The influence of temperature and spatial distribution on the susceptibility of southern leopard frog eggs to disease.

Since host defenses to infectious disease are often costly, one would expect hosts to use their defenses only when the threat of infection is high. Southern leopard frogs (Rana sphenocephala) at Ellenton Bay in South Carolina (USA) have an extended breeding season and their eggs are exposed to a wide range of temperatures depending on the time of year when they are laid. Adults aggregate their egg masses in cold temperatures, but separate them in warm temperatures. The spatial aggregation of egg masses may insulate eggs from cold temperatures, but may also affect the transmission of pathogens between the eggs. I examined the effects of temperature, pathogens, and spatial distribution on the survival of R. sphenocephala in the egg stage. Field observations found that temperature had little effect on the number of infected eggs within egg masses, but that egg masses in colder water were more likely to be aggregated together. In a controlled laboratory experiment, the presence of aquatic oomycetes led to higher mortality in cold temperatures than they did at warmer temperatures. Infectious disease may be a selective force that favors R. sphenocephala adults that aggregate their offspring when the threat of disease is high. Host aggregation can reduce the risk of infection when pathogens have a slow generation time that hinders them from responding to an abundance of hosts.