tiate torpor in early fall (Fitzgerald and Lechleitner 1974;
Rayor et al. 1987; Hoogland 1999)— once again months
after completion of our experiments. Utah prairie dogs also
hibernate. Adult females usually initiate torpor in August or
September, but adult males attain maximal body mass and
initiate hibernation as early as July (Wright-Smith 1978;
Hoogland 2003b, unpublished data)— soon after experi-
ments with the stuffed badgers. Body mass of adult male
Utah prairie dogs when juveniles are emerging from their
natal burrows averages 147% of body mass during the
breeding season; for adult male black-tailed and Gunni-
son’s prairie dogs, by contrast, body mass at first juvenile
emergences averages only 108% and 129%, respectively,
of body mass during the breeding season (fig. 37.3). Like
obese Belding’s ground squirrels (Trombulak 1989), obese
male Utah prairie dogs that are ready to initiate hibernation
cannot run at full midseason speed, and they sometimes trip
and fall when racing toward a burrow.
I hypothesize that male Utah prairie dogs remain silent
in response to badgers in June because obesity makes call-
ing too risky. Specifically, males do not call shortly after
first juvenile emergences because they cannot run quickly,
and therefore might be unable to reach safety after an alarm
call would make them more vulnerable to detection by a
predator. In terms of Hamilton’s (1964) rule, the cost of
calling (i.e., increased vulnerability) is greater than the re-
cipient’s benefit, which is devalued by the coefficient of ge-
netic relatedness (i.e., crb). Data from 2004 support this
hypothesis. As in previous years, students and I introduced
badgers in June 2004, when males were obese. But we also
introduced badgers in April 2004, soon after the breeding
season, when males were still lean and fast. The mean call-
ing frequency of obese males in June (N38) was only
6%, but the mean calling frequency by lean males in April
(N28) was 19% (P0.043, Mann-Whitney U test).
These results indicate that obesity is a proximate factor that
influences alarm calling of male Utah prairie dogs. In ulti-
mate terms, the silence in June of males that are surrounded
by kin remains puzzling.
Increased vulnerability associated with obesity might
explain why a male Utah prairie dog away from a burrow
entrance does not utter an alarm call to warn nearby kin.
But why don’t males call after reaching the safety of a bur-
row entrance, perhaps while partially submerged in the en-
trance? I have no answer for this pertinent question.
Body mass is one easy-to-quantify factor that might af-
fect a prairie dog’s vulnerability to predation, and thus its
probability of giving an alarm call. Perhaps other factors
that are more difficult to quantify also are important. Mi-
nor injuries, weak eyesight, and poor health, for example,
might reduce the probability that an individual will give an
alarm call. Might these other factors that affect vulnerabil-
ity help to explain the extreme variation in alarm calling
that I have observed among individual prairie dogs, and
that other researchers have observed among individuals of
other species?
As noted previously, breeding male prairie dogs consis-
tently have someoffspring in the home territory in June for
our experiments with the badgers. But litters of all three
species sometimes show multiple paternity, and the resident
breeding male is cuckolded whenever another male sires
some of the juveniles in his territory. Might interspecific dif-
ferences in the frequency of multiple paternity help to ex-
plain interspecific differences in the frequency of alarm call-
ing by breeding males? Specifically, is natural selection more
likely to favor calling by breeding males when 75% of
the juveniles in the home territory consistently are offspring
than when only 25% are offspring? If multiple paternity
and cuckoldry are rampant, so that juvenile beneficiaries of
alarm calls by breeding males frequently will include more
non-offspring than offspring, then perhaps natural selection
will favor mechanisms less dangerous than alarm calling
by which breeding males can increase reproductive success.
Consistent with this hypothesis, the calling frequency by
breeding males is highest among black-tailed prairie dogs,
for which the frequency of multiple paternity is lowest, and
the calling frequency is lowest among Utah prairie dogs, for
which the frequency of multiple paternity is highest (see be-
low). Note, however, that the estimated frequencies of mul-
tiple paternity for Gunnison’s and Utah prairie dogs are al-
most identical (77% versus 82%), but calling frequencies
for breeding male Gunnison’s and Utah prairie dogs differ
substantially (37% versus 7%).
In summary, data in this chapter show that kinship is an
important predictor of alarm calling for three species of
prairie dogs, as it is for many other animals. I have identi-
fied two other factors — demography and body mass — that
also affect the frequency of alarm calling. Data from prairieAlarm Calling, Multiple Mating, and Infanticide among Black-Tailed, Gunnison’s, and Utah Prairie Dogs 443Figure 37.3 Body mass of adult male prairie dogs at the beginning of the
breeding season versus body mass of adult males after first juvenile emergences.
Shown here are means 1 SE; the number above each SE line indicates the
number of males weighed. P-values are from Mann-Whitney U test.