Ann. Zool. Fennici 46: 395–415 ISSN 0003-455X (print), ISSN 1797-2450 (online) Helsinki 18 December 2009 © Finnish Zoological and Botanical Publishing Board 2009 Carnivory is positively correlated with latitude among omnivorous mammals: evidence from brown bears, badgers and pine martens Egle Vulla1, Keith A. Hobson2, Marju Korsten1, Malle Leht3, Ants-Johannes Martin4, Ave Lind4, Peep Männil5, Harri Valdmann1 & Urmas Saarma1* 1) Department of Zoology, Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia (*corresponding authors’ e-mail: [email protected]) 2) Environment Canada, 11 Innovation Blvd., Saskatoon, Saskatchewan, Canada S7N 3H5 3) Department of Botany, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 5, 51014 Tartu, Estonia 4) Department of Plant Protection, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 5, 51014 Tartu, Estonia 5) Centre of Forest Protection and Silviculture, Rõõmu tee 2, 51013 Tartu, Estonia Received 17 Nov. 2008, revised version received 4 May 2009, accepted 16 Mar. 2009 Vulla, E., Hobson, K. A., Korsten, M., Leht, M., Martin, A.-J., Lind, A., Männil, P., Valdmann, H. & Saarma, U. 2009: Carnivory is positively correlated with latitude among omnivorous mammals: evidence from brown bears, badgers and pine martens. — Ann. Zool. Fennici 46: 395–415. Omnivores exploit numerous sources of protein and other nutrients throughout the year, and meat is generally considered a high-quality resource. However, it is unknown if there is any general association between latitude and carnivorous behavior in omnivorous mammals. We examined the relative importance of meat and other dietary components, including anthropogenic food items, in the diet of brown bears (Ursus arctos) in Estonia using conventional scat- and stomach-content analyses as well as stable-isotope (δ15N, δ13C) analyses. When food habits of brown bears in Estonia were compared with those of other populations in central and northern Europe, the propor- tion of animal prey in the diet was positively correlated with latitude. Further compari- son with the data on the diet of two other omnivorous mammals, the European badger (Meles meles) and the European pine marten (Martes martes), provides evidence that increased carnivory towards northern latitudes may be a general adaptation in omnivo- rous mammals. Introduction gies and ecological roles. Compared with mam- mals exhibiting other feeding strategies, omni- Geographic variation in the diet plays an impor- vores are more flexible and can select among tant role in determining the abundance and dis- numerous food items. However, omnivores tribution of mammal species, as well as shaping are not as efficient as herbivores in digesting their evolutionary adaptations, life-history strate- and assimilating plant food or as efficient as 396 Vulla et al. • ANN. ZOOL. FENNICI Vol. 46 carnivores in feeding on animals (Chapman & conflict in itself (Sagøret al. 1997), but may also Reiss 1999). Thus, omnivores must either obtain increase the local density of bears (Fedriani et al. large quantities of food or select high-quality 2001) and reduce their wariness toward humans food items in order to satisfy their nutritional (Swenson 1999). The issue of so-called problem demands. For omnivorous animals, the con- bears is likely to remain serious since much of sumption of different food items is influenced the brown bear range world-wide coincides with by spatio-temporal variation in the availability of areas of high human density. Therefore, detailed potential food items. Omnivorous species occur- knowledge of the brown bear diet and consump- ring over large heterogeneous areas may there- tion of anthropogenic food items is essential in fore be predicted to exhibit geographic variation order to minimize conflicts between bears and in their feeding habits, with locally-available, humans. high-quality food items favored. However, infor- Conventional methods for evaluating the diet mation on geographic variation in the diet of of mammals — based on the analysis of scat or omnivorous mammals living under natural con- stomach contents — are limited when studying ditions is scarce due to the difficulty of identify- omnivores, since these species consume a vari- ing the potentially large number of food items ety of food types that are digested to a differing that are often efficiently digested and reduced to degree. A valuable addition to these approaches is small fragments. a stable-isotope analysis, which allows estimates To date, relatively few studies carried out of dietary inputs in cases where there are isotopi- wide-scale comparisons of the food habits of cally distinct dietary options available. Although widespread omnivorous mammals: two mustel- the use of two stable isotopes allows for deter- ids, the European badger (Meles meles) (Goszc- mination of inputs from only three isotopically zynski et al. 2000) and the European pine marten distinct sources, the recent development of prob- (Martes martes) (Zalewski 2004), being the abilistic models has made it possible to estimate only examples. These studies found that animal- the range of dietary inputs from more than three derived food items were consumed more fre- sources (Phillips & Gregg 2003). For omnivorous quently in northern as compared with southern species like brown bears, which have access latitudes (in the northern hemisphere). However, to a broad range of isotopically distinct foods, it is not known whether a similar trend would be this approach may be particularly useful. There observed in a large omnivorous mammal, such is currently a considerable interest in applying as the brown bear (Ursus arctos); nor did the stable-isotope methods to deciphering the diets previous studies investigate seasonal changes of extant and extinct bears and other wildlife in in the diet composition, which could help us to North America and Europe (Hildebrand et al. understand in greater detail the variables that 1999, Hobson et al. 2000, Felicetti et al. 2003, influence the diet in regions with seasonally vari- Bocherens et al. 2004, Urton & Hobson 2005). able climate. Our study can provide important data regarding The diet of the European brown bear was European bears in this context. well studied in recent decades (Appendix 1). In this study, we analyzed the diet of the The diet composition and the availability of suit- Estonian brown bear population during differ- able sources of food strongly affects brown bear ent seasons and compared it with that of other population productivity (Hildebrand et al. 1999), European populations, where dietary analyses habitat use (Nomura & Higashi 2000), and the have been conducted using the same methods spatial structure of populations (McLoughlin et (see Appendix 1). In particular, we considered al. 2000). In areas with high human population whether there exists a latitudinal gradient of car- density, such as Europe, brown bear ranges often nivory (with a larger proportion of animal food overlap with areas subject to human activity. items in the diet at higher latitudes) in the brown These areas often provide aggregated and easily bear and other omnivorous mammals for which accessible food items for brown bears, such as the data were available. The specific aims of this livestock, grain and fruit. The consumption of study were: (i) to estimate the occurrence and anthropogenic food by bears may be a source of energetic contribution of different food items ANN. ZOOL. FENNICI Vol. 46 • Carnivory is correlated with latitude among omnivorous mammals 397 Fig. 1. Map of Estonia with marked study areas (1 = Lääne-Virumaa, 2 = Ida-Virumaa, 3 = Jõge- vamaa), which together constitute the core area of brown bears in Estonia. to the Estonian brown bear diet using analysis study area was 297–332, giving a population of fecal samples and stomach contents, with density of 33.3 ± 4.7 (mean ± SD) bears per particular attention paid to items of anthropo- 1000 km2 (Estonian Ministry of the Environment genic origin; (ii) to compare the results of these 2005). The densities of potential prey species for conventional analyses with those derived from brown bear in the study area were the follow- the stable-isotope analysis of brown bear hair ing (individuals per 1000 km2): moose (Alces samples; and (iii) to place our results in the alces) 218.1, wild boar (Sus scrofa) 274.5, roe context of previous work on other brown bear deer (Capreolus capreolus) 509.8 and red deer populations in Europe and of other omnivorous (Cervus elaphus) 4.1 (Estonian Ministry of the mammals. Environment 2005). Cereals, apples, and pota- toes were available to foraging bears at wild- boar feeding sites. The density of feeding sites Material and methods in the study area was 0.06 feeding sites km–2. Carcasses of domestic animals (cows, pigs) were Study area occasionally available at informal burial sites. Brown bear scat and stomach samples were col- lected from three counties in north-east Estonia: Fecal and stomach analysis Lääne-Virumaa, Ida-Virumaa and Jõgevamaa (59°N, 26°E) (Fig. 1), which constitute the core A total of 142 bear scats and 12 bear stom- area for the Estonian brown bear population achs were collected by local hunters and project (Valdmann et al. 2001). Bogs and coniferous staff during 2003–2004. The scats were collected forests are characteristic of the eastern part of the according to Dahle et al. (1998). The scats and study area, while an agricultural landscape con- stomachs were kept frozen until further analysis. taining patches of both coniferous and deciduous To investigate seasonal differences in the diet forests dominates in the western part. Forest composition, the activity period of brown bears covers 57% of the study area, while 18.5% is during the year was divided — according to used for agriculture (Estonian Ministry of the Dahle et al. (1998) — into three seasons based Environment 2005). The density of the human on the availability of major food items: spring population is 8.6 individuals km–2.