Color profile: Generic CMYK printer profile Composite Default screen 1650 Potential mechanisms of phenotypic divergence in body size between Newfoundland and mainland black bear populations Shane P. Mahoney, John A. Virgl, and Kim Mawhinney Abstract: Phenotypic variation in body size and degree of sexual size dimorphism of North American black bears (Ursus americanus) was quantified for populations from New Brunswick, Quebec, Ontario, Maine, Alaska, and the island of Newfoundland. Based on a model of island biogeography developed by Case, we predicted that body size should be larger in Newfoundland bears than in mainland populations. The presence of few large predators and mini- mal competition between herbivore prey on Newfoundland allow an appropriate test of the model (i.e., food availability for bears may differ between populations on the mainland and in Newfoundland). In addition, sexual-selection theory predicts that the coevolution of polygyny and large size will be coupled with an increase in sexual size dimorphism. Therefore, we also predicted that among the six populations, male body mass should scale hyperallometrically with female body mass (i.e., slope > 1). Analysis of deterministic growth curves indicated that bears from Newfoundland attained greater asymptotic body size 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 difference between males was 37%. However, we found that sexual size dimorphism did not increase disproportionately with body mass among the six populations, which refuted our second prediction. We discuss a range of abiotic and biotic selection pressures possibly responsible for larger body size in Newfoundland bears. We suggest that the ability to exploit seasonally abundant and spatially dispersed dietary protein by female and male black bears on the island has been and is still a primary environmental factor selecting for large body size in Newfoundland bears. Although the relationship between sexual size dimorphism and body size is tenuous (slope ≤ 1), it does suggest that (an)other adaptive mechanism(s), opposing sexual selection for extreme male size, explain(s) a large amount of the variation in sexual size dimorphism among black bear populations. Résumé : La variation phénotypique de la taille et de l’importance du1660 dimorphisme sexuel de la taille a été quantifiée chez des populations nord-américaines d’Ours noirs (Ursus americanus) du Nouveau-Brunswick, du Québec, de l’Ontario, du Maine, de l’Alaska et de Terre-Neuve. D’après un modèle de biogéographie insulaire élaboré par Case, nous avons prédit que la taille des ours de Terre-Neuve devait être supérieure à celle des ours des populations conti- nentales. La présence limitée de prédateurs de grande taille et la compétition minimale entre les proies herbivores à Terre-Neuve sont des conditions appropriées pour tester le modèle (i.e., la disponibilité de la nourriture peut être diffé- rente chez les populations insulaires et les populations continentales). De plus, la théorie de la sélection sexuelle prédit que la coévolution de la polygynie et d’une grande taille devait s’accompagner d’une augmentation de l’importance du dimorphisme sexuel de la taille. Nous avons donc prédit en outre que, chez les six populations, la masse corporelle des mâles devait être hyperallométrique par rapport à la masse des femelles (i.e., pente > 1). L’analyse des courbes de croissance déterministes indique que les ours de Terre-Neuve atteignent une taille asymptotique supérieure à celle des ours des populations continentales, ce qui vérifie notre première prédiction. En moyenne, la différence relative entre la masse asymptotique des femelles insulaires et celle des femelles des populations continentales a été évaluée à 55 % et la différence relative entre les mâles, à 37 %. Cependant, le dimorphisme sexuel de la taille n’a pas augmenté de façon disproportionnée en fonction de la masse corporelle chez les six populations étudiées, ce qui infirme notre deuxième prédiction. Nous examinons une série de pressions de sélection possibles, abiotiques aussi bien que biotiques, qui pour- raient être responsables de la taille plus grande des ours de Terre-Neuve. Nous croyons que la capacité des ours mâles et femelles d’exploiter des sources saisonnières abondantes et éparses de protéines alimentaires dans l’île a été et demeure le facteur environnemental déterminant de la sélection en faveur d’une grande taille chez les ours de Terre- Neuve. Bien que la relation entre le dimorphisme sexuel de la taille et la taille elle-même soit ténue (pente ≤ 1), elle Received August 16, 2000. Accepted July 31, 2001. Published on the NRC Research Press Web site at http://cjz.nrc.ca on September 7, 2001. S.P. Mahoney. Wildlife Division, Department of Forest Resources and Agrifoods, P.O. Box 8700, Building 810, St. John’s, NF A1B 4J6, Canada John A. Virgl.1 Ecological Developmental and Statistical Analysis, 222 Haight Place, Saskatoon, SK S7H 4W2, Canada. Kim Mawhinney. Parks Canada, 1869 Upper Water Street, Halifax, NS B3J 1S9, Canada. 1Corresponding author (e-mail: [email protected]). Can. J. Zool. 79: 1650–1660 (2001) DOI: 10.1139/cjz-79-9-1650 © 2001 NRC Canada J:\cjz\cjz79\cjz-09\Z01-122.vp Thursday, August 30, 2001 11:43:14 AM Color profile: Generic CMYK printer profile Composite Default screen Mahoney et al. 1651 indique tout de même qu’un ou plusieurs autres mécanismes évolutifs qui s’opposent à la sélection sexuelle favorisant le gigantisme des mâles expliquent une grande partie de la variation du dimorphisme sexuel de la taille chez les popu- lations d’ours noirs. [Traduit par la Rédaction] breeding system (Ralls 1977). For example, in monogamous Introduction Mahoney et al. species, individuals are typically small to medium-sized, male Within mammalian species, body size typically varies across parental investment can be high, and there is little or no sexual latitudinal and longitudinal gradients (McNab 1971; Ralls size dimorphism. Conversely, extreme polygyny is associated and Harvey 1985; Geist 1987; Brown 1995). Explanations with large body size, minimal male parental investment, and for the observed latitudinal pattern in body size include ad- a high degree of sexual size dimorphism. Comparative studies aptations for temperature, primary productivity, seasonal un- that regress male body mass on female body mass among predictability of food resources, and prey size. While the primate species have generally shown that male body mass classic correlation between increase in body size and decrease scales hyperallometrically (i.e., slope > 1; Fairbairn and in temperature (Bergmann’s rule) is controversial (Geist 1987), Preziosi 1994) with female body mass (Clutton-Brock et al. a number of studies do support this hypothesis (McNab 1971; 1977; Leutenegger 1978). Sexual selection for large body Burnett 1983; Owen 1989; Quin et al. 1996). Alternatively, size in males, and the associated advantage in terms of increased Rosenzweig (1968) demonstrated that size in mammalian mating opportunities, appears to be the primary mechanism carnivores was explained more by primary productivity than that explains the variation in sexual size dimorphism be- by temperature, and suggested that highly productive envi- tween monogamous and polygynous species (Fisher 1958; ronments should select for larger body size. Boyce (1979) Clutton-Brock et al. 1977; Ralls 1977; Leutenegger 1978). linked increasing latitude with primary productivity and sea- Therefore, an increase in body size for polygynous species sonality, and predicted that variability in food resources should be correlated with an increase in the degree of sexual should select for longer fasting endurance, which is posi- size dimorphism. However, environmental factors, such as tively correlated with body size. Finally, given the relation- spatial and temporal variation in availability of high-quality ship between maximum prey size and predator body size food resources, availability of receptive females, and length (Schoener 1969; Vézina 1985), an increase in prey size with of the mating period, acting on both female and male body latitude may also be coupled with an increase in predator size can constrain sexual selection for increasing size in size (Ralls and Harvey 1985). All of these environmental males (Fisher 1958; Clutton-Brock et al. 1977; Ralls 1977). factors, operating through evolutionary time and space, have We investigated geographic variation in body size and sex- likely contributed to the patterns of body-size variation in ual size dimorphism in North American black bears (Ursus species (Gould 1996). americanus) from five populations on the mainland and the Studies have also shown a link between body-size varia- population on the island of Newfoundland. Bears on New- tion and biogeographical isolation, with insular populations foundland have coexisted with wolves (Canis lupus) and car- often being larger or smaller than mainland populations ibou (Rangifer tarandus) since the end of the Wisconsin ice (Foster 1964; Case 1978). While lagomorphs, ungulates, foxes, age, except during the last 80 years, when wolves have been raccoons, and snakes tend to be relatively smaller on islands, extirpated from the island (Dodds 1983). Moose (Alces alces), other groups such as cricetid rodents, bears, and iguanid liz- which were introduced around the turn of the