measure these potentially important inbreeding costs in
mice, studies in Drosophila(Charlesworth and Charles-
worth 1987) were beginning to suggest that competitive
conditions revealed fitness declines that were effectively in-
visible or underestimated in the lab.
To measure the fitness consequences for progeny of full-
sib matings, Meagher et al. (2000) bred wild-caught mice to
create an F2 generation derived from either outbred or full-
sib matings. During these laboratory matings we found ap-
proximately the same 10% effect of inbreeding on litter size
(fig. 5.3) that the two previous studies had demonstrated
(Lynch 1977; Connor and Bellucci 1979). However, when
we measured the adult fitness of these offspring, we found
an additional 500% effect in males! Outbred males had five
times more offspring than inbred males. Figure 5.3 com-
pares the relative decline in reproductive success (fitness) of
inbred animals to outbred animals for the two previous
house mouse studies (which measured only litter size) and
our study, which went on to measure the adult fitness of
those inbred offspring (and outbred controls). The dramatic
fitness declines in inbred males were due both to a 41% re-
duced ability to gain territories and to decreased survival.
Reduced survival rates were particularly important for ter-
ritorial inbred males, where 90% had died by the end of the
experiment, as compared to only 24% of outbred territorial
males. These results suggest that inbred males had difficulty
maintaining territories, as well as gaining them. Significant
fitness declines were also found for inbred females (fig. 5.3),
but these declines were four times smaller than the observed
male declines. We attributed these gender differences to the
fact that males compete aggressively over territories, and
nonterritorial (subordinate) males have little reproductive
success (20%). In contrast, females had no limiting re-
sources. It remains an open question whether the fitness
consequences of inbreeding in females would approach that
of males if they had to compete over critical resources such
as food.
Figure 5.4 shows the relative reproductive success of in-
bred and outbred males over time. This analysis demon-
strates that the relative differences were increasing at the
end of the experiment, indicating that our inbreeding de-
pression estimates were conservative. If we had allowed the
populations to continue to obtain lifetime reproductive suc-
cess measures, the fitness differences between inbred and
outbred animals would have been much larger, because at
40 weeks almost all (90%) of inbred territorial males were
dead, whereas only 24% of outbred territorial males had
died. Male lifetime fitness effects are estimated by extrapo-
lating the slope of our curves between weeks 25 – 40 out to
80 weeks; outbred males would produce 25 times more off-
spring than inbred males. This dramatic 96% fitness decline
suggests that only one generation of full-sib matings is ef-
fectively lethal to inbred sons in the face of competition
from non-inbred conspecifics.Fitness consequences of inbreeding in other rodents,
measured both in lab and nature
There is only one comparable study for rodents where mea-
sures of inbreeding depression were taken both in the labo-
ratory and in natural or seminatural population conditions.
Jiménez et al. (1994) captured wild Peromyscus leucopus
(white-footed mouse) from a study site in Illinois. This wild-
derived outbred colony was maintained for two to three
generations before experimental pairings of either unrelatedSexual Selection: Using Social Ecology to Determine Fitness Differences 63Figure 5.3 Relative fitness of inbred animals compared to outbred controls as
measured by weaning success (inbred divided by outbred fitness). The laboratory
data are from studies by Lynch (1977; dotted line), Connor and Belucci (1979;
dashed line), and Meagher et al. (2000; dash-dotted line). The population data
are all from Meagher et al. 2000; (solid lines) and are broken down by gender
components and the mean of both genders (males females).
Figure 5.4 Comparison of reproductive success over time for inbred and out-
bred males for both caged matings and in seminatural populations. The caged
matings represented equal numbers of the four possible mating types of inbred
and outbred males and females.