Reduced Starvation Risks and Habitat Constraints Promote Cooperation in the Common Mole-Rat, Cryptomys hottentotus hottentotus: A Computer-Simulated Foraging Model

We used a simulation model, incorporating empirical data from an arid and a mesic locality, to assess the influence of group size and habitat characteristics on foraging success and energetic returns of the common mole-rat, and thereby gain insight into social evolution within the bathyergids. The most significant finding of the model was that per capita energetic returns for foraging mole-rats decreased with increasing group size, suggesting that there is no simple energetic benefit to be gained from increased colony size; rather, cooperative foraging served to reduce the risks of starvation, particularly in the arid locality. The model suggests that the energetic rewards gained from foraging at a mesic locality are almost three times greater than those gained from an arid locality, where food resources are widely dispersed and patchy. Absolute food availability was less important in determining foraging success than the spatial distribution of food. The food distribution pattern also influenced burrow system architecture in the model, with systems changing from a long and dominantly linear pattern to a shorter, more reticulated pattern as resources become more dense and evenly distributed, Soil hardness was an important determinant of energetic returns in the model as it influenced digging rates and energetic expenditure. Hence the foraging success of mole-rats may be expected to fluctuate as a function of patterns of precipitation. The results indicate that the spatial dispersion of food resources and sporadic rainfall in arid areas interact to elevate foraging costs and reduce energetic returns, ultimately constraining foraging efficiency in the common mole-rat. Increased group size and cooperative foraging lower foraging costs and reduce the risks of unproductive foraging, and as such represent an evolutionarily stable adaptation to foraging and survival in arid areas. Our findings provide persuasive support for the Aridity Food-Distribution Hypothesis as an explanation for bathyergid social evolution.