sume that it must exist if skewed sex ratios are detected.
This is faulty logic.
Is it necessary to present a proximate mechanism of ma-
nipulation to provide support for one of the hypotheses of
facultative manipulation? Again, the answer is “NO,” pro-
vided there is evidence of facultative manipulation. Even
when such evidence is presented, care must be taken to
be able to differentiate among competing explanations. As
Cockburn et al. (2002) point out “literature is littered with
studies described as ‘consistent with’ the prediction of one
hypothesis or another.” These authors go on to note that
the real distinction among predictions of competing hy-
potheses are slight and require focus on key predictions. If
this is the case, then it is the study carefully designed to dis-
criminate among the various hypotheses that will provide
the most insight.
Beyond the need to identify mechanisms and demon-
strate facultative adjustment, we must also carefully con-
sider the limitations to models of sex ratio adjustment. Pen
and Weissing (2002) argue that likely explanations for rela-
tively little progress in understanding sex ratio variation in
vertebrates, as compared to haplodiploid systems, include:
(1) chromosomal sex determination hindering parental ma-
nipulation; (2) longer life spans and overlapping genera-
tions, opening the door for tradeoffs between current and
future reproductive effort; (3) opportunity for both parents
to influence sex ratios (most male influence is likely prior to
gestation); and (4) substantial costs for manipulation if sex
ratios are altered at any stage of development beyond es-
tablishment of the primary sex ratio. These authors call for
development of explicit models that include costs as a nec-
essary step toward a mechanistic theory of sex allocation.
In mammals, the extended period of parental investment
during gestation and lactation can set the stage for parent-
offspring conflict (sensuTrivers 1974) where parents and
offspring have different sex ratios and parental allocation
optima, which further complicate modeling attempts. If we
are to make the most of Pen and Weissing’s (2002) reco-
mmendations, then rodents are a near-ideal mammalian
system to test developing models. They have shorter life
spans than many other mammalian taxa and can be studied
within large enclosures or nest boxes where lifetime repro-
ductive success can be measured. However, if the work pre-
sented by Gosling (1986) is any indication, the differences
in investment of one sex relative to the other may be slight,
and may require long-term data sets to provide the neces-
sary statistical power to demonstrate empirically.
From this review it should be obvious that theories con-
cerning facultative manipulation of sex ratios are well es-
tablished. From these theories we have specific predictions
and their underlying assumptions. We have data from many
different taxa exhibiting skewed sex ratios. We even have
a number of mechanisms that have been shown to result in
skewed sex ratios in various instances. However, we lack a
clear link between theory, prediction, data, and mechanism.
Demonstration of facultative adjustment of parental invest-
ment, according to predictions and in line with necessary
conditions of one of the facultative models of sex ratio the-
ory, will identify systems in which to seek mechanisms for
adjustment. Until such a link is established, it is more par-
simonious to conclude that biased sex ratios result from
causes other than facultative manipulation.SummaryAlthough instances of skewed sex ratios are well docu-
mented in various rodents, our understanding of the causes
of such skews is incomplete. Facultative manipulation of
sex ratios in mammals is of considerable interest as this
topic has been of central importance in understanding fun-
damental principles of other social species, most notably
the hymenopteran insects. Hypotheses proposed for facul-
tative manipulation include differential parental allocation
depending upon maternal condition and cost of offspring of
either sex, differential allocation to minimize competition
for local resources, and advantage of favoring offspring of
one sex over the other at different times within the repro-
ductive season because of resource availability or probabil-
ity of the offspring’s reproduction within that same breed-
ing season. Skewed sex ratios may also arise because of
differential viability of embryos under adverse conditions
experienced by the mother during gestation. Although hy-
potheses for sex ratio adjustment make specific predictions
about the direction of skew expected, empirical data link-
ing predictions and results are few. Most importantly, prox-
imate mechanisms for manipulation of sex ratios have not
been identified and tied to specific instances of biased ra-
tios. A direct linking of theory, proximate mechanisms, and
advantages of sex ratio adjustment over reproductive life-
time is essential before facultative sex ratio manipulation is
more than an intriguing possibility in rodent ecology.138 Chapter Eleven