Wolf (Canis lupus)

[Sully]

The effect of group size on reproduction in cooperatively

breeding gray wolves depends on density

Abstract


In cooperatively breeding species, large group size is often positively related to

reproductive success and group persistence. We have a poor understanding, how-

ever, of how group sizes within a population affect reproduction particularly as

density varies. We hypothesized that at low densities, wolves in both small and

large groups would have similar reproductive rates. At high densities, however,

wolves in small groups would have lower reproductive rates compared to those in

large groups. Using empirical data from radio-collared wolves in Idaho and Yel-

lowstone National Park, WY, USA (1996–2012), we compared reproductive rates

(i.e. proportion reproducing, litter size, pup survival) among small and large groups

of wolves as density uctuated within the populations. Reproductive rates were

generally lower for individuals in small groups compared to those in large groups,

particularly as density increased. Pup survival, however, was slightly higher for

wolves in small groups compared to large groups except at very high densities.

Polygamy increased with density regardless of group size, suggesting a polygamy

threshold for wolves. Large group size resulted in less parturition failure, more

breeding females per group, larger litter sizes, and ultimately more pups recruited

per group. Large group size appears advantageous for several, but not all, aspects
of reproduction particularly when population density is high. Reply