ligate outbreeder. In this species, a clear physiological block
to reproduction is observed only in nonbreeding females
(Bennett et al. 1996; Molteno and Bennett 2000). As with
the naked mole-rat, ovulation is blocked, although possibly
through a different physiological mechanism, at the level of
the ovary (Bennett et al. 1999). Nonbreeding males are not
physiologically suppressed in the same way as naked mole-
rats; however, they do possess increased proportions of
immature sperm (Maswanganye et al. 1999). Although the
significance of these sperm abnormalities for fertility is un-
clear, nonbreeding males make no attempt to mate with
their female colony mates, presumably as a result of an in-
cest avoidance mechanism, as they are usually close kin.
Both wild and captive colonies in which the breeding fe-
male has died will remain reproductively quiescent (some-
times for years) until a foreign, unrelated, individual be-
comes available or dispersal /fragmentation of the colony
occurs (Jarvis and Bennett 1993, Rickard and Bennett,
1997). However, new genetic studies indicate that in the
wild, conditions exist in which nonreproductive females
may come into contact with unrelated males, even when
they do not disperse from their natal colony. Multiple and
unidentified paternity was found to be widespread, and im-
migrants of both sexes were identified. In addition, un-
related opposite-sex nonbreeders were identified in two col-
onies out of the eighteen studied, yet all the Damaraland
mole-rat colonies studied to date contained a single breed-
ing female (Burland et al. 2004). Incest avoidance alone is
therefore insufficient to maintain the high levels of repro-
ductive skew identified in this species, supporting the ob-
servation of a physiological block among females.
Other social species in the genusCryptomysin which
there is a single breeding female appear to lack a physiolog-
ical block to reproduction in either sex; e.g., the Mashona
mole-rat, (Cryptomys darlingi;Bennett et al. 1997), the Gi-
ant mole-rat (Cryptomys mechowi;Bennett et al. 2000),
Ansell’s mole-rat, (Cryptomys anselli;Burda 1995), and the
common mole-rat (Cryptomys h. hottentotus;Spinks et al.
2000). The kin structure of colonies in these species is as-
sumed to be predominantly the breeding pair and their off-
spring. In such cases, maintenance of reproductive skew
could be achieved by incest avoidance alone. However, de-
tailed genetic data are lacking in all but the common mole-
rat, in which parentage analysis has revealed that while only
a single breeding female was found per colony, both extra-
pair and extra-colony paternity were common (Bishop et al.
2004). The presence of both adult and subadult foreign
conspecifics within colonies means that inbreeding avoid-
ance may not be sufficient to maintain reproductive skew,
and that in the absence of physiological suppression, other
factors now need to be considered.
Clearly, the interplay between many factors has poten-
tially influenced both phenotype and social evolution in the
Bathyergidae. Figure 36.4 displays these in a hierarchical
flow chart, which attempts to link cause and effect among
these factors. While sociality can be seen as an adaptation
to arid environments in subterranean bathyergids, the way
in which ecological constraints may influence social group
formation may vary markedly among different niches in a
given environment. For example, aridity has been shown to
favor a solitary lifestyle in surface-active desert rodents like
kangaroo rats (Randall, chap. 31 this volume).Seasonal aspects of reproduction
In some species of mole-rat, environmental cues as well as
social factors play a role in regulating reproductive and life-
history traits. Many of the cues available to terrestrial ani-
mals normally used by seasonally breeding organisms (e.g.,
changing annual photoperiod) may be precluded, as Afri-
can mole-rats rarely come to the surface. Thus the onset of
breeding may be triggered by other physical cues, such as
changes in temperature, changes in moisture content, or the
associated sudden flush of vegetation associated with abun-
dant precipitation (Bennett et al. 1988).
All of the solitary-dwelling mole-rats of southern Afri-
can breed seasonally. While many of the social species of
bathyergids reproduce throughout the year (Bennett et al.
1991), two exceptions to this are known to occur. The com-
mon mole-rat inhabits a winter rainfall region and rears
young during the southern hemisphere summer (late No-
vember to January; Spinks et al. 1997, 1999). In contrast,
the highveld mole-rat (Cryptomys hottentotus pretoriae)
occurs in a summer rainfall zone and rears the young in the
winter (early June to August; Janse van Rensburg et al.
2002, 2003). One of the challenges that now remains is to
link both seasonal and social effects and examine in detail
the variation in mating patterns, philopatry, and lifetime re-
productive success across the family.SummaryThere is extensive social diversity within the taxonomic di-
versity of the Bathyergidae, with convergent gains and /or
losses of sociality. Inter- and intraspecific studies generally
support an ecological constraints model for the ultimate
cause of sociality, whereby foraging and dispersal risks, in
part resulting from rainfall patterns, lead to natal philo-
patry, cooperation, and skew in reproduction. Sociality
may be maintained at a proximate level by kin selection,
but the high relatedness observed in naked mole-rats is not436 Chapter Thirty-Six