T
he family bathyergidae(African mole-rats)
are subterranean rodents that range in sociality from
species that are strictly solitary to the naked mole-
rat, arguably the most social of all rodents. This chapter
will review our current understanding of social evolution in
the Bathyergidae within the framework of phylogeny, bio-
geography, and speciation. Beginning first with a brief re-
view of their palaeontology, we will use this in combination
with molecular phylogenetic data to build a picture of the
adaptive radiation of the family. With reference to both
inter- and intraspecific comparisons, we will explore the ev-
idence for a relationship between sociality, and ecological
constraints, focusing in particular on the possible role of
habitat aridity and food distribution.
Species Diversity in the Bathyergidae
Species descriptions and distributions
Despite their cryptic nature, many bathyergid holotypes
were named and described by naturalists in the early days
of African exploration. Summarizing these studies, Eller-
man (1940) listed a total of sixty-two species in five genera,
as follows: Heterocephalus(n4); Heliophobius(n8);
Georychus(n3);Bathyergus(n3);Cryptomys(n
49). Recent molecular phylogenetic studies of both nuclear
and mitochondrial genes have produced congruent trees that
support the high species diversity of these early descriptions
(e.g., Allard and Honeycutt 1992; Faulkes, Bennett et al.
1997, Faulkes et al. 2004; Huchon et al. 1999; Walton et al.
2000; Ingram et al. 2004). Figure 36.1 gives approximate
ranges for the five extant genera of the Bathyergidae.
The solitary mole-rats (Heliophobius, Bathyergus,and
Georychus) are generally restricted to regions of higher pre-
cipitation (greater than 400 mm per annum). Of these, He-
liophobiushas the widest distributional range; it occurs
in the sandy soils of savannas and woodlands of south-
ern Kenya, throughout Tanzania and parts of southeastern
Zaire, through Malawi to central Mozambique (fig. 36.1).
These areas are characterized by a higher annual rainfall,
which on average exceeds 900 mm. The other two gen-
era are much more restricted in their ranges. Bathyergusis
endemic to some coastal areas of South Africa and south-
ern Namibia, whereas Georychusoccurs in disjunct popu-
lations within South Africa (fig. 36.1).
In contrast, the two social genera (Cryptomysand Het-
erocephalus) are found in both mesic and xeric regions. Ex-
tant populations of Heterocephalusare found in the arid
regions of East Africa from the Rift Valley of Ethiopia east-
ward into the north of Somalia, and from east of Lake
Turkana in Kenya eastward to the coast of Somalia, and
south as far as Tsavo National Park in Kenya. These areas
are characterized by low (less than 400 mm per annum) and
unpredictable rainfall, with on average only 4 months per
year having more than 25 mm of rain. This rainfall is ap-
proximately the quantity required to soften the soil at the
depth of foraging tunnels and facilitate burrowing (Jarvis
et al. 1994). The Cryptomysgenus is the most speciose and
widely distributed of all the extant bathyergids. Molecular
and morphological data strongly suggest that the genus
should be considered as two distinct lineages, the Crypto-