not appear to play any significant role in diseases such as
plague (Yersinia pestis). Mice transmit Salmonellabut in
general appear to be genetically resistant to this bacterium,
as measured by the failure of inoculations to cause illness
or death. Further study of the disease resistance of rats and
mice could prove quite significant with regard to the evolu-
tionary history and socioecology of these widespread and
commensal species, because disease /parasite resistance con-
veys advantages in terms of inhabiting areas where there is
regular exposure to particular pathogens.
Summary and Unanswered Questions
Rats and mice are similar with respect to their reproductive
biology, timing of sexual maturation, gestation, and litter
size when adjusting for body size (table 32.1). Both species
also exhibit flexible spatial distributions and mating sys-
tems that are generally polygynous or promiscuous, can live
either commensally or in feral populations, and tolerate
high densities. These traits have undoubtedly contributed to
their worldwide distribution, as well as preadapted them to
laboratory breeding. Their comparison also highlights dif-
ferences however — some stemming from their biology, oth-
ers likely stemming from differences in research efforts. For
example, rat density is usually described in terms of ani-
mals per inhabitant rather than (as for mice) surface area,
and detailed behavioral data on social interaction in natu-
ral or seminatural circumstances are more available for rats
(partly reflecting their greater behavioral repertoire — see
http://www.ratbehavior.org) while life-history data in the wild is
more abundant for mice. Nevertheless, given the broad sim-
ilarity in their reproductive biology, an obvious question is:
why are population explosions more associated with mice
than rats? Research efforts in ecology have so far not re-
solved this deceptively simple question.
An important factor in the ecological success of rats and
mice is the way that they are both able to colonize new en-
vironments; a noticeable difference, however, is that mice
are reportedly attracted to novelty (neophilic), whereas rats
tend to avoid novel objects or foods (neophobic). Although
the rats’ extreme caution is often viewed as a feature of diet
selection, in that it may assist in the development of con-
ditioned aversion, it is worth considering that it may be
linked instead to a general antipredation response. Rat col-
onies, for example, can also react strongly to the presence
of a new food container(fig. 32.4), and Inglis et al. (1996)
and Beck et al. (1988) showed that the pattern of feeding
was not consistent with general diet assessment (see also
Galef, chap. 18 this volume). Moreover, neophobia has a
cost, that of delaying the exploitation of a resource — al-
though this also seems to be subject to environmental plas-
ticity: rat populations can afford to be most neophobic
in stable environments where food is plentiful (e.g., as inComparative Social Organization and Life History of Rattusand Mus 391Figure 32.7 Rats have evolved an innate, and sensible, aversion to cat odors, which is present even among individuals (e.g.,
laboratory rats) that have never seen a cat for generations. However, this can be manipulated by Toxoplasma gondii,a para-
site that can infect rats and that will benefit from finding its way back to its definitive host, the cat. Photo by M. Berdoy.