example, S. beecheyican have long active seasons and ex-
tended co-occurrence of mothers and their young (up to
12 months; Dobson and Davis 1986), and thus maternal
behavior may directly or indirectly influence the antipreda-
tor behaviors ofS. beecheyijuveniles more so than responses
of S. beldingijuveniles, which have only a 3 – 4 month de-
velopmental period prior to autumnal immergence (Mor-
ton and Tung 1971; Maxwell and Morton 1975; pers.
obs.). Experiences with mothers may also affect differen-
tially the development of behavior in species with similar
active seasons but different growth rates (e.g., S. beldingi
and S. mollis;Morton and Tung 1971; Rickart 1986), or in
populations with varying active-season lengths (M. T. Bron-
son 1979; Joy 1984; Dobson and Davis 1986). In addition
to physiological (Morton and Tung 1971; Maxwell and
Morton 1975) and social (Armitage 1981) adaptations to
the length of the growing period, then, selection may favor
more experience-dependent behaviors in slowly maturing
species than in species with accelerated growth (fig. 17.3A–
F). This possibility has not yet been examined systemati-
cally in rodents.
Development across the lifespan
Development does not end at puberty — it continues
throughout the life span —yet few researchers have exam-
ined aging in rodents in natural contexts. This is largely be-
cause animals die from extrinsic causes such as starvation
or predation well before they senesce. It is important to
note that senescence is not the same as aging. Aging is the
postmaturation decline in survivorship and fecundity (po-
tential for reproduction) that accompanies advancing age.
Senescence is the bodily changes that cause the decline. Lab
rodents have been used as models for human aging, but this
work rarely considers ecological or evolutionary correlates
of aging. There are some exceptions, however, including
recent work examining how behavioral syndromes (see the
following) influence health outcomes and longevity in rats.
Juvenile rats that are neophobic continue to be so through-
out their lives, have increased stress responses and more
tumors, and die at an earlier age than neophilic rats (Cavi-
gelli and McClintock 2003). This accelerated aging or early
death does not necessarily reduce lifetime fitness, as neo-
phobic animals in the field may actually experience better
health during their reproductive prime, and reduced expo-
sure to predators.
Some rodent species are well suited for studying senes-
cence, in particular to test evolutionary predictions that se-
lection against it would be expected in species with low
rates of mortality (e.g., threat of predation) or high fecun-
dity (Williams 1957). For example, fossorial naked mole-
rats (Heterocephalus glaber) are protected from climatic
changes and most predators, and breeding females can pro-
duce very large litters. As expected by senescence theory,
then, H. glabercan live at least 10 years in the wild and
26 years in captivity, a remarkably long period for a small
rodent (35 g; Braude 2000; Sherman and Jarvis 2002).
Broussard et al. (2003) examined age-related investment in
somatic development and reproduction in female Colum-
bian ground squirrels (S. columbianus), which must allo-
cate energy to both somatic and reproductive investments
during short active seasons. Yearlings that emerge from hi-
bernation at low weight tend to invest more in continued
growth and weight gain rather than reproduction, whereas
some old females (6 years) show a significant decline in
reproductive output relative to other age classes.
As yet, however, there is scant evidence of reproductive
senescence in other free-living rodents. Given the variation
in ecological niches that rodents inhabit, comparative anal-
yses of senescence could take advantage of ensuing vari-
ation in extrinsic sources of mortality, such as presence
of pathogens, predators, and nest sites, similar to Austad’s
(1993) work on aging in mainland and island populations
of opossums. Although rodents are relatively short lived
compared with other mammals, researchers could fruitfully
explore the selective potential for senescence and aging in
sympatric species, for example, or within a species across
populations spread along gradients of elevation or latitude
(e.g., Dobson and Davis 1986). Further, there has been
little focus on changes in social behaviors as animals age.
Some older female rodents will shift their territories to make
room for their reproductive daughters, occasionally mov-
ing to less-optimal spaces (Sherman 1976; Berteaux and
Boutin 2000; pers. obs.). Whether this reflects an inten-
tional bequeath of territory or a lack of competitive ability
in an older animal is unclear, as is the net influence on her
lifetime reproductive success, particularly when consider-
ing the indirect benefits she would receive if her daughters’
reproductive success is improved by inheriting her old site
(see Lambin 1997 for a treatment of territory bequeathal).Endocrine disruptors
Endocrine disruptors, chemicals in the environment that
bind to steroid-hormone receptors and act as agonists or
antagonists of endogenous hormones, have been the recent
focus of both field and laboratory research with rodents.
Even at very low levels (e.g., 1 part per trillion) these disrup-
tors can have profound effects on reproductive functioning,
particularly if exposure occurs during fetal development,
such as when estrogen mimics are present during the or-
ganizational phase of gonadal development. Endocrine dis-204 Chapter Seventeen