Social group size influences pathogen transmission in salamanders

Individuals within animal societies are expected to mitigate the costs and enhance the benefits associated with group living. For example, sociality can facilitate the sharing of beneficial microbes among individuals, but can also increase transmission of pathogens, representing a major cost of group living.  We examine the costs of sociality in the California slender salamander (Batrachoseps attenuatus), a terrestrial salamander which naturally forms close social aggregations.  We investigate whether innate sociality (e.g., skin-to-skin contact) increases an individual’s transmission risk of Batrachochytrium dendrobatidis (Bd), a fungal pathogen that emerged throughout the salamander’s range over the last 50 years and has decimated hundreds of amphibian species globally. We found that in captivity, B. attenuatus exhibit random mixing within social groups, resulting in high contact rates and high potential for Bd transmission. Our experimental infection trials resulted in 50% mortality after one month in moist conditions.  In order to test how group size affects pathogen transmission, we manipulated social group size and found a marked effect on the spread Bd among individuals; a single, uninfected individual contracted Bd much more rapidly in larger groups of infected individuals. Surprisingly, this did not translate into a more rapid death rate or higher pathogen infection loads.  Our results show that the innate behavior of group formation represents a per-individual risk of socially acquired directly transmissible pathogens and is magnified in larger social groups. This study highlights one important cost of sociality in terrestrial salamanders and underscores the general susceptibility of social animals to novel invasive pathogens.