DOI: 10.5061/dryad.cjsxksn96
title: How to estimate body condition in large lizards? Argentine black and white tegu (Salvator merianae, Duméril and Bibron, 1839) as a case study

description: 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.
description: Species description 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
 (<em>Crocodylus</em> <em>acutus</em>) and gopher
 tortoises (<em>Gopherus</em>
 <em>polyphemus</em>). 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. Statistical methods
 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.
 <strong>Additional information on the following analyses
 using this data is available within the associated
 manuscript.</strong>