Costs and benefits of temporary egg desertion in a rocky shore frog with male-only care

Abstract

Egg attendance imposes costs on parents, including decreased food intake and increased mortality risks. By concentrating parental activities when egg predation is greater and abiotic conditions are less stressful, parents may decrease these costs. Here, we quantify the costs and benefits of temporary egg desertion in the frog Thoropa taophora, whose males care for eggs on rocky shores. We tested hypotheses on the effect of the period of the day (day vs. night) and breeding site (exposed vs. protected from sunlight) on the frequency of temporary desertion, water loss, and egg predation. Using naturalistic observations, we show that parental males deserted their clutches more often during daytime and in exposed sites. Using a field experiment with agar models simulating adult males, we show that water loss was greater during daytime and in exposed sites. Finally, using field observations and a male removal experiment, we show that male presence improves egg survival and that egg predation was higher during the night, with no effect of breeding site. Because the main egg predators are conspecifics, which are mostly inactive when parental males are not attending their clutches, the costs of temporary desertion in terms of egg loss are relatively low when compared with the benefits related to decreased exposure of parental males to stressful abiotic conditions. In conclusion, we predict that temporary offspring desertion should be found especially among ectotherms, when periods of harsher environmental abiotic conditions for the parents are coincident with periods of low offspring demand for protection against predators. To take care of eggs is costly for the parents because they are exposed to predators or stressful environmental conditions. In order to avoid these costs, parents may leave the eggs temporarily unattended. Although temporary egg desertion has been reported for many species, the selective forces favoring its evolution are still poorly understood. Here, we investigated the costs and benefits of temporary egg desertion in a frog whose males care for eggs on rocky shores, an extreme environment for animals with permeable skin. Our findings lead us to conclude that temporary egg desertion should evolve when periods of harsher environmental conditions for the parents are coincident with periods of low offspring demand for protection against predators.