Effects of larval density on hematological stress indices in salamanders

In animals with complex life cycles, the quality of juvenile environments is important in shaping the longer-term fitness of individuals. Larval density is a major factor governing quality of larval environments in amphibians, with high densities leading to reduced growth rates, smaller size at metamorphosis, and potentially long-lasting postmetamorphic effects. A little-studied effect of larval density is its impact on physiological stress of postmetamorphic individuals. We used a hematological approach, involving counts of specific white blood cells types (neutrophils and lymphocytes) that covary with corticosterone, to estimate stress levels in recently metamorphosed spotted salamanders (Ambystoma maculatum) that were reared in three different larval densities in outdoor mesocosms. In replicated treatments consisting of 12, 25, or 50 larvae, survival was, as expected, lowest and size at metamorphosis smallest in the highest density mesocosms. In addition, surviving salamanders from high-density treatments had significantly higher neutrophil to lymphocyte ratios, indicative of high levels of stress hormones (corticosterone). This trend was not a result of density-related differences in body condition as these did not vary with density. Further, estimated stress levels were similar regardless of whether the salamanders metamorphosed early or late, suggesting that the density effect on stress is long-lasting even once realized density has been reduced through mortality or early metamorphosis. These results may be important in understanding amphibian population dynamics, since research on other vertebrate taxa demonstrates that high hematological stress indicators lead to reduced growth, survival, and increased disease susceptibility in vertebrate animals.