of the major period of maternal investment (viz., weaning
weight) should reflect how close they are to completing de-
velopment to adult body size. Thus the ratios of neonatal
mass and of weaning mass to adult body mass should re-
flect how precocial (as opposed to altricial) offspring are at
the end of these periods of maternal investment. We found
a significant association between our indices of precociality
and scores on the first axis of the PCA for data that were
not adjusted for body mass and phylogeny (table 8.3). Fast
species appeared to have more precocial offspring and slow
species more altricial young. When adjustments were made
to PC1 scores for body mass and phylogeny, however, these
patterns disappeared. Thus the role of precociality as a third
major axis of life history (Gaillard et al. 1989) was not sup-
ported. In their discussion of mammalian Orders, Gaillard
et al. (1989) suggested that more fully developed young
(precocial species) should have slow life histories. Our mea-
sure of how close young are at birth and weaning to adult
size placed precocial species (viz., those with relatively large
young) at the fast end of the continuum, but only when data
were not adjusted for body size and phylogeny. This result
could be due to a difference between comparing Orders of
mammals and species within the Rodentia, or due to our
more specific definition of precocial.
The proportion of the life cycle of rodents that is spent
in the prereproductive period, measured by a/v, was not as-
sociated with the fast-slow continuum, as reflected by either
unadjusted life histories or when adjustments for body
mass and phylogeny were made (table 8.3). Surprisingly,
this aspect of the timing of life history was most closely as-
sociated with the second PC of the adjusted (for body mass
and phylogeny) data set, an axis that primarily reflected the
magnitude of fecundity (though the association of a/vand
monly approached statistical significance). Rodents with
long developmental periods relative to their lifespan are
those with high levels of fecundity. If the first principal com-
ponent of the life cycle represented the fast-slow continuum,
then this pattern of long development and high fecundity is
statistically independent of the continuum.
SummaryWe examined whether rodent species exhibit a fast-slow
continuum of life histories via analyses of life table data. We
also examined whether a tertiary axis of rodent life history
reflects a dichotomy between altricial and precocial species.
Life histories were encapsulated by five key measures of
populations: age at maturity, reproductive lifespan, juvenile
and adult survival, and fecundity. Precociality was mea-
sured by how close neonatal and weaned rodents were to
their adult body mass. We used principal component analy-
ses to quantify the fast-slow continuum, both without and
with statistical adjustments for body mass and family-level
phylogenetic relationships. If the PCA reflected the fast-
slow continuum, then high, positive factor loadings were
expected for variables that reflect the tempo of life history:
age at maturity, reproductive life span, and juvenile and
adult survival.
We found that for rodents, the first principal component
of a PCA may provide an excellent reflection of the empiri-
cally observed fast-slow continuum of mammalian species.
This reflection of the fast-slow continuum appeared to hold
when influences of body size and phylogeny were at least
partly statistically controlled. Scores on the first PC might
provide the best measure of where species occur along the
fast-slow continuum, since our PCAs combined influences
of the key life-history traits that make up the continuum.
Rodents appear to vary in life cycle, from those that live
short lives to those with long lives. Reproductive success,
measured as offspring produced over a set interval time, ap-
pears to vary from high to low, respectively, with the con-
tinuum, in unadjusted data. But reproduction varied inde-
pendently of the continuum in data that were statistically
adjusted for body mass and phylogeny. The degree of pre-
cociality that species exhibit also appears to vary along the
fast-slow continuum, with fast species exhibiting greater
precociality. This pattern, however, was only significant
when influences of body size and phylogeny were allowed
to exhibit their natural variation.Fast and Slow Life Histories of Rodents 105