Females
Female black-tailed prairie dogs reap at least five benefits
from infanticide (Hoogland 1995): (1) removal of future
competitors from themselves and their offspring, (2) in-
creased sustenance, because infanticide almost always in-
cludes cannibalism, (3) larger area in which to forage, be-
cause victimized females no longer defend a territory
around the home nursery burrow, (4) more help with alarm
calling, defense of the coterie territory, and excavation and
maintenance of burrows, because victimized mothers are
emancipated from maternal duties, and (5) increased safety
of offspring, because victimized mothers are unlikely to kill.
Increased sustenance from cannibalism is probably the pri-
mary benefit of infanticide, but I have not rigorously evalu-
ated the relative importance of different payoffs.
As for mammals in general (Clutton-Brock et al. 1989)
and for ground-dwelling squirrels in particular (Michener
1989), lactation probably is physiologically stressful for
prairie dog mothers. In late winter and early spring, when
new vegetation is still sparse, mothers not only must feed
themselves, but also must produce milk for their offspring.
Perhaps some mothers can only wean a litter if they can ob-
tain crucial additional sustenance — e.g., protein, rare min-
erals —via infanticide and cannibalism sometime during
lactation. If so, then perhaps infanticide by females should
be more common in species in which lactation is more
stressful. To investigate this possibility, I have compared fe-
male body mass during the breeding season versus female
body mass at first juvenile emergences (i.e., just before
weaning). I predicted that lactation would depress body
mass for all three species, and that the reduction would be
most pronounced for black-tailed prairie dogs (the only
species in which females engage in nonparental infanticide).
Figure 37.9 shows an unexpected result: females of all three
species are heavierat first juvenile emergences than during
the breeding season. Female Gunnison’s and Utah prairie
dogs are heavier by 21% and 31%, respectively, however,
but female black-tailed prairie dogs are heavier by only
1%. These results support the notion that lactation is more
stressful for black-tailed prairie dogs than for Gunnison’s
and Utah prairie dogs. If so, the reason might be that vege-
tation in early spring is less abundant for black-tailed prai-
rie dogs than for the other two species (fig. 37.6).Males
Infanticide and cannibalism by male black-tailed prairie
dogs occur after a male invades a new territory with either
preemergent (unweaned) juveniles or emergent juveniles
that have been coming aboveground for 4 weeks; in-
vaders most commonly are yearlings. The invading male
is usually unrelated, or only distantly related, to his victims
(29/39 74%). Infanticides of unweaned juveniles occur
underground, but killings of emergent juveniles frequently
occur aboveground. Killers usually eliminate all juveniles of
a victimized litter (29/39 74%), and such infanticide af-
fects 7% of litters (Hoogland 1995).
For Utah prairie dogs, infanticidal males are residents
of the territory in which killing occurs (48/48 100%),
and killing and cannibalism usually occur aboveground af-
ter a male emerges from a nursery burrow with a live un-
weaned juvenile (45/48 94%). Marauders are of three
types: (a) male that copulated with the victimized mother
(N9), (b) breeding male that did not copulate with the
victimized mother (N19), and (c) nonbreeding yearling
male (N20). Marauders usually kill only one juvenile
within a litter (N22 different killers), but occasionally448 Chapter Thirty-Seven
Table 37.2 Cannibalism for black-tailed, Gunnison’s, and Utah prairie dogs
Does killer consume victim
Species Sex of killer in 90% of cases?
Black-tailed Male Yes?
Female Yes
Gunnison’s Male Males do not kill
Female Females do not kill
Utah Male Yes
Female Females do not kill
SOURCE: Hoogland 1985, 1995, 1999, 2001.
NOTES: Infanticide by male Utah prairie dogs usually occurs aboveground, so verifica-
tion of frequent cannibalism is easy. Infanticide by female black-tailed prairie dogs oc-
curs underground, but a bloody face and other diagnostic signs indicate that cannibal-
ism consistently follows infanticide. When infanticide by male black-tailed prairie dogs
occurs aboveground, cannibalism is sporadic; when males kill underground, diagnostic
signs of cannibalism are common. Figure 37.9 Body mass of female prairie dogs during the breeding season
versus female body mass at first juvenile emergences. Shown here are means
1 SE; the number above each SE line indicates the number of females weighed.
P-values are from Mann-Whitney U test.