Evolution of African mole‐rat sociality: burrow architecture, rainfall and foraging in colonies of the cooperatively breeding Fukomys mechowii

Abstract

African mole rats (Bathyergidae) offer an excellent system with which to test theories relating to the evolution and maintenance of sociality in mammals. The aridity food distribution hypothesis (AFDH) suggests that, within the bathyergids, sociality has evolved in response to patterns of rainfall, its effects on food distribution, and the subsequent costs and risks of foraging and dispersal. Here, in the first detailed study of burrow architecture in a social mole-rat species, with data from 32 burrows, we show that in the giant mole-rat Fukomys mechowii burrow fractal dimension increases with colony size and is higher during the rainy season than during the dry season. The mass of food in the burrow increases with fractal dimension and is higher during the rainy season than during the dry season. These results link for the first time colony size, burrow architecture, rainfall and foraging success and provide support for two assumptions of the AFDH, namely that (1) in arid conditions burrowing may be severely constrained by the high costs of digging; and (2) the potential risks of failing to locate food may be mitigated by increases in colony size.

Seasonal changes in burrow geometry of the common mole rat (Rodentia: Bathyergidae)

“…We also found no difference in the proportion of foraging tunnels produced in winter compared to summer, which suggests that mole-rats forage during all seasons (Skliba et al, 2009) and may not need to rely on food stores or caches, although there were a greater number of food stores present in the winter burrow systems. In agreement with this observation, Sichilima et al (2008) found fewer food stores in summer than in winter which suggests that mole-rats may use the food stores in spring during the breeding season. The food storage method could be dependent upon food availability as C. h. hottentotus have been found to farm geophytes yet there was no evidence of geophyte farming but rather they tended to create typical food storage areas as in Thomas et al (2009;.…”

“…Previous studies investigating fractal dimensions of mole-rat burrow systems have primarily focused on associations between environmental characteristics (namely seasonal differences) and burrow structure (Spinks & Plaganyi, 1999;Sumbera et al, 2003Sumbera et al, , 2012Sichilima et al, 2008;Thomas et al, 2009;. The current study indicates that relatively low fractal dimensions were found both in winter and summer in a mesic region.…”

“…The greatest amount of habitat exploration and excavation took place in winter with burrow systems being longer and covering a greater area as has been found in, F. mechowii (Sichilima et al, 2008), B. suillus (Thomas et al, 2009) and G. capensis (Thomas et al, 2012). This is most likely because during winter the soil is moist and friable and can be worked easily in comparison to summer and burrows can be extended by the extrusion of mounds .…”

“…Seasonal data with respect to burrow system characteristics are available for three solitary species of mole-rat: Bathyergus suillus (Thomas et al, 2009;, Heliophobius argenteocinereus (Sumbera et al, 2008), and Georychus capensis (Thomas et 4 al., 2012) and for one social species, Fukomys mechowii (Sichilima et al, 2008;Sumbera et al, 2003). The few existing studies investigating burrow architecture and geometry in C. h.…”

“…In this study we investigate how intra-colony size and foraging behaviour (as determined by burrow structure and geometry) in a mesic environment may vary during two seasons, winter (wet) and summer (dry) as part of on-going research into foraging behaviour in bathyergids (Thomas et al, 2009;Sichilima et al, 2008;Thomas, et al, 2012;Le Comber et al, 2002;Romañach & Le Comber, 2004). We predicted that the winter burrow systems of colonies would: (1) be longer and cover a greater area as the soil is more friable and less energetically costly to excavate than during summer; (2) have a higher fractal dimension (an indicator of efficiency of habitat exploration) (Le Comber et al, 2002) as burrowing is tougher and more energetically costly during summer thus it is necessary to forage more efficiently than in winter; (3) have proportionally more mounds than summer burrows (indicating more active burrowing) and (4) contain proportionally more shallow tunnels that are used for foraging than the summer burrow systems.…”