Potential mechanisms of phenotypic divergence in body size between Newfoundland and mainland black bear populations

Citation
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
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Animal Sciences
Journal title
CANADIAN JOURNAL OF ZOOLOGY-REVUE CANADIENNE DE ZOOLOGIE
ISSN journal
0008-4301 → ACNP
Volume
79
Issue
9
Year of publication
2001
Pages
1650 - 1660
Database
ISI
SICI code
0008-4301(200109)79:9<1650:PMOPDI>2.0.ZU;2-E
Abstract
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.