(Gileva and Federov 1991; Eskelinen 1997). The role of X
chromosomes in reducing local mate competition has been
further criticized based on the argument that the extent of
segregation distortion of Y-bearing sperm is consistent with
random mating, because selection for female-biased sex ra-
tios in response to local mate competition would eliminate
pressure for segregation distortion favoring Y sperm (Bul-
mer 1988). Moreover, in populations subjected to repetitive
inbreeding the female-biased sex ratios tend to disappear
(Jarrell 1995; Gileva 1998). Although its role in minimizing
local mate competition in mammals is unclear, an evolu-
tionary explanation for this mechanism of sex determina-
tion offers a continuing challenge.
With the exception of X sex determination in those spe-
cies listed, no proximate mechanisms for sex ratio adjust-
ments are documented in any of the studies examining lo-
cal resource competition in rodents. Further, meta-analyses
of data on primates suggest that differences reported for
those taxa may not deviate from random expectations, and
hence provide no support for facultative adjustment of sex
ratios in response to local competition (Brown and Silk
2002; but see West et al. 2002 for cautions concerning use
of meta-analyses of such studies). As with data concern-
ing the Trivers-Willard hypothesis, in the absence of mech-
anisms of sex ratio adjustment or evidence that skews are
facultative responses by parents to local conditions, support
for the local resource competition hypothesis in rodents is
compromised.
First cohort advantage hypothesis
Based on his work with Virginia opossums (Didelphis vir-
giniana), Wright et al. (1995) proposed that male-biased
sex ratios early in the reproductive season might occur in a
population if males are more likely than females to breed in
their year of birth. This would be especially true given that
smaller size does not affect female reproductive success as
strongly as that of males in this species, and because males
may have only one year for breeding due to high mortality
rates. Wright et al. (1995) termed this the first cohort ad-
vantage (FCA) hypothesis. While numerous rodent studies
show no seasonal changes in sex ratios (e.g., Havelka and
Millar 1997), others are at least partially consistent with
the FCA predictions (table 11.2). For example, Goundie
and Vessey (1986) observed female-biased spring litters and
male-biased fall litters in Peromyscus leucopusborn in nest
boxes.
It is important to note that the sex ratio trend in rodents
typically is opposite that of opossums, in that either early
litters are female biased and late season litters are male bi-
ased, or both (McShea and Madison 1986; Lambin 1994c;
Bond et al. 2002). The opposite trend in ratios in rodents
as compared to opossums may come about if early born fe-
males in short-season environments are able to get off a lit-
ter in the year of their birth, whereas it is unlikely for males
to do the same. Alternatively, late-born males are more likely
to breed the following season if maturation is delayed until
the spring (Boonstra 1989). McAdam and Millar (1998)
concluded that good-quality femalePeromyscus manicula-
tusin the Kananaskis Valley of Alberta could maximize their
inclusive fitness by producing female-biased litters early in
the season.
Data from Gosling’s (1986) study of naturally repro-
ducing coypus complicates interpretation of seasonal differ-
ences of sex ratios in rodents. In this species males are 15%
larger than females as adults, and thus presumably more
costly to rear. Given the dimorphism in this polygynous
species and the fact that single adult males are dominant to
and at least partially exclude other males within the range
of a group of females (Gosling 1986), it would seem that the
likelihood of males breeding in their natal year is exceed-
ingly remote. To this point there is similarity with the small-
rodent systems described by McShea and Madison (1986),
Lambin (1994b, 1994c) and McAdam and Millar (1998),
and we would expect female-biased early litters. However,
Gosling found male-biased litters in summer months, and
that both litter size and mean number of female embryos
per litter were significantly smaller in younger as compared
to older females. In this case, younger females were produc-
ing more males than females, whereas older females pro-
duced a more even sex ratio. This pattern was a quandary
for Gosling as well, but he had sufficient statistical power
with his data set of ca. 1,500 dissected females to conclude
that skewed sex ratios resulted from abortion of female-
biased small litters in younger mothers, whereas there was
no similar tendency for abortion in older mothers. The
take-home message with regard to seasonal differences in
sex ratios is that although seasonal differences are apparent
in some rodent species, we must carefully consider and dis-
criminate among competing explanations before we can de-
termine the underlying cause.Nonfacultative Adjustment?Most hypotheses have been couched in terms of parental
manipulation of investment. Framing questions in this way
may constrain our interpretation of available data. As has
already been discussed, without identifying a proximate
mechanism for adjustment, or at least evidence that sex
ratio adjustment is facultative, the existence of spatially or
temporally biased ratios is equally consistent with explana-
tions that do not rely on parental manipulation. More re-
cent treatments have backed away from reliance on faculta-136 Chapter Eleven