dogs and other rodents show that alarm calling is a com-
plicated behavior that defies simple explanation.
Multiple Mating by Females
Like females of other species, female prairie dogs com-
monly copulate with more than one male. Behavioral ob-
servations indicate that 33% of female black-tailed prairie
dogs copulate with two or more males each year, and that
65% of female Gunnison’s and Utah prairie dogs copulate
with two or more males each year (fig. 37.4). Consistent
with these interspecific differences, the minimal frequency
of litters with multiple paternity is only 5% for black-tailed
prairie dogs, but is 77% and 82%, respectively, for Gun-
nison’s and Utah prairie dogs. Some of this disparity might
result because detection of multiple paternity for black-
tailed prairie dogs results from use of starch-gel electropho-
resis (Hoogland 1995), whereas detection for Gunnison’s
and Utah prairie dogs results from use of microsatellites
(Haynie et al. 2003).
If 65% of female Gunnison’s and Utah prairie dogs cop-
ulate with two or more males, how can frequencies of mul-
tiple paternity for both species be 65% (i.e., 77% and
82%, respectively)? Two possible explanations come to
mind. First, for some of the litters that we sampled for mul-
tiple paternity, we had no information on the mother’s
number of sexual partners (Haynie et al. 2003). Second, de-
spite our long hours and best efforts, students and I prob-
ably failed to detect certain copulations. Failure was prob-
ably more likely when a copulation was especially short or
secretive, or when numerous females from the same and
different clans were sexually receptive on the same day.
Why the interspecific differences in the frequency of mul-
tiple mating by females? Figure 37.5 provides one possible
explanation. Female Gunnison’s prairie dogs that copulate
with several males rear larger litters than do females that
copulate with only one male; multiple regression shows that
this effect is real, and does not result simply because heav-
ier, older, more fecund females are more likely to copulate
with several males (Hoogland 1998b). Female Utah prairie
dogs also might rear larger litters by copulating with several
males, but sample sizes to this point are too small for ac-
ceptance or rejection of this hypothesis (fig. 37.5). Female
black-tailed prairie dogs, by contrast, evidently do not en-
hance litter size by copulating with several males (fig. 37.5).
The trends in figure 37.5 thus might explain why female
black-tailed prairie dogs are less likely than female Gunni-
son’s and Utah prairie dogs to copulate with second and
third males. Why incur the risks of copulation if additional
matings do not enhance reproductive success?
How might copulating with several males increase lit-
ter size for prairie dogs? As noted earlier, multiple mat-
ing commonly leads to multiple paternity. Multiple pater-
nity promotes genetic diversity among littermates and thus
maximizes the advantages of sexual reproduction (Williams
1975; Hamilton and Zuk 1982; Seger and Hamilton 1988;
Birkhead and Moller 1992). In an unpredictable environ-
ment that includes diseases and parasites, genetic diversity
might be especially important to fetal and neonatal surviv-
orship. Multiple copulations also might enhance the qual-
ity of offspring via intrauterine sperm competition (Parker
1984b; Madsen et al. 1992; Birkhead et al. 1993; Keller
and Reeve 1995)— so that mortality of fetuses and neonates
is less likely and larger litters at first emergence are more
likely.444 Chapter Thirty-Seven
Figure 37.4 Number of different males with whom female prairie dogs
copulate.
Figure 37.5 Litter size for prairie dogs versus mother’s number of sexual part-
ners. Shown here are means 1 SE; the number above each SE line indicates
the number of females for which I have information on both number of copula-
tions and litter size at first juvenile emergence. P-values are from Spearman rank
correlation test.