Walia Special Edition on the Bale Mountains (2011) - Zoologische ...

Cooperative Breeding
Role of helpers at the den
Studies of wolf social behaviour conducted during 1988-1992 included detailed observations
of behaviour at 20 dens where pups emerged. In those, all wolves helped to rear the litter of the
dominant female, guarding the den, chasing potential predators, and regurgitating or carrying rodent
prey to feed the pups (Sillero-Zubiri 1994). Given the high degree of relatedness among group
members (Randall et al. 2007), subordinate wolves may increase the indirect component of their
inclusive fitness by acting as helpers or, if competition within the group is intense, subordinates
may be induced to help as a ‘payment’ for remaining in the territory. Ethiopian wolf males tend to
help throughout their lives and never disperse; the dominant male at least shared paternity, whereas
subordinate males appear generally to have no probability of fathering the pups, nevertheless they
still help. Subordinate females help more intensely than do males for one or two years before
dispersing or inheriting the breeding position. The balance of costs and benefits to all participants
in a cooperative breeding system have been widely debated (e.g., Solomon and French 1997). One
aspect of the debate is whether groups containing many helpers deliver more food and care to the
young than do smaller groups.
The development of the young wolves is divisible into three broad stages (Sillero-Zubiri
1994). First, early denning (birth to four weeks) when the pups are confined to the den and are
entirely dependent on milk. Second, mixed nutritional dependency (week five to week 11) when
milk is supplemented by solid foods such as rodents provisioned by all pack members until pups are
completely weaned. And third, post-weaning dependency (week 12 to six months) when the pups
subsist almost entirely on solid foods supplied by breeders and non-breeding helpers. Juveniles are
considered independent after six months, when they cease receiving appreciable quantities of food
from adults. Juvenile become subadults and are ‘recruited’ at one year of age.
Although the mother and putative father spend more time at the den on average than do
other wolves, some non-breeders spend more time at the den than do the breeders themselves. The
proportion of time pups were left unattended declined significantly as the number of helpers in the
pack increased. Pack size may thus influence anti-predator behaviour, because baby-sitters were
active in deterring and chasing potential predators. Unattended young might be taken by spotted
hyaenas Crocuta crocuta, domestic dogs, honey badgers Melivora capensis and eagles Aquilla
verreauxi, A.rapax. On the other hand, no evidence has been found that increases in pack size result
in measurable increases in numbers of pups at any age.
In the original 1982-1992 study observations were made of nine wolf packs during the
breeding season to quantify the amount of solid food provisioned to pups. Non-maternal food
provisioning for 17 litters constituted 67% of feedings observed other than nursing. Independent of
the number of donors, there were significant differences in the rate of food contributions per hour
by individuals of different breeding status, sex or age (F = 9.08, P < 0.0001; Table 2). Breeders
(5,119)
contributed significantly more food than did non-breeders, and females more than males. Breeding
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