Sociality and particularly advanced forms of sociality such as cooperative breeding (living in permanent groups with
reproductive division of labour) is relatively rare among vertebrates. A suggested constraint on the evolution of sociality is
the elevated transmission rate of parasites between group members. Despite such apparent costs, sociality has evolved
independently in a number of vertebrate taxa including humans. However, how the costs of parasitism are overcome in
such cases remains uncertain. We evaluated the potential role of parasites in the evolution of sociality in a member of the
African mole-rats, the only mammal family that exhibits the entire range of social systems from solitary to eusocial. Here we
show that resting metabolic rates decrease whilst daily energy expenditure and energy stores (i.e. body fat) increase with
group size in social Natal mole rats (Cryptomys hottentotus natalensis). Critically, larger groups also had reduced parasite
abundance and infested individuals only showed measurable increases in energy metabolism at high parasite abundance.
Thus, in some circumstances, sociality appears to provide energetic benefits that may be diverted into parasite defence. This
mechanism is likely to be self-reinforcing and an important factor in the evolution of sociality.