Measuring selective pressures on aquatic and terrestrial reproduction using a vertebrate with reproductive mode plasticity

Background/Question/Methods

The movement to land is one of the major transitions in the history of life and led to major diversifications of plants and animals. Essential to this transition was the ability to reproduce out of water. In animals, terrestrial egg-laying has evolved independently in multiple phyla. Hypotheses for this repeated trend include selection from 1) aquatic predation, 2) low oxygen in water, and/or 3) increased developmental rates out of water. Using the Neotropical treefrog Dendropsophus ebraccatus, which has plasticity in reproductive mode and can lay both aquatic and terrestrial eggs, I measured selective pressures on aquatic eggs from oxygen deprivation and aquatic predation and on terrestrial eggs from desiccation and terrestrial predation. Based on environmental conditions, D. ebraccatus makes decisions about aquatic or terrestrial egg-laying, permitting the first ever experimental measures of the ecological factors which select for aquatic and terrestrial reproduction within a single species. I measured weather patterns and mortality of aquatic and terrestrial D. ebraccatus eggs over the first two days post-oviposition, after which embryos can hatch and escape predators with high success.

Results/Conclusions

Under most weather conditions experienced by D. ebraccatus, mortality of terrestrial eggs is lower than mortality of aquatic eggs, meaning that terrestrial reproduction is favored in most environments. However, under certain weather conditions (low rainfall and high air temperature), terrestrial mortality increases to a point where aquatic eggs become favored over terrestrial ones. Furthermore, aquatic predators increased the probability of death 323% over that of oxygen deprivation alone, whereas terrestrial predators only increased the probability of egg mortality by 20% over desiccation risk alone. Thus it appears, at least in D. ebraccatus, that aquatic predation is the strongest selective agent for terrestrial egg-laying, whereas desiccation risk is the greatest selective force for aquatic reproduction. This is the first study to quantify the environmental factors that select for aquatic and terrestrial reproduction in any animal, and provides a framework for understanding historic and current selection for terrestrial reproduction in other lineages of animals.