Non-invasive body condition monitoring and relationships between thermal responses and behaviour in captive and wild Nile crocodiles (Crocodylus niloticus)

Abstract

As ectotherms, crocodilians achieve and maintain a preferred body temperature by seeking or avoiding heat in the environment. Temperature management influences aspects of crocodile welfare, production, and reproduction and strongly determines the success of crocodile farming operations. Concerns surrounding the temperature management of commercially farmed Nile crocodiles in South Africa have been raised by the NSPCA (National Society for the Prevention of Cruelty to Animals). Wild Nile crocodiles in the Kruger National Park have been affected by pansteatitis outbreaks leading up to and during winter months since 2008. Cooler winter temperatures have been implicated as a potential contributor to the recurrence of this disease. This study evaluated minimally invasive Nile crocodile body condition assessments, thermal experiences and behaviours of wild and captive Nile crocodiles, and the effect of temperature on nest site selection in commercial settings. Non-invasive data capture techniques were employed and developed to ensure the crocodiles natural behaviours, and therefore thermal experiences, were not disrupted or altered. This included drones, an Internet of Things system of abiotic loggers, telemetry tracking and temperature logging systems, and iButtons. Seven drone-based morphometrics were measured for 288 commercially farmed Nile crocodiles across two farms in South Africa, and a subsequent body condition assessment was conducted using two of these morphometrics. A small and relatively inexpensive drone (DJI Mavic Mini) was selected for this study, after a vigilance assessment concluded that it was minimally disruptive to normal crocodile behaviour. Crocodiles on one farm were slimmer than those on the other farm prior to the drone flights. Crocodile condition was assessed with the following UAV-captured measures: total length (TL), snout-hindlimb length (SHL), snout-neck length (SNL), neck width (NW), belly width (BW), total surface area (SA), and perimeter. A body condition index (BCI) was calculated for each crocodile by measuring the relationship between total length and belly width, with the equation: BCI = (BW/TL) * 10, derived from photogrammetrically processed orthophotos in GIS. The BCI values were then normalized to form a body condition score (BCS) with the equation: BCS = (BCI/1.27) * 4 + 1. The body condition score ranked crocodiles from 1–5, with 1 being thin or emaciated and 5 being fat or obese relative to other crocodiles. The majority of crocodiles in the study had a BCS of 3, with few animals scoring a 1 or 5. The farm housing noticeably thinner crocodiles prior to the study had no BCS 5 occurrences, while the other farm had no BCS 1 occurrences. This UAV-based body condition score could be applied to large wild or captive populations for a fast-paced health and welfare evaluation. Thermal behaviours of captive Nile crocodiles on a commercial crocodile farm in South Africa were assessed using a method which transformed relative thermal maps (produced by a DJI Mavic 2 Enterprise Dual drone) into a predictive model where temperatures were derived to within 2.6 °C per pixel of a processed ortho-photo. Thermal behavioural data was extracted from the drone imagery and juxtaposed with climate and abiotic thermal data from the pen. Site (concrete, water, nest, and grass/sand) selection by crocodiles varied with season, time of day, and daily climatic conditions. During high ambient and pen temperatures in sum-mer crocodiles sought refuge in water bodies, which only accounted for 20% of the pens total surface area. During low ambient and pen temperatures in winter the crocodiles prioritized basking on the concrete and grass/sand areas, abandoning the water bodies. The results suggest a need for increased shading over the land areas during summer, and heating of the waterbodies during winter. These alterations would increase the amount of thermally viable areas within the pen, allowing the crocodiles to exercise heat seeking and avoidance behaviours during both seasons. This method provides a non-invasive assessment of the pen design from a thermal perspective, which can then inform improved temperature management. Thermal behaviours of 16 wild Nile crocodiles (233–429 cm TL) within Kruger National Park were assessed. Each crocodile was fitted with sensors that recorded external temperatures (attached onto the crocodiles’ backs), internal temperatures (stomach), activity (accelerometer), and location (GPS coordinates). Internal body temperatures were strongly influenced by ambient air temperatures and humidity, as well as the immediate external environment temperature. Variations in internal temperatures and activity levels were dependent on sex, size, and season. Seasonal temperature (internal, external, and ambient) and activity lev-el fluctuations varied significantly. Internal temperature averages were low from April to July. Activity means declined from March till May and remained relatively low before increasing again between November and December. Reduced internal temperature and activity levels coincided with outbreaks of pansteatitis, high-lighting a potential correlation. As ectotherms, the environmental temperatures available to crocodiles play a critical role in successful thermoregulation and dependent biological processes (metabolism, digestion, re-production, immune function, muscular function, development, and behaviour). This study expanded the park's understanding of the temperatures experienced by these wild crocodiles and will guide future management efforts with respect to thermal comfort and thermal opportunities. Nile crocodile nesting behaviours were monitored over a single breeding season on a commercial farm in South Africa. Drones (a DJI Mavic Mini 2 SE and a Mavic 2 Enterprise Dual) were used for overhead behavioural and temperature monitoring, whilst iButtons were placed at ground level, recording nest temperatures at various depths and locations around the breeder pen. The farm recorded the numbers of eggs laid and hatched in each nest. The complex interplay between climate, nest thermal properties, crocodile nesting behaviours, and pen layout was inspected. Nesting locations within the pen varied in orientation, thermal properties, distance from waterbodies, distance from a nearby tourist centre, and proportion of grass coverage. Significant variations in nest site selection occurred across nesting locations, with a distinct predilection for certain sections while others were scarcely utilized. When considering nest occupancy versus confirmed egg deposition, behavioural variations were also noted between sections. The characteristics of preferred nesting sites in farmed settings need to be identified and met to ensure that all nesting female crocodiles have sufficient nesting opportunities. Stress due to nest site competition could result in a decrease in laying and hatching success and is a welfare concern. Given the thermal challenges faced by crocodiles during summer and winter seasons, and the variations in crocodile farm setups in South Africa, guidelines for dynamic and innovative management strategies of both farmed and wild populations are necessary. The findings of these studies inform both farming and conservation efforts regarding the thermal experiences of Nile crocodiles in South Africa. Farming regulations may require updating with regards to temperature/pen design and management, whereas wildlife management initiatives could benefit from wild Nile crocodile thermal experience awareness.