Sexual dimorphism in baseline urinary corticosterone metabolites and their association with body-condition indices in a peri-urban population of the common Asian toad (Duttaphrynus melanostictus)

Morphometrics of Xenopus laevis Kept as Laboratory Animals

Morphometric data that provide information on body conditions can be used to monitor the health and well-being of animals. In laboratory animals, they can help to evaluate the stress due to experiments or treatments, following the 3R principles. The aim of the present study was to obtain morphometric data of male and female African clawed frogs, Xenopus laevis, as the bases for body condition evaluations. Adult frogs (n = 198) were weighed and standardized photographs were taken. The photographs were used to determine several measurements (length, cranial width, caudal width, thigh width). In addition, a triangle was drawn to outline each frog’s simplified body form, and the triangle surface was calculated. In conclusion, the triangle surface drawn on the dorsal plane of each frog correlated with the body weight of the females. There were significant differences between the body weights and sizes of male and female frogs, with males being smaller (p < 0.001). Based on the morphometric data, females could be assigned to five groups in which an assessment of the animal’s well-being is feasible.

A Review of Non-Invasive Sampling in Wildlife Disease and Health Research: What’s New?

Simple Summary

The interest in wildlife research has increased in the last decades as more scientists work within a One Health framework that regards human, livestock and wildlife health as connected entities. To minimise the impact of research on wildlife, collecting samples with as little disturbance of the animals as possible is important. In our review, we assess the use of so-called non-invasive sampling and summarise which samples can be used successfully when carrying out research on wildlife diseases and health status. Our results show that interest in minimally invasive sampling has steadily increased since the 2010s. Topics able to employ these methods include disease research, but also stress and other hormone assessments, pollution studies, and dietary studies. At the moment, such methods are mainly used to collect samples from land mammals, however, they can also be used in a wide range of other animals. Ever more capable analytical methods will allow for an even wider use of such “animal-friendly” sampling methods.

Abstract

In the last decades, wildlife diseases and the health status of animal populations have gained increasing attention from the scientific community as part of a One Health framework. Furthermore, the need for non-invasive sampling methods with a minimal impact on wildlife has become paramount in complying with modern ethical standards and regulations, and to collect high-quality and unbiased data. We analysed the publication trends on non-invasive sampling in wildlife health and disease research and offer a comprehensive review on the different samples that can be collected non-invasively. We retrieved 272 articles spanning from 1998 to 2021, with a rapid increase in number from 2010. Thirty-nine percent of the papers were focussed on diseases, 58% on other health-related topics, and 3% on both. Stress and other physiological parameters were the most addressed research topics, followed by viruses, helminths, and bacterial infections. Terrestrial mammals accounted for 75% of all publications, and faeces were the most widely used sample. Our review of the sampling materials and collection methods highlights that, although the use of some types of samples for specific applications is now consolidated, others are perhaps still underutilised and new technologies may offer future opportunities for an even wider use of non-invasively collected samples.

URINARY CORTICOSTERONE CONCENTRATIONS IN FREE-RANGING AND MANAGED CANE TOADS (RHINELLA MARINA)

Approximately 40% of amphibian species are threatened with extinction. The welfare of amphibians maintained under managed care as assurance populations is of vital importance to guard against extinction and provide a viable source population for future reintroduction. To manage amphibian species properly ex situ, it is important to understand how stress levels change over time when animals are removed from the wild and placed into managed environments. Corticosterone was analyzed in urine samples from free-ranging cane toads (Rhinella marina, n = 55) in Miami, FL, and under managed care for 22 (n = 48), 50 (n = 11), 81 (n = 25), or 119 (n = 10) days. Concentrations of corticosterone in free-ranging toad urine averaged 1.74 ± 0.195 ng/ml urine specific gravity (sp. gr.), which was greater (P < 0.05) than other time points (day 22: 0.77 ± 0.114 L; day 81: 0.85 ± 0.191 ng/ml sp. gr.; day 119: 0.58 ± 0.093 ng/ml sp. gr.), except day 50 (0.91 ± 0.274 ng/ml sp. gr.), which was not different from free-ranging or managed care values. Thus, corticosterone was lower in cane toads under managed care compared with those sampled in the wild, suggesting that managed care is not a stressor for this species.

Influence of temperature, size and confinement on testosterone and corticosterone levels in breeding male spined toads (Bufo spinosus)

Winter breeding amphibians opportunistically engage in reproductive behaviour when environmental conditions become favourable. In such explosive breeding systems, males strongly compete for reproductive females. Although most research has been oriented on species in which males use mating calls to attract females, many high-density explosive breeding amphibians do not rely on mating calls. In such systems, larger and stronger males are thought to have significant advantages to access reproductive females. Testosterone (T) is expected to increase with the physical attributes that facilitate access to females, while increased corticosterone levels (CORT) are needed to sustain the energetic requirements associated with mating behaviour. In this study, we investigated how environmental temperature, and body size influence testosterone and corticosterone in males of an explosive winter-breeding species with low investment in mating call, the spined toad (Bufo spinosus). We found that both baseline CORT and T were positively correlated with environmental temperature. Interestingly, despite a remarkable range of variation in CORT and T, there was no evidence that either hormone was correlated with body size. Finally, we found no effect of confinement (13 h of captivity) on circulating CORT and T levels. This suggests that breeding male toads may be relatively insensitive to stress in order to maintain reproductive effort during their short mating period. Future studies should investigate both the influence of the phenology of breeding and the social interactions on these hormonal levels.

Interrelationship among annual cycles of sex steroids, corticosterone and body condition in Nyctibatrachus humayuni

Synergism between extrinsic and intrinsic factors is crucial for the seasonality of reproduction. Environmental factors such as photoperiod and temperature activate the hypothalamus-pituitary-gonadal axis leading to the secretion of steroid hormones that are crucial for reproduction. Sex steroids are not only essential for the maturation of gonads, but also for development of secondary sexual characters in males and reproductive behaviour of both the sexes. In the present study, we quantified the urinary testosterone (UTM) and corticosterone (UCM) metabolites in males and urinary estradiol metabolites (UEM) and UCM in females of Nyctibatrachus humayuni for two consecutive years to determine annual and seasonal variation in the levels of sex steroids, corticosterone and body condition index (BCI). The results show that sex steroids were highest during the breeding season and lowest during the non-breeding season in both the sexes. An increase in UTM and UEM was observed in males and females respectively during the breeding season. Testicular histology showed the presence of all stages of spermatogenesis throughout the year indicating that spermatogenesis is potentially continuous. Ovarian histology showed the presence of vitellogenic follicles only during the breeding season indicating that oogenesis is strictly seasonal. In males, UCM levels were highest during the breeding season, while in females their levels were highest just prior to the breeding season. In males, BCI was highest during the pre-breeding season, declined during the breeding season to increase again during the post-breeding season. In females, BCI was comparable throughout the year. In males, UTM levels were positively correlated with UCM levels but negatively correlated with BCI. Interestingly, UEM, UCM and BCI were not correlated in females. These results indicate that N. humayuni exhibits an associated pattern of reproduction. Quantification of urinary progesterone metabolites (UPM) during the breeding season showed UPM levels were higher in post-spawning females, suggesting the significance of progesterone in ovulation. Further, non-invasive enzyme immunoassay has been successfully standardized in N. humayuni for the quantification of urinary metabolites of steroid hormones.

A novel method for the measurement of glucocorticoids in dermal secretions of amphibians

Amphibians have been declining in both diversity and abundance due in large part to habitat degradation and the prevalence of emerging diseases. Although stressors can suppress the immune system, affecting an individual's health and susceptibility to pathogens, established methods for directly collecting stress hormones are not suitable for rapid field use or for use on threatened and endangered species. To overcome these challenges, we are developing an innovative method to collect and measure amphibian glucocorticoid secretions using non-invasive dermal swabs. We tested this methodology using multiple terrestrial, semi-aquatic and fully aquatic species. We swabbed the dorsal side of each animal six times and then induced a stressor of either hand-restraint, ACTH injection, or saline as a control. We then repeated swab collection immediately after the stressor and at 15, 30, 45, 60, 90 and 120 min intervals. Cortisol enzyme immunoassay detected changes in cortisol post-stressor. We also tested this methodology in the field and were successfully able to detect glucocorticoids from multiple species at varying life stages. When using in the field, capture technique should be considered since it may impact stress levels in certain species. Upon further testing, this novel method may be used to greatly increase our understanding of amphibian health especially as disease and environmental changes continue to impact fragile populations.