grove and Wissel 1988; Lovegrove 1991; Jarvis et al. 1994;
Faulkes et al. 1997). This model has become known as
thearidity-food-distribution hypothesis(AFDH), and con-
trasts markedly with the proposition put forward by Ran-
dall (chap. 31 this volume), in which scattered food re-
sources appear to restrict group formation in semifossorial
desert rodents.
The AFDH proposes that cooperative breeding and eu-
sociality in African mole-rats evolved as an adaptive re-
sponse to a combination of the patterns of rainfall and the
distribution of food and the subsequent costs and risks of
foraging and dispersal /independent reproduction. Mole-
rats generally burrow when the substrate is softened by
rain, and in H. glaberand C. damarensisheavy rainfall trig-
gers a frenzy of digging activity (Brett 1991; Jarvis et al.
1998). Thus when rainfall is both low and unpredictable,
the opportunities for extending the burrow to search for
food and /or to disperse and form new colonies are limited.
Furthermore, in such habitats the plants are arid adapted.
The consequence of this is that they tend to produce swollen
roots and tubers to store their reserves, and this makes an
excellent food and moisture resource for mole-rats. Such
plants reproduce vegetatively and therefore tend to occur in
patchily distributed and /or widely dispersed clumps com-
pared to those in mesic regions. This increased dispersion of
food increases the risk of unsuccessful foraging when indi-
viduals are blindly excavating energetically costly foraging
burrows. It is suggested that this cost can be offset by the
cooperative foraging seen in social mole-rats. Clearly these
high costs, but potentially large benefits, are very different
from the challenges facing a surface-foraging desert rodent.
Many such semifossorial rodents are folivores and /or grani-
vores and are thus exploiting food resources that are also
often scattered but of low quality, and hence can only sup-
port solitary foragers. Physiological constraints imposed by
surface activity are also different and more variable than
the highly stable environment of the subterranean niche
of mole-rats. The need to meet these different physiologi-
cal demands may also lead to divergence in social behavior
in semifossorial versus subterranean rodents (see Randall,
chap. 31 this volume). One consequence of high diurnal sur-
face temperatures in deserts is that many mammals forage
at night, resulting in a reduction in visual cues of food, al-
though other sensory modalities remain unimpaired. Again,
this contrasts with the subterranean niche, where sensory
cues to food resources are presumably severely limited or
nonexistent, leading to largely blind foraging. In a study of
Damaraland mole-rats, Jarvis et al. (1998) have shown that
initial foraging is indeed done blindly, as tunnels often
missed geophyte-rich areas, burrowing past them as little as
one meter away. Conversely, Heth et al. (2002) argue that
chemosignals from plants (known as kairomones) may at-
tract burrowing mole-rats. They have shown that in T-maze
choice tests performed in captivity, Cryptomys anselliand
Heterocephalus glaberpreferred to dig in soil in which food
plants had previously been growing. Such a preference sug-
gests that the animals were attracted by kairomones from
the plants that were contained within the soil. In the wild
it seems likely that any such effects act over a short range
and possibly after rains have washed chemosignals from
the plants into the surrounding soil. In accordance with the
hypothesis that social species are able to forage more effi-
ciently than solitary species, Le Comber et al. (2002) mea-
sured the fractal dimension of burrows to quantify the com-
plexity of their architecture and found evidence that the
burrows of social mole-rats explore their surrounding area
more thoroughly.
Further evidence in support of the AFDH comes from
comparative studies across the family, which showed con-
vergence in sociality among unrelated taxa in similar hab-
itats. Specifically, three ecological variables were signifi-
cantly correlated with social group size: geophyte density,
the mean number of months per year that rainfall was
greater than 25 mm (the quantity that is estimated as suffi-
cient to penetrate the ground enough to stimulate burrow-
ing), and the coefficient of rainfall variation (Faulkes et al.
1997). Comparative analysis of this kind makes indepen-
dent contrasts with no a priori assumption of the character
state of the common ancestor. Burda et al. (2000) suggest
that if the AFDH is supported we should see evidence of di-
vergence in social structure among related taxa in different
habitats, as well as convergence in behavior among unre-
lated, divergent taxa. Spinks et al. (1998, 2000) have indeed
shown that within a single species (Cryptomys h. hotten-
totus) the social structure varied along an aridity gradi-
ent according to the predictions of the AFDH. Colonies of
this species that occurred in more arid regions had a social
structure more similar to that of the eusocial species: there
was a greater degree of philopatry, and increased reproduc-
tive skew and overlap in generations. In a different study of
C. damarensis,a link between the environment and social
traits in keeping with the AFDH was also apparent. In this
case, the average value for within-colony relatedness be-
tween individuals was found to be greater in more arid
regions when compared to a region of higher rainfall (Bur-
land et al. 2002). Such an observation fits with the hypoth-
esis that philopatry is increased when ecological constraints
on dispersal are higher. When the costs of dispersal are con-
stantly high, offspring will benefit from staying at home un-
til ecological conditions improve and the benefit to cost
ratio is favorable for dispersal (see Nunes, chap. 13 this vol-
ume). It follows that conditions that allow for reproductionAfrican Mole-Rats: Social and Ecological Diversity 433