Social Evolution in Mole-rats

Naked mole-rats (Heterocephalus glaber) have received a lot of scientific attention because they are the only mammals with a eusocial mating system. Like honey bees, naked mole-rats have colonies with a single breeding “queen,” a few breeding males, and numerous non-breeding “workers” who forage and maintain the complex burrow system. It is not surprising then, that these unusual mammals have received the lion’s share of attention from scientists.

As fascinating as naked mole-rats are, they are but one of 22 species of mole-rats. Interestingly, mole-rats exhibit a very wide range of social behavior, from the eusocial naked mole-rat to species that are completely solitary. This variation in social structure makes them an ideal group for studying the factors that influence the evolution of social behavior in mammals.

Five scientists from the Czech Republic, Germany, and Malawi (Lovy et al., 2012) studied two mole-rat species that live in different ecological habitats in the same region of Malawi. The silvery mole-rat (
Heliophobius argenteocinereus, Figure 1) is a solitary species that lives in high altitude grassland habitats, whereas Whyte’s mole-rat (Fukomys whytei) is social and lives in drier, lower-altitude woodlands. The authors sought to tease out what ecological factors drive the evolution of social behavior.

Silvery_mole-rat_(Heliophobius_argenteocinereus),_by_Chris_Faulkes
Figure 1. A slivery mole-rat (Heliophobius sp). (from Chris Faulkes)

Because mole-rats are fossorial, soil quality and food availability are likely to play important roles in shaping their social systems (Figure 2). Whyte’s mole-rats in Malawi live in harsh habitats where soils are harder and food is relatively scarce. In contrast, silvery mole-rats inhabit cooler grasslands where soils are easier to burrow through and where food biomass was four times greater than in the woodlands.

mole rat graph 2
Figure 2. A principal component analysis showing the relationship between food availability and soil paramters for Heliophobius (NR, North Rumphi; NRa, North Rumphi alluvium; and FL, Fort Lister.) and for Fukomys (J, Jalawe). Open symbols represent burrow systems numbered from lowest to highest altitude. Solid symbols represent locality centroids. (From Lovy et al., 2012)

Although both solitary and social species of mole-rat coexist in the Nyika Plateau, Malawi, there is niche differentiation between silvery and Whyte’s mole-rats. What factors are responsible for separating the niches of these two species? The authors suggest that the solitary species could not survive in the poor, hard soils of the drier woodlands where patchy underground tubers are in short supply.

In addition,
Heliophobius mole-rats, living in afromontane grasslands are subject to colder temperature and consequently have thicker fur and tolerate low temperatures better. Thus, it may be that Fukomys are ill-prepared to compete with Heliophobius at the cooler, higher-altitude grassland sites. In sum, neither food availability, nor soil density alone explain the observed niche differentiation and social structure in these mole-rats. Rather, it is likely to be a combination of thermoregulatory and competitive abilities, perhaps shaped by ecological factors, that explains the evolution of different social systems.

References

Lövy, M., Šklíba, J., Burda, H., Chitaukali, W., & Šumbera, R. (2012). Ecological characteristics in habitats of two African mole-rat species with different social systems in an area of sympatry: implications for the mole-rat social evolution Journal of Zoology, 286 (2), 145-153 DOI: 10.1111/j.1469-7998.2011.00860.x