with local communities (Mobaraki 2015). The Mugger's habit of moving between different habitats depending on local climatic and seasonal conditions means that they often turn up in remote areas (Abtin & Mobaraki 2016). Because this population persists at the extreme edge of the species distribution, environmental limits or conditions are likely to affect its biology and population dynamics, as well as its susceptibility to potential threats (Mobaraki et al. 2019).
Like many other crocodilians around the world, monitoring of the C. palustris population in Iran has been based on day and night-time (spotlight) surveys. During surveys, crocodiles are usually partly submerged, with only the head exposed to observers. Where observers have considerable experience with carrying out surveys and capturing large numbers of differentsized crocodiles, estimating body length from size of the head comes somewhat naturally (C. Manolis, pers. comm. 2021). However, this is not the case in many situations, including that in Iran.
Researchers working on morphometric relationships of different species of crocodilians have invariably noted that head length (HL) relative to total length (TL) tends to change little across size classes (e.g., Webb & Messel 1978;Magnusson 1983;Webb et al. 1983;Montague 1984;Hutton 1987;Verdade 2000;Wu et al. 2006;Platt et al. 2011;Fukuda et al. 2013;Edwards et al. 2017). Whitaker & Whitaker (2008) examined the HL/TL ratio for a number of crocodilian species, and confirmed the average ratio to be around 1:7, but this ratio was found to be closer to 1:8 for large (>4 m TL) species such as Crocodylus porosus, due to ontogenic changes associated with the head with increasing body size (Webb & Messel 1978;Whitaker & Whitaker 2008;Britton et al. 2012;Fukuda et al. 2013). In this study, we aim to quantify the relationship between TL and HL for Iranian Muggers, as well as examining the ratio between these two morphometric measures.
Material and Methods: Natural and artificial ponds are Fifty-three Mugger crocodiles of varying sizes and sex were captured using fishing nets or ropes, during surveys or the translocation of nuisance specimens, in 2018 and 2019. Head length (HL) was measured from the anterior tip of the snout to the rear edge of the cranial platform (parietal bone). Total length (TL) was measured from the anterior tip of the snout to posterior tip of the tail along the back of the animal. Both measurements were taken using fiberglass tape, and the results rounded to the nearest cm. We excluded any specimens that were missing the tip of their tail. Sex was determined for 30 individual by examination of the cliteropenis, but as this sample size was considered small, the effect of sex was not examined further. Statistical methods: The measurements were first tested for normality using Shapiro-Wilk (SW) and Kolmogorov-Smirnov (KS) tests. Linear regression analysis was used to examine the relationship between TL and HL, and between the TL/HL ratio and body size (TL and HL).
Discussion and Conclusion: As with other crocodilians, the relationship between HL and TL in Iranian Muggers was linear and thus provides a means to estimate size from heads/skulls found in the field or as museum specimens. The mean TL/HL ratio in this study (7.03) was similar to that obtained by Whitaker & Whitaker (2008) in their review of other crocodilian species. For all but perhaps the most longirostrine species, there appears a similarity in TL/HL ratio for crocodilians of moderate size.
The increasing TL/HL ratio with increasing size in our sample of Muggers merits further investigation, particularly with inclusion of data from larger Muggers (>3 m TL), since in some other species there is an increase in the TL/HL ratio in the largest crocodiles (Whitaker & Whitaker 2008;Fukuda et al. 2013). More

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accurate data on additional head measurements (e.g., see Webb & Messel 1978) is likely to have provided more insights into how head shape changes with size in our sample of Muggers, and future work will take this into consideration. In view of the relatively harsh environment in which Iranian Muggers occur, similar studies in other range states, namely Pakistan, Nepal, India, and Sri Lanka, may provide a clearer picture of potential morphometric differences across localities or populations for Muggers.