Sp. Mahoney et al., Potential mechanisms of phenotypic divergence in body size between Newfoundland and mainland black bear populations, CAN J ZOOL, 79(9), 2001, pp. 1650-1660
Citations number
53
Categorie Soggetti
Animal Sciences
Journal title
CANADIAN JOURNAL OF ZOOLOGY-REVUE CANADIENNE DE ZOOLOGIE
Phenotypic variation in body size and degree of sexual size dimorphism of N
orth American black bears (Ursus americanus) was quantified for populations
from New Brunswick, Quebec, Ontario, Maine, Alaska, and the island of Newf
oundland. Based on a model of island biogeography developed by Case, we pre
dicted that body size should be larger in Newfoundland bears than in mainla
nd populations. The presence of few large predators and minimal competition
between herbivore prey on Newfoundland allow an appropriate test of the mo
del (i.e., food availability for bears may differ between populations on th
e mainland and in Newfoundland). In addition, sexual-selection theory predi
cts that the coevolution of polygyny and large size will be coupled with an
increase in sexual size dimorphism. Therefore, we also predicted that amon
g the six populations, male body mass should scale hyperallometrically with
female body mass (i.e., slope > 1). Analysis of deterministic growth curve
s indicated that bears from Newfoundland attained greater asymptotic body s
ize than populations on the mainland, which supports our first prediction.
On average, the relative difference in asymptotic body mass between females
from the island and mainland populations was 55%, while the relative diffe
rence between males was 37%. However, we found that sexual size dimorphism
did not increase disproportionately with body mass among the six population
s, which refuted our second prediction. We discuss a range of abiotic and b
iotic selection pressures possibly responsible for larger body size in Newf
oundland bears. We suggest that the ability to exploit seasonally abundant
and spatially dispersed dietary protein by female and male black bears on t
he island has been and is still a primary environmental factor selecting fo
r large body size in Newfoundland bears. Although the relationship between
sexual size dimorphism and body size is tenuous (slope <= 1), it does sugge
st that (an)other adaptive mechanism(s), opposing sexual selection for extr
eme male size, explain(s) a large amount of the variation in sexual size di
morphism among black bear populations.