nonself antigens; therefore female mating preferences may
be influenced by her MHC haplotype and that of her po-
tential mate. Mate choice based on differences in MHC
haplotypes can increase fitness due to decreased inbreed-
ing (Brown 1999), because the MHC acts as an indicator
of overall genetic similarity between mates. Therefore, the
detection of MHC similarity can act to prevent matings
between close relatives. In addition, mates with different
MHC genotypes are more compatible, higher-quality mates,
because their offspring will have a more heterogeneous im-
mune response (Doherty and Zinkernagel 1975; but see
Penn and Potts 1999) and females are less likely to suffer fe-
tal mortality. How the MHC complex affects reproduc-
tion in rodents is discussed more thoroughly in Carroll and
Potts, chap. 5, this volume.
In half of the published studies (N6), female house
mice display odor preferences and mating preferences
for males whose MHC haplotype differs from their own
(table 4.1). These results were not consistently found when
inbred congenetic strains of house mice were tested, nor
were wild female house mice apparently using MHC hap-
lotypes as a basis for mate choice (Eklund 1997; 1999).
Carroll and Potts (chap. 5, this volume) argue that labora-
tory studies do not give the true picture of mate choice
based on MHC haplotypes, and that seminatural enclosures
are better for answering this question. When mice were re-
leased into seminatural enclosures, they did show disassor-
tative mating due to settlement of male-female pairs and ex-
traterritorial matings by females. Extraterritorial matings
by females accounted for approximately 75% of the excess
heterozygotes in the population (Carroll and Potts, chap. 5,
this volume) and were primarily responsible for the defi-
ciency of t/thomozygotes in the populations (Potts et al.
1992). When mice differed at only one MHC locus against
a wild-type background, mating was random rather than
dissasortative. This result strongly suggests that other ge-
netic loci may be important in the expression of olfactory
cues that affect mate choice (Carroll and Potts, chap. 5, this
volume). More studies are necessary to determine whether
or not MHC antigen-presenting alleles are the source of
MHC-based female mate choice.
Choosing when multiple traits are involved
Although most studies of female mate choice focus on a
single trait, investigators recognize the potential complex-
ity of mate choice. A choosy individual will likely consider
more than one trait when assessing the quality of potential
mates (Burley 1981), and therefore the preference displayed
may be a trade-off among these traits. In only a few stud-
ies have investigators designed mate choice tests so that the
results of females’ choices show the relative importance
of various traits. Two studies show that the typical prefer-
ence for dominant males can be altered if another factor is
included. Mihalcin (2002) showed that the preference for
dominant males versus subordinate males changed when
parasite infection was also included as a factor. Mihalcin
found that female house mice preferred odors from subor-
dinate nonparasitized males to those of dominant para-
sitized males. Thus infection status of the male was more
important than dominant status. In another study, female
meadow voles spent more time with males that had good
spatial ability and low dominance ranks as compared to
males with poor spatial ability and high dominance ranks
(Spritzer et al. 2005). These results suggest that spatial abil-
ity may be relatively more important than dominance rank
to females.
Most recently, Roberts and Gosling (2003) showed that
indicators of male quality and mate complementarity can
both have important although different roles in female mate
choice. In some strains of house mice, rate of scent mark-
ing (an indicator of male quality) was a better predictor of
female preference, as measured by time spent with odors,
than was MHC dissimilarity. MHC dissimilarity appeared
to be important only when differences in scent marking
were small. Thus in these experiments each trait signals
separate, sometimes conflicting, adaptive values, forcing fe-
males to trade off between males with different combina-
tions of traits.
These types of studies have significant implications for
sexual selection theory. First, they demonstrate the impor-
tance of female choice in contributing to the maintenance
and variability of sexually selected traits (Roberts and Gos-
ling 2003). These studies also give insight into selection for
the complex decision-making process of females faced with
choosing among males with a combination of traits. A fe-
male’s choice is likely to be determined by the relative fit-
ness benefits resulting from choosing the best male. Finally,
the impact of multiple male traits on female choice may ex-
plain some of the lack of consistency in the results of previ-
ous studies where only one trait was examined.Multiple Mating by FemalesIn general, the hypotheses proposed to explain the func-
tional significance of mating with more than one male
(multi-male mating; MMM) are similar to those proposed
to explain female mate choice (Schwagmeyer 1984; Keller
and Reeve 1995; Yasui 1997, 2001; Jennions and Petrie
2000). These benefits of mating with multiple males include
assurance of fertilization (can include a guard against sperm48 Chapter Four