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

they moved towards independence. Additionally, juveniles aged 10-12 months had lower survival
in territories with a greater degree of overlap with neighbouring packs, indicating they may be
vulnerable to foraging competition and intraspecific aggression, particularly during the mating
season when territorial incursions increase (Tallents 2007).
Allo-suckling
The most extreme manifestation of cooperative care by Ethiopian wolves involves nursing the
offspring of the dominant female, or allo-suckling (Sillero-Zubiri 1994, Sillero-Zubiri et al. 2004).
Of the 20 successful breeding attempts observed eight dens had a subordinate female acting as allosuckler.
These females, two years-old females or older, often were closely related to the breeder and
at least two showed signs of pregnancy. Allo-suckling obviously has the potential to confer benefits
to infants, and reduce the mother’s energetic costs. For example pups with access to two lactating
females were suckled significantly more often (0.43 ± 0.05 SE bouts per hour versus`0.26 ± 0.03;
t = 2.78, df = 60, P = 0.007). Pups whose mother was assisted by an allo-suckler received a higher
energetic input per capita until weaning (weeks 4 to 18) and enjoyed better survival than did those
nursed by their mother alone. Dominant females apparently benefit from allo-suckling by sharing
the costs of lactation, and thus lowering their per capita suckling frequency, without affecting a
reduction in the pups’ overall milk intake. For a more detailed treatment of energetic contributions
of allo-suckling see Sillero-Zubiri et al. (2004).
Although one might expect up to double the average litter size of pups in those with two dens
that was not the case; mean litter size on emergence from 12 dens with only one nursing female was
5.1 ± SE 0.35, significantly larger than that from eight dens with an allo-suckler, 2.6 ± SE 0.39 (t =
4.88; df = 16; P = 0.0002). Tallents’ data (2007) supports these results, with a lower proportion of
the litter surviving from three to eight months in packs with more adult and subadult females. The
presence of an allo-suckler was associated with distinct social unease in the pack and evident tension
between the dominant and subordinate females, suggesting that female aggression inside the den
may have an influence on pup mortality prior to emergence (Sillero-Zubiri 1994).
The foregoing results raise several interesting puzzles, which we hope the continuing
research of the Ethiopian Wolf Conservation Programme will resolve. First, to the extent that the
allo-sucklers do indeed make a long-term contribution to pup survival, this is initially disguised
by the counter-intuitive earlier effect of litter reduction. Although the helpers in general, and allosucklers
in particular, appear to work assiduously for the well-being of the pups, and notwithstanding
the rather large size of our data set, demonstrating any survival benefit is difficult at best. Perhaps
such benefits are conditional upon circumstances. One intriguing speculation is that males nursed
by two females do well: one such male grew up to acquire the dominant male position in his pack,
another became dominant male in a pack with six adult males, and three survived a rabies epizootic
in which nearly all other pack members perished (Sillero-Zubiri et al. 1996b). Additionally, the
benefit of the presence of an allo-suckler in a pack of Ethiopian wolves might be contingent on
the availability of prey. As suggested above, in a good year, unassisted females may not need help,
but allo-suckler assistance might be important in harsh years. In a scenario where the chances of
<strong>Walia</strong>-<strong>Special</strong> <strong>Edition</strong> on the Bale Mountains 74