Singly laid mole salamander (Ambystoma talpoideum) eggs resist mortality from water mold infection

Parasites and pathogens are noteworthy forces in evolutionary and population biology. Water molds are known to infect and kill amphibian eggs, and masses placed in clusters sometimes suffer greater losses. Thus, selection may favor single oviposition behavior in which females scatter individual eggs to avoid contact with dead eggs, thereby lowering the chances of hyphal spread. I tested the hypothesis that eggs in physical contact with dead eggs are more susceptible to death via pathogen infection using a singly laying urodele, the mole salamander, whose congeners mostly lay eggs in masses. Focal eggs were placed in replicated containers with variable egg contact (physically touching or separated) by treatment eggs that varied in status (dead or alive). The effect of egg arrangement was significant, with focal eggs surviving better to hatching when not contacting treatment eggs. Although treatment egg status (dead vs. alive) was not influential on its own, the interaction between egg contact and status was an important determinant of focal egg survival. Only when eggs were physically touching was it important whether they were dead or alive. Moreover, when eggs were touching there was a significant positive association between focal egg survival and the number of surviving treatment eggs. Water mold infected most dead eggs in all treatment groups. This study shows that mole salamander eggs are more likely to die when touching other eggs that have perished, most likely because of a greater chance of water mold infection. Such mortality may have contributed to diversification of egg-laying behavior in amphibians and shows a potentially compounding fitness cost of diminished fertilization success and embryo viability.