How to estimate body condition in large lizards? Argentine black and white tegu (Salvator merianae, Duméril and Bibron, 1839) as a case study
McCaffrey, Kelly et al. (2023), How to estimate body condition in large lizards? Argentine black and white tegu (Salvator merianae, Duméril and Bibron, 1839) as a case study, Dryad, Dataset, https://doi.org/10.5061/dryad.cjsxksn96
Body condition is a measure of the health and fitness of an organism represented by available energy stores, typically fat. Direct measurements of fat are difficult to obtain non-invasively, thus body condition is usually estimated by calculating body condition indices (BCIs) using mass and length. The utility of BCIs is contingent on the relationship of BCIs and fat, thereby validation studies should be performed to select the best-performing BCI before application in ecological investigations. We evaluated 11 BCIs in 883 Argentine black and white tegus (Salvator merianae) removed from their non-native range in South Florida, United States. Because the length-mass relationship in tegus is allometric, a segmented linear regression model was fit to the relationship between mass and length to define size classes. We evaluated percent, residual, and scaled fat and determined percent fat was the best measure of fat because it was the least associated with snout-vent length (SVL). We evaluated performance of BCIs with the full dataset and within size classes and identified Fulton’s K as the best-performing BCI for our sampled population, explaining up to 19% of the variation in fat content. Overall, we found that BCIs: 1) maintained relatively weak relationships with measures of fat and 2) splitting data into size classes reduced the strength of the relationship (i.e., bias) between percent fat and SVL but did not improve the performance of BCIs. We postulate that the weak performance of BCIs in our dataset was likely due to the weak association of fat with SVL, the body plan and life-history traits of tegus, and potentially inadequate accounting of available energy resources. We caution against assuming that BCIs are strong indicators of body condition across species and suggest that validation studies be implemented, or that alternative or complementary measures of health or fitness should be considered.
The Argentine black and white tegu is a large teiid lizard native to eastern and central South America that has become established in several areas of central and southern Florida, United States. Tegus can be classified as omnivores as well as generalist meso-predators which consume fruit, plant material, and animal prey (arthropods, gastropods, reptiles, birds, and small mammals), with evidence of diet changing based on seasonal abundance. Tegus are also known to scavenge upon carrion and regularly consume eggs of ground-nesting vertebrates such as alligators, turtles, and birds. This species is considered invasive in Florida with potential negative impacts on native fauna through egg predation and competition for burrowing sites. To combat this threat, management programs have been established in Florida to reduce the population of invasive tegus and halt their expansion into ecologically sensitive areas, such as nesting sites of threatened species such as American crocodile (Crocodylus acutus) and gopher tortoises (Gopherus polyphemus).
Collection, euthanasia, and necropsy
We received tegus collected through trapping and removal efforts performed by the Florida Fish and Wildlife Conservation Commission (FWC) and the University of Florida (UF) from 2012 through 2018. Once obtained, tegus were humanely euthanized using captive bolt or firearm immediately followed by pithing, and frozen until necropsy. Tegus were thawed prior to necropsy and examined for general health and condition by visually inspecting all internal organs and the body exterior for any abnormalities or deformities that may affect body mass, body length, or fat mass. We obtained measurements of snout-vent length (SVL) to the nearest 0.1 cm using a flexible measuring tape, and total body mass using a digital scale to the nearest g. Coelomic wet-fat mass was obtained by removing and weighing the discrete abdominal fat bodies to the nearest 0.0001 g. The average timespan a tegu was held between euthanasia and necropsy was approximately 263 ± 258 days (range 0–1,847 days). The research protocol was approved by the University of Florida Animal Research Committee and University of Florida Institutional Animal Care and Use Committee and protocol numbers are listed in the Acknowledgements.
To reduce potential bias in our dataset, we excluded data from tegus with incomplete or unreliable necropsies due to decay, unknown sex, physical abnormalities (missing or abnormal limbs or tails (e.g., regenerated tail), and scoliosis), or if the time spent in captivity prior to euthanasia was more than 4 days. We also removed animals whose wet-fat mass was equal to 0, which could not be transformed with natural log. All data analyses were performed in R.
Additional information on the following analyses using this data is available within the associated manuscript.
Florida Fish and Wildlife Conservation Commission, Award: AWD07127
South Florida Water Management District, Award: AWD07368
U.S. Fish and Wildlife Service, Award: AWD06940
National Park Service, Award: AWD02657
U.S. Army Corps of Engineers, Award: AWD08785
University of Florida
U.S. Geological Survey, Award: AWD00438