Rodent Societies: An Ecological & Evolutionary Perspective

equally synchronous? In particular, are female black-tailed
prairie dogs less synchronous than female Gunnison’s and
Utah prairie dogs, and might this difference be another rea-
son why mate-guarding is easier for male black-tailed prai-
rie dogs?
Two lines of evidence indicate that female black-tailed
prairie dogs breed less synchronously than female Gunni-
son’s and Utah prairie dogs (Hoogland 1995, 1999, un-
published data). First, even though students and I observe
approximately the same number of estrous females each
year for all three species, the mean interval between the first
and last copulation is consistently about 5 weeks for black-
tailed prairie dogs, but is only 3 – 4 weeks for both Gun-
nison’s and Utah prairie dogs (P0.001 for black-tailed
versus Gunnison’s, P0.001 for black-tailed versus Utah,
and P0.100 for Gunnison’s versus Utah, Mann-Whitney
U test). Second, and more important, females of the same
coterie of black-tailed prairie dogs are less likely to come
into estrus on the same day than are females of the same
clan of Gunnison’s and Utah prairie dogs. For sixty-nine fe-
male black-tailed prairie dogs in 1981, for example, only
12% copulated on a day when another female of the home
coterie also copulated. For seventy-eight female Gunnison’s
prairie dogs in 1994 and fifty-three female Utah prairie
dogs in 2004, by contrast, 27% and 36%, respectively, syn-
chronized estrus on the same day with at least one other
female of the home clan (P0.020 for black-tailed versus
Gunnison’s, P0.001 for black-tailed versus Utah, and
P0.276 for Gunnison’s versus Utah, 2 2 chi-square
test; for all three species, I used data from the year when
students and I observed the highest number of copulations).
As for yellow-bellied marmots (M. flaviventris;Armitage
1986), male and female prairie dogs probably have a con-

flict of interest regarding the optimal number of different

sexual partners per female: the copulating male gains by

precluding matings with other males, but the female prob-

ably benefits from additional matings. Figure 37.4 shows

that neither sex for any of my study species is completely

winning the conflict of interest.

In summary, female black-tailed prairie dogs probably

are less likely than female Gunnison’s and Utah prairie dogs

to copulate with more than one male for two reasons. First,

the enhancement of litter size from second and third cop-

ulations is probably lower for female black-tailed prairie

dogs. Second, because of shorter vegetation, fewer entrances

per mating burrow, and reduced reproductive synchrony,

male black-tailed prairie dogs probably more easily can

monopolize estrous females and thereby deter copulations

by additional males.

Infanticide

For black-tailed prairie dogs, nonparental infanticide af-

fects 39% of litters and is the major cause of juvenile mor-

tality (N130 observed cases). Females are the more com-

mon killers (Nthirty-eight different killers), but males

also kill (Ntwenty different killers; fig. 37.8). By con-

trast, during my 7-year study of Gunnison’s prairie dogs, I

never observed a single unequivocal case of infanticide. For

Utah prairie dogs, infanticide affects 15% of litters, but

only males kill (Nforty-eight cases, involving twenty-five

different killers; fig. 37.8). For both species that show kill-

ing of juveniles, cannibalism usually follows infanticide

(table 37.2). What might account for the striking interspe-

cific differences depicted in figure 37.8? I first will investi-

gate this question for females, then for males.

Alarm Calling, Multiple Mating, and Infanticide among Black-Tailed, Gunnison’s, and Utah Prairie Dogs 447

Figure 37.7 After copulating with a female, black-tailed prairie dog male-05
tries to preclude copulations with other males by sequestering the female in
burrow 40. Photo by John L. Hoogland.

Figure 37.8 Infanticide by male and female prairie dogs.