Data from: Egg predators improve the hatching success of salamander eggs

A common challenge that oviparous animals face is securing survivorship
during the vulnerable embryonic stage. One of the parental investment
strategies to improve survivorship is providing physical structures to
protect the embryos. In amphibians, there is a notable diversity in
jelly-layer structures surrounding eggs. Previous studies show that these
jelly layers provide eggs with protection against egg predators, egg
pathogens, and desiccation. However, few studies examined the cost–benefit
relationship of the jelly-layer structures. By using the predator–prey
interaction between wood frog (Lithobates sylvaticus) tadpoles and spotted
salamander (Ambystoma maculatum) eggs as a model system, we tested three
hypotheses: (1) having the outer jelly layers would be costly to the
embryos, (2) the relative benefit of the structural egg defense would
become apparent and increase as the intensity of egg predation increases,
and (3) a certain degree of predation would increase the hatching success
of salamander embryos by mechanically thinning the thick outer jelly
layers and increasing oxygen diffusion throughout an egg mass. To test
these hypotheses, we conducted a factorial experiment in which we crossed
four egg-predation levels with two jelly-layer conditions, intact or
removed. We found that the jelly layers were essential in protecting
spotted salamander embryos from wood frog tadpoles but that the associated
cost was apparent in no-predation treatments. The differential
survivorship between intact eggs and eggs without jelly layers showed that
the fitness advantage of jelly layers increased as the level of predation
increased. Finally, the hatching success of intact egg masses was highest
under the high predation conditions. These results imply that the
evolution of the jelly-layer thickness occurred under constant
egg-predation pressure. Given this predator–prey coevolution, egg
predators may play a critical role in improving the hatching success of
salamander embryos under certain conditions.