Assessing Stress Levels of Captive-Reared Amphibians with Hematological Data: Implications for Conservation Initiatives

Abstract

Larval amphibians are increasingly being reared for conservation initiatives to bolster declining populations. Few researchers, however, have asked whether reared individuals are functionally equivalent to their wild counterparts. Compared with those in the wild, amphibians reared in captivity may develop in relatively stress-free environments, because they are usually fed ad libitum, raised in the absence of predators and pathogens and in controlled environments. Thus, with few challenges throughout development, would their resting stress levels or reactions to acute stressors be normal? We addressed this question by rearing Litobates sphenocephalus and Ambystoma opacum from eggs and 10-day-old larvae through to late larval stages in artificial pond environments and by determining their ratios of neutrophils (N) to lymphocytes (L) (two leukocytes that covary with stress hormones) before and after a standardized stressor. We obtained similar samples from wild-caught larvae of equivalent developmental stages and from the same source pond. Baseline and stress-induced N∶L ratios of reared L. sphenocephalus were statistically similar to those of wild individuals. In contrast, baseline N∶L ratios of reared A. opacum were slightly higher than those of wild individuals. In general, the magnitude of the leukocyte response to stress for both species (a 3-fold increase in N∶L over baseline), was similar to that of wild individuals, suggesting that captive-reared amphibians are capable of mounting a normal physiological stress response. Although this last point provides support for the use of captive-rearing for conservation and research purposes, the unusually high baseline N∶L ratios of reared salamanders will require additional research to determine the functional meaning.