T
he characteristics ofa species’ life cycle, or
life history, are typical patterns of growth, reproduc-
tion, and survival. Such traits vary greatly among
species (Roff 1992; Stearns 1992). The study of life-history
traits is important to ecology because changes in these traits
reveal the ways in which populations respond to changes in
environment. Life-history traits are important to the study
of evolution because changes in these traits represent fitness
differences of individuals, which result in genetic change
within populations. Studies of the evolution of life-history
traits combine ecological and evolutionary influences to re-
veal how particular life histories evolved and how popula-
tions can be expected to respond to future changes in the
environment.
Body size has proved to be a controversial characteristic
in interspecific studies of life histories. Early modeling of
the evolution of life histories considered body size to be an
important life-history trait (MacArthur and Wilson 1967).
In this context, body size was found to be associated with
a suite of life-history traits, and the suite was interpreted
in an adaptive context that compared species that often ex-
hibit high population growth rates (r-selected species) to
species exhibiting relatively little fluctuation in population
size (K-selected species). Subsequent research suggested that
body size was not a covarying life-history trait, but a causal
influence on life histories that could explain the r-Kcontin-
uum (Western 1979; Western and Ssemakula 1982; Peters
1983; Schmidt-Nielson 1983; Calder 1984). Evolutionary
causes of interspecific patterns of body size were not di-
rectly addressed, but processes like competition might rea-
sonably be invoked as an explanation (e.g., Brown 1995).
Comparisons of life histories among species might reveal
evolutionary patterns and environmental circumstances
associated with those patterns (e.g., Gaillard et al. 1989,
2000). In searching for evolutionary patterns of life histo-
ries, Read and Harvey (1989) suggested that mortality pat-
terns and adaptive responses to them provide the best way
to identify meaningful life-history patterns. In their view,
body size is an obfuscating characteristic that needs to be
controlled statistically before adaptive patterns of life histo-
ries can emerge. When influences of body size are removed,
associations of life-history traits are revealed that reflect
adaptations to relatively greater mortality, particularly dur-
ing the prereproductive period (Promislow and Harvey
1990, 1991). This analysis produces a continuum from spe-
cies that have relatively shorter lives to those with longer
lives, and other life-history traits like reproduction adjust
to this fast-slow continuum via tradeoffs. The fast-slow
continuum is an empirical observation, basically the well-
known “mouse to elephant” differences in life span and re-
lated demographic rates that have been noted in mammals.
The continuum remains when differences among species in
adult body mass are removed; thus, body size may not be a
key determinant of this aspect of life histories (Harvey and
Purvis 1999).
In the present review, we consider whether the life histo-
ries of rodent species adhere to the fast-slow continuum. In
studies of mammalian life histories, Oli and Dobson (2003)
and Oli (2004) suggested that the fast-slow continuum
might be best reflected by the ratio of fertility to age at ma-
turity (viz., the F/aratio). At both ends of a scale from high
F/a(fast life cycles) to low F/a(slow life cycles), mammal-