Description
Excavating sand is a costly but crucial behavior in subterranean animals such as the social Damaraland mole-rats (Fukomys damarensis). These mammals dwell underground in arid environments, where digging large networks of tunnels are necessary to find food. They are cooperative breeders, living in groups where a pair of dominant individuals monopolizes reproduction. It is still unclear whether sociality evolved in mole-rats due to important ecological constraints on dispersion or through the indirect benefits of cooperative behaviors such as digging. In cooperative societies, non-breeding "helpers" often dedicate less time to cooperative activities as they get older (and larger). This pattern is evident in the digging behaviors of social mole-rats, where older helpers are frequently labeled as "lazy." However, most studies that assess cooperative contributions only consider the time individuals spend on cooperative tasks, overlooking how the quality or efficiency of task performance might change with age or size. In this study, we use a standardized assay of digging performance in captive Damaraland mole-rats to demonstrate that larger individuals are more efficient diggers. In a natural setting, this efficiency should enable larger helpers to extend tunnel systems disproportionately quickly, thereby providing access to subterranean tubers that all group members can consume. Combined with analyses of behavioral time budgets, the performance curves we generate suggest that the contributions of older helpers have been underestimated and may be even more significant if the relative benefits of being larger increase further in more compacted soils. We recommend that future studies of cooperative behavior, in mole-rats and other species, incorporate both time and performance metrics to accurately assess individual contributions.
