Hidden beneath small mounds in the Kalahari Desert in southern Africa, Damaraland mole-rats (Cryptomys damarensis, pictured) have developed a remarkable caste system. In the life cycle of these animals, which is spent entirely underground, a single ‘queen’ female mates with one or two unrelated males. The rest of the colony members generally invest their efforts in caring for successive litters of young, hunting for food and maintaining the colony's intricate network of tunnels.
These worker mole-rats are divided into two types: ‘frequent’ and ‘infrequent’ workers, the latter being evidently lazy types that may comprise as much as 40% of the community but do less than 5% of the work. Elsewhere in this issue, M. Scantlebury et al. describe how they have followed up circumstantial evidence for the reasons behind this division of labour, and show that in certain situations the layabouts spring into action (pages 795).
Mole-rat workers are thought to postpone their own reproduction (sometimes indefinitely) because of the difficulties of setting up a new colony in the rock-hard soil. Extensive burrowing, and so the chance of meeting a mate from another colony, is restricted to brief periods, maybe once or twice a year, when heavy rains soften the soil.
Is this when infrequent workers pay their dues? To find out, Scantlebury and colleagues examined individuals they trapped at burrow entrances. By measuring the body fat, daily energy expenditure and resting metabolic rate of several individuals during a dry period and after rainfall, the authors show that the infrequent workers are fatter and expend far less energy than the other workers when it is dry. Following rainfall, however, they display bursts of effort not shown by the other colony members. Scantlebury et al. propose that, by conserving their energy during dry periods and then digging furiously after it has rained, the fat workers have a good chance of dispersing far enough to find a mate.
As the authors point out, funnelling extra resources into a dispersive caste may well be a sensible strategy for the colony as a whole. These apparent layabouts may spend most of their time reaping the benefits of colony life, such as food and protection, without pulling their weight. But they seem to give good returns when it comes to exploiting environmental conditions to ensure long-term survival of the colony's gene pool.