Data for Seasonal variation mediates group size foraging benefits in a cooperatively breeding rodent

Cooperation and group living has been suggested to facilitate survival in harsh environments and under challenging conditions. However, group living may also be associated with costs, particularly in cooperative breeders, where individuals may compete for limited opportunities to breed. How the costs and benefits of cooperative group living affect cooperative breeders in varying environments is still unclear. Here, we show that in cooperatively breeding Natal mole-rats (<i>Cryptomys hottentotus natalensis</i>) body condition improves with increasing group size during the cold, dry winter season, when the benefits of cooperation are particularly important. In contrast, during the warm, wet summer season, when cooperative foraging may be less important, body condition declines with increasing group size. Additionally, body condition of reproductive females improves with helper number during both seasons. Our data suggests that large group sizes may yield benefits to all group members during environmentally harsh periods and may invoke costs to non-breeders during periods where food is plentiful. Group living in Natal mole-rats is likely promoted by a combination of collective foraging, thermoregulatory benefits and potentially indirect fitness benefits through improved body condition of the breeding female.