Temperature and host density influence susceptibility of 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.  This expectation was tested in the host-pathogen system of the southern leopard frog (Rana sphenocephala) and the pathogenic aquatic oomycetes that infect their eggs.  I examined the effects of temperature, infectious disease, 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 eggs in colder water had more egg masses laid adjacent to them.  In a controlled field experiment, eggs had lower mortality in the presence of pathogens only when they were clumped together with other egg masses.  In the laboratory, the presence of two strains of oomycetes added to egg mortality only at low temperatures.  Infectious disease may be a selective force that favors R. sphenocephala frog adults that aggregate their offspring when the threat of disease is high.