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Effect of watercourse characteristics on food-caching behaviour by European beaver, Castor fiber
ANIMAL BEHAVIOUR, 2004, 67, 643e646 doi:10.1016/j.anbehav.2003.07.008
Effect of watercourse characteristics on food-caching behaviour by European beaver, Castor fiber ¨ RA N H ART MA N & A G NETA AX ELSS ON GO
Department of Conservation Biology, Swedish University of Agricultural Sciences (Received 6 August 2002; initial acceptance 11 October 2002; final acceptance 30 July 2003; MS. number: 7435R)
We studied the relations between watercourse depth and width, and the occurrence, time of initiation and finite size of beaver food caches. Water depth was significantly greater by lodges with a food cache than at lodges lacking a food cache. There was no significant difference in watercourse width between the two groups. Date of initiation of the food cache was not related to water depth, but beavers living by wider watercourses started constructing food caches before those living on narrow watercourses. Finite size of food caches was significantly related to watercourse width and time of initiation. Results are discussed in the contexts of behavioural plasticity in overwintering strategies and habitat selection. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
In northern environments, winter is usually a period of food shortage and cold stress for most animals. Watershed freezeup is a problem especially for semiaquatic animals such as beavers (Castoridae), confining them to a life in lodges, burrows and under ice for extended periods. Beavers’ strategy for surviving winter conditions incorporates several physiological and behavioural components: thickening of fur (Scholander et al. 1950), body fat storage (Aleksiuk & Cowan 1969a; Djoshkin & Safonov 1972), construction of insulated lodges or burrows (Stephenson 1969), opportunistic searching for food during winter (Semyonoff 1953; Novakowski 1967), communal huddling (Stephenson 1969; Wilsson 1971) and changes in activity pattern (Semyonoff 1953; Potvin & Bovet 1975). Research has also intensely explored whether beavers have the ability to enter a depressed metabolic state during winter (Aleksiuk & Cowan 1969a, b; MacArthur & Dyck 1990, 1994; Smith et al. 1991, 1994; Bovet 1993). The most conspicuous overwintering behaviour in beavers, however, is the construction of large, partially submerged food caches. Neither the North American species of beaver, Castor canadensis, nor the European species, C. fiber, always builds a food cache. Both species are less apt to build food caches in places with milder climates, such as at lower elevations and lower latitudes. In North America, food cache construction is not frequently seen at low altitudes south of 38(N latitude (Hill 1982). Beavers living at lower
Correspondence: G. Hartman, Department of Conservation Biology, Swedish University of Agricultural Sciences, Box 7002, 750 07 Uppsala, Sweden (email: [email protected]). 0003e3472/03/$30.00/0
elevation in Colorado, where streams rarely become heavily ice covered, do not regularly cache food, but beavers at higher altitudes do (Yeager & Rutherford 1957). The influence of ice conditions on food cache construction has also been reported in studies from Wyoming (Collins 1976) and Virginia (Echternach & Rose 1987) in the U.S.A. and in Switzerland (Stocker 1985). In Voronezh, Belorussia, where average temperature is below freezing from November to March, 20% of beaver settlements lacked a food cache (Djoshkin & Safonov 1972), and in Bø, Norway, which has a similar temperature regime as Voronezh, 25% of the settlements lacked a food cache (F. Rosell, personal communication). In the mild climate of the Rhone valley, France (mean annual temperatureGSD ¼ 17G6(C), beavers do not build food caches (Blanchet 1959). However, Blanchet reported that a beaver pair that was caught there in February and released in Versoix, Switzerland (mean annual temperatureGSD ¼ 9G6(C), built a food cache the following autumn. It has been suggested that falling temperatures during autumn trigger caching behaviour (Semyonoff 1953; Wilsson 1971; Hodgdon 1978). The size of a food cache might also be a reflection of climate. In Voronezh, cache size varied from ‘a dozen twigs’ to 7.5 m3 (mean 4.6 m3) per colony (Djoshkin & Safonov 1972, pp. 58e60), but in the harsher climate of West Siberia, Vasin (2001) observed that beaver colonies store nearly 10 m3 of woody vegetation per member. Other studies have also suggested this connection between colony size and cache size (Djoshkin & Safonov 1972; Easter-Pilcher 1990). However, Osmundsen & Buskirk (1993) found no correlation between cache size and colony size.
643 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
644
ANIMAL BEHAVIOUR, 67, 4
Yet another factor might influence food-caching behaviour. Wilsson (1971) observed that food caches were smaller and less well constructed on small tributaries than on large rivers. In a study of winter activity of beavers in northern Sweden, Lavsund (1989) found that beavers living on a lake and by a 10-m-wide stream built food caches and showed no activity above ice from November to the beginning of May. However, beavers living on a small creek did not build a food cache, but were active above ice gathering food throughout winter, except during periods of very low temperature. Lavsund suggested that the creek provided little room for a sufficient food cache because of the low water depth in combination with a thick ice layer. Based on these findings, our aim in this study was to identify the relations between water depth and watercourse width and the occurrence, time of initiation and size of beaver food caches in southwestern Sweden. METHODS
settlements with a food cache (XGSD ¼ 1:1G0:43 m) and 0.3e1.5 m by those lacking a food cache (0.7 G 0.09 m; Table 1). Watercourse width varied greatly in both groups (settlements with a food cache: range 3e770 m, X GSD ¼ 118G195 m; settlements lacking a food cache: range 3e255 m, XGSD ¼ 52G71 m; Table 1). Watercourse width was correlated with water depth (Pearson correlation: r37 ¼ 0:36, P ¼ 0:02). The lack of independence between water depth and watercourse width complicated the interpretation of statistical results such as assessment of the relative importance of predictor variables. Including both depth and watercourse width in a binary logistic regression (model: c22 ¼ 12:5, P ¼ 0:002) suggests that only watercourse depth was significantly related to cache occurrence (log-likelihood ratio test, log depth: c21 ¼ 11:6, P ¼ 0:0007; log width: c21 ¼ 0:38, P ¼ 0:54). Initiation of food cache-building was evenly distributed from the end of September to the end of November (weeks 38e48). Time of initiation was significantly negatively
Study Area The study was carried out in the vicinity of the town Deje (59(36#N, 13(28#E) in the province of Va¨rmland in Sweden. The area is in the southern part of the boreal zone and is dominated by coniferous forest. The most frequent tree species are Scots pine, Pinus sylvestris, Norway spruce, Picea abies, birch, Betula pubescens and B. pendula, willows, Salix spp., and black alder, Alnus glutinosa. Average temperature is below freezing for 4 months of the year (DecembereMarch). Beavers were exterminated in Va¨rmland during the 19th century and reintroduced in the 1920s (Fries 1940). Average population density in 1992 was estimated at 0.2 beaver settlements/km2 (total county area; Hartman 1994), and at 0.3 beaver settlements/km2 in 1999 (G. Hartman, unpublished data).
Procedure We surveyed the study area on foot during the summer of 1996. We measured water depth by each known beaver settlement 2 m in front of the lodge entrance, three to four times during summer and once in November. Watercourse width at each settlement was measured with a thin rope or calculated from a map if the watercourse was too wide to throw a rope across it. To estimate the time when caching was initiated, we visited all settlements at least once a week from September to freezeup in the last week of November (week 48). We estimated an index of finite food cache volume by measuring maximum breadth, total length, water depth and cache height above the surface at the time of freezeup (breadth ! length ! depth C height above surface). Data were analysed using binary logistic regression and multiple regressions. Skewed data were log-transformed. Relationships were checked for linearity. RESULTS We identified 39 beaver settlements. At 26 of these, beavers built a food cache. Water depth was 0.5e2.3 m by
Table 1. Winter food cache size (m3), water depth outside the lodge (m), watercourse width (m) and week of initiation of food cachebuilding, by beaver settlements in Va¨rmland, Sweden Lodge no. Cache size (m3) Depth (m) Width (m) Week 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
d d d d d d d d d d d d d 4.0 1.2 0.6 9.2 10.5 6.8 1.2 40.5 10.8 * 12.0 20.6 30.9 47.2 45.0 17.6 6.6 49.5 1.2 * 4.7 93.6 * 3.6 36.0 55.2
Asterisks indicate missing values.
0.3 0.4 0.4 0.4 0.5 0.5 0.6 0.6 0.6 0.7 1.0 1.2 1.5 0.5 0.6 0.6 0.6 0.7 0.7 0.8 0.9 0.9 0.9 1.0 1.0 1.0 1.0 1.0 1.1 1.1 1.1 1.2 1.2 1.2 1.3 1.4 1.8 2.0 2.3
3 4 4 7 45 95 4 40 255 12 12 95 95 5 5 12 105 4 4 10 10 70 100 4 91 100 500 590 12 95 170 3 95 100 770 100 3 100 12
d d d d d d d d d d d d d 44 48 48 45 42 43 46 38 39 38 44 39 39 38 39 46 44 40 44 45 45 42 44 48 43 42
HARTMAN & AXELSSON: FOOD CACHING BY BEAVER
related to watercourse width in a multiple linear regression (t ¼ 2:78, P ¼ 0:01), but was not related to depth outside the lodge (t ¼ 0:19, P ¼ 0:85). The fit of the overall regression was poor (R2 ¼ 0:19) but significant (log time ¼ 46:1 2:16 log width þ 0:70 log depth, F2;23 ¼ 3:96, P ¼ 0:033). Thick ice prevented estimation of finite food cache size by three settlements which had to be excluded. Estimates of finite food cache size ranged from 0.6 to 94 m3 (XGSD ¼ 22G5:0 m3 , N ¼ 23). In a multiple linear regression, finite size was significantly, positively related to both watercourse width (t ¼ 3:34, P ¼ 0:003) and water depth (t ¼ 2:14, P ¼ 0:045). The overall model was consequently also significant (log cache size ¼ 0:409 C 0:424 log widthC1:29 log depth, R2 ¼ 0:44, F2;20 ¼ 9:79, P ¼ 0:001). Finite size of the food cache was also significantly related to time of initiation (linear regression: log cache size ¼ 7:20 0:144 time, R2 ¼ 0:54, F1;21 ¼ 27:13, P ¼ 0:0001). DISCUSSION Our results indicate that occurrence, time of initiation and finite size of food caches were related to watercourse characteristics in the population of European beavers that we studied. Water depth in front of the lodge was a strong predictor of occurrence of food caches but a weak predictor of finite cache size. A possible explanation is that sufficient depth will be one of several factors triggering collecting behaviour. However, in those cases where beavers built a food cache, the onset of this behaviour was connected more closely to watercourse width than to depth. One explanation as to why beavers on wide waters seemed to begin constructing food caches earlier and have larger caches may be that wider watercourses (lakes and still water areas in streams) usually have a slower current and thereby freeze sooner, and wider watercourses will generally be ice covered longer than narrower watercourses. However, we did not measure water current in this study. These suggested differences in caching behaviour in relation to water depth and width support Lavsund’s (1989) hypothesis that, if water volume is insufficient to hold a functional food cache, beavers might not even attempt to establish one and may instead rely on other means of winter survival. An alternative explanation is that settlements without food caches may have been temporary settlements of dispersing subadults. However, more than one beaver was observed at some settlements during field work, and several lodges were active in the following years, contradicting this explanation. Our results suggest that, in our study area and among the variables we studied, the occurrence of food caches is related mainly to water depth outside the lodge, but the relative time of initiation and thereby the finite size of food caches is determined mainly by the width of the watercourse. The relation between time of initiation and finite cache size was also observed in a study of North American beaver by Osmundsen & Buskirk (1993). If there is a relation between colony size and size of food cache (Djoshkin & Safonov 1972; Easter-Pilcher 1990;
Vasin 2001), and the results of this study are valid, then one would also predict a relation between colony size and watercourse characteristics. Furthermore, the results imply that beavers may have to move to new sites as colony size increases. This hypothesis has yet to be tested, but it could explain some of the movements of entire families that have been observed in both C. fiber (Wilsson 1971) and C. canadensis (Nordstrom 1972; Svendsen 1989). Although the two beaver species are similar in appearance and behaviour and are therefore often referred to as if they were identical, they may differ in more than chromosome number. In Karelia, where both species occur, North American beavers built lodges and dams more frequently than did European beavers (Danilov & Kan’shiev 1983). This difference in building behaviour might also apply to cache building. Some support for this difference is that beavers in Massachusetts, which has a climate similar to that of Va¨rmland, always build food caches (Busher 1996). There is consensus that beavers in colder climates build food caches more often than beavers in mild climates. Consequently, there is also consensus that food caches are more important and possibly necessary for beavers’ chances of surviving winter in harsh climates. The influence of water characteristics on habitat selection by beavers has been reported (Beier & Barret 1987; Dieter & McCabe 1989; Hartman 1996), but it has not been shown whether the effect of water characteristics on habitat selection depends on latitude. If this were true, one would also expect relatively lower population densities in colder climates simply because the necessity of food caches would make fewer sites suitable. Beavers inhabit a wide climatic range and a variety of habitats. A similar ecological plasticity could be expected for winter strategies in particular, expressed as behavioural differences at the population level as well as at the individual level. The relations suggested by our results should be viewed in the context of all other factors determining winter behaviour of beavers. Several of our cases also deviate remarkably from the pattern. We cannot expect one simple, general description of the winter strategies of beavers. To understand the strategic possibilities and constraints of how beavers cope with winter survival, one should study the question as a multivariate problem, including factors such as climate, species, colony size and composition, fat deposits, inactivity and metabolic rate, availability of food species suitable for caching, availability of alternative food, over-ice activity, and site characteristics including water depth and watercourse width. The ecological plasticity of beavers may arguably show that they are flexible in their behaviour and can alter their strategy at the individual level, depending on these factors.
Acknowledgments We thank Peter Busher, Andrew Kitchener and two anonymous referees for their comments on the manuscript.
645
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ANIMAL BEHAVIOUR, 67, 4
References Aleksiuk, M. & Cowan, I. M. 1969a. Aspects of seasonal energy expenditure in the beaver (Castor canadensis) at the northern limits of its distribution. Canadian Journal of Zoology, 47, 471e481. Aleksiuk, M. & Cowan, I. M. 1969b. The winter metabolic depression in Arctic beavers (Castor canadensis) with comparison to California beavers. Canadian Journal of Zoology, 47, 965e979. Beier, P. & Barret, R. H. 1987. Beaver habitat use and impact in Truckee river basin, California. Journal of Wildlife Management, 51, 794e799. Blanchet, M. 1959. Was machen die ‘Taler-Biber’ an der Versoix? Heimatschutz, 53, 119e123. Bovet, J. 1993. Comment: missed opportunities in the study of activity and body temperature of beaver. Canadian Journal of Zoology, 72, 567e569. Busher, P. E. 1996. Food caching behaviour of beavers (Castor canadensis): selection and use of woody species. American Midland Naturalist, 135, 343e348. Collins, T. C. 1976. Population characteristics and habitat relationships of beavers (Castor canadensis) in northwest Wyoming. Ph.D. thesis, University of Wyoming. Danilov, P. I. & Kan’shiev, V.Ya. 1983. The state of populations and ecological characteristics of European (Castor fiber) and Canadian (Castor canadensis) beavers in the northwestern U.S.S.R. Acta Zoologica Fennica, 174, 95e97. Dieter, C. D. & McCabe, T. R. 1989. Factors influencing beaver lodge-site selection on a prairie river. American Midland Naturalist, 122, 409e411. Djoshkin, W. W. & Safonov, W. G. 1972. Die Biber der Alten und Neuen Welt. Wittenberg Lutherstadt: A. Ziemsen Verlag. Easter-Pilcher, A. 1990. Cache size as an index to beaver colony size in northwestern Montana. Wildlife Society Bulletin, 18, 110e113. Echternach, J. L. & Rose, R. K. 1987. Use of woody vegetation by beavers in southeastern Virginia. Virginia Journal of Science, 38, 226e232. Fries, C. 1940. Ba¨verland. Stockholm: Nordisk Rotogravyr. Hartman, G. 1994. Long-term development of a reintroduced beaver population in Sweden. Conservation Biology, 8, 713e717. Hartman, G. 1996. Habitat selection by European beaver (Castor fiber) colonizing a boreal landscape. Journal of Zoology, 240, 317e325. Hill, E. P. 1982. Beaver. In: Wild Mammals of North America: Biology, Management and Economics (Ed. by J. A. Chapman & G. A. Feldhammer), pp. 256e281. Baltimore, Maryland: Johns Hopkins University Press. Hodgdon, H. E. 1978. Social dynamics and behavior within an unexploited beaver (Castor canadensis) population. Ph.D. thesis, University of Massachusetts.
Lavsund, S. 1989. Winter activity of beaver (Castor fiber) in Sweden. Fifth International Theriological Congress Abstracts, 1, 303. MacArthur, R. A. & Dyck, A. P. 1990. Aquatic thermoregulation of captive and free-ranging beavers (Castor canadensis). Canadian Journal of Zoology, 68, 2409e2416. MacArthur, R. A. & Dyck, A. P. 1994. The search for a metabolic depression in the beaver: ‘lost opportunities’ or red herrings? A reply to J. Bovet. Canadian Journal of Zoology, 72, 569e571. Nordstrom, W. R. 1972. Comparison of trapped and untrapped beaver populations in New Brunswick. M.S. thesis, University of New Brunswick. Novakowski, N. S. 1967. The winter bioenergetics of a beaver population in northern latitudes. Canadian Journal of Zoology, 45, 1107e1118. Osmundsen, C. L. & Buskirk, S. W. 1993. Size of food caches as a predictor of beaver colony size. Wildlife Society Bulletin, 21, 64e69. Potvin, C. L. & Bovet, J. 1975. Annual cycle of patterns of activity rhythms in beaver colonies (Castor canadensis). Journal of Comparative Physiology, A, 198, 243e256. Scholander, P. F., Walters, V., Hock, R. & Irving, L. 1950. Body insulation of some arctic and tropical mammals and birds. Biological Bulletin, 99, 225e236. Semyonoff, B. T. 1953. Beaver biology in winter in Archangel province. In: Translations of Russian Game Reports. Vol. 1. Beaver 1951e1955, pp. 71e92. Ottawa: Canadian Wildlife Service. Smith, D. W., Peterson, R. O., Drummer, T. D. & Sheputis, D. S. 1991. Over-winter activity and body temperature patterns in northern beavers. Canadian Journal of Zoology, 69, 2178e2182. Smith, D. W., Drummer, T. D. & Peterson, R. O. 1994. Reply: studies of beaver activity and body temperature: a historical perspective. Canadian Journal of Zoology, 72, 572e574. Stocker, G. 1985. Biber (Castor fiber) in der Schweiz. Swiss Federal Institute of Forestry Research, 274, page 105. Stephenson, A. B. 1969. Temperatures within a beaver lodge in winter. Journal of Mammalogy, 50, 134e136. Svendsen, G. E. 1989. Pair formation, duration of pair bonds, and mate replacement in a population of beavers (Castor canadensis). Canadian Journal of Zoology, 67, 336e340. Vasin, A. 2001. Beavers of the north of West Siberia. In: Proceedings of the First Euro-American Beaver Congress (Ed. by P. Busher & Y. Gorshkov), pp. 51e60 Kazan: Transactions of Volga-Kama National Nature Zapovednik. Wilsson, L. 1971. Observations and experiments on the ethology of the European beaver (Castor fiber). Viltrevy, 8, 142e144. Yeager, L. E. & Rutherford, W. H. 1957. An ecological basis for beaver management in the Rocky Mountain region. In: Transactions of the 22nd North American Wildlife Conference (Ed. by J. B. Trefethen), pp. 269e300. Washington, D.C.
Effect of watercourse characteristics on food-caching behaviour by European beaver, Castor fiber ¨ RA N H ART MA N & A G NETA AX ELSS ON GO
Department of Conservation Biology, Swedish University of Agricultural Sciences (Received 6 August 2002; initial acceptance 11 October 2002; final acceptance 30 July 2003; MS. number: 7435R)
We studied the relations between watercourse depth and width, and the occurrence, time of initiation and finite size of beaver food caches. Water depth was significantly greater by lodges with a food cache than at lodges lacking a food cache. There was no significant difference in watercourse width between the two groups. Date of initiation of the food cache was not related to water depth, but beavers living by wider watercourses started constructing food caches before those living on narrow watercourses. Finite size of food caches was significantly related to watercourse width and time of initiation. Results are discussed in the contexts of behavioural plasticity in overwintering strategies and habitat selection. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
In northern environments, winter is usually a period of food shortage and cold stress for most animals. Watershed freezeup is a problem especially for semiaquatic animals such as beavers (Castoridae), confining them to a life in lodges, burrows and under ice for extended periods. Beavers’ strategy for surviving winter conditions incorporates several physiological and behavioural components: thickening of fur (Scholander et al. 1950), body fat storage (Aleksiuk & Cowan 1969a; Djoshkin & Safonov 1972), construction of insulated lodges or burrows (Stephenson 1969), opportunistic searching for food during winter (Semyonoff 1953; Novakowski 1967), communal huddling (Stephenson 1969; Wilsson 1971) and changes in activity pattern (Semyonoff 1953; Potvin & Bovet 1975). Research has also intensely explored whether beavers have the ability to enter a depressed metabolic state during winter (Aleksiuk & Cowan 1969a, b; MacArthur & Dyck 1990, 1994; Smith et al. 1991, 1994; Bovet 1993). The most conspicuous overwintering behaviour in beavers, however, is the construction of large, partially submerged food caches. Neither the North American species of beaver, Castor canadensis, nor the European species, C. fiber, always builds a food cache. Both species are less apt to build food caches in places with milder climates, such as at lower elevations and lower latitudes. In North America, food cache construction is not frequently seen at low altitudes south of 38(N latitude (Hill 1982). Beavers living at lower
Correspondence: G. Hartman, Department of Conservation Biology, Swedish University of Agricultural Sciences, Box 7002, 750 07 Uppsala, Sweden (email: [email protected]). 0003e3472/03/$30.00/0
elevation in Colorado, where streams rarely become heavily ice covered, do not regularly cache food, but beavers at higher altitudes do (Yeager & Rutherford 1957). The influence of ice conditions on food cache construction has also been reported in studies from Wyoming (Collins 1976) and Virginia (Echternach & Rose 1987) in the U.S.A. and in Switzerland (Stocker 1985). In Voronezh, Belorussia, where average temperature is below freezing from November to March, 20% of beaver settlements lacked a food cache (Djoshkin & Safonov 1972), and in Bø, Norway, which has a similar temperature regime as Voronezh, 25% of the settlements lacked a food cache (F. Rosell, personal communication). In the mild climate of the Rhone valley, France (mean annual temperatureGSD ¼ 17G6(C), beavers do not build food caches (Blanchet 1959). However, Blanchet reported that a beaver pair that was caught there in February and released in Versoix, Switzerland (mean annual temperatureGSD ¼ 9G6(C), built a food cache the following autumn. It has been suggested that falling temperatures during autumn trigger caching behaviour (Semyonoff 1953; Wilsson 1971; Hodgdon 1978). The size of a food cache might also be a reflection of climate. In Voronezh, cache size varied from ‘a dozen twigs’ to 7.5 m3 (mean 4.6 m3) per colony (Djoshkin & Safonov 1972, pp. 58e60), but in the harsher climate of West Siberia, Vasin (2001) observed that beaver colonies store nearly 10 m3 of woody vegetation per member. Other studies have also suggested this connection between colony size and cache size (Djoshkin & Safonov 1972; Easter-Pilcher 1990). However, Osmundsen & Buskirk (1993) found no correlation between cache size and colony size.
643 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
644
ANIMAL BEHAVIOUR, 67, 4
Yet another factor might influence food-caching behaviour. Wilsson (1971) observed that food caches were smaller and less well constructed on small tributaries than on large rivers. In a study of winter activity of beavers in northern Sweden, Lavsund (1989) found that beavers living on a lake and by a 10-m-wide stream built food caches and showed no activity above ice from November to the beginning of May. However, beavers living on a small creek did not build a food cache, but were active above ice gathering food throughout winter, except during periods of very low temperature. Lavsund suggested that the creek provided little room for a sufficient food cache because of the low water depth in combination with a thick ice layer. Based on these findings, our aim in this study was to identify the relations between water depth and watercourse width and the occurrence, time of initiation and size of beaver food caches in southwestern Sweden. METHODS
settlements with a food cache (XGSD ¼ 1:1G0:43 m) and 0.3e1.5 m by those lacking a food cache (0.7 G 0.09 m; Table 1). Watercourse width varied greatly in both groups (settlements with a food cache: range 3e770 m, X GSD ¼ 118G195 m; settlements lacking a food cache: range 3e255 m, XGSD ¼ 52G71 m; Table 1). Watercourse width was correlated with water depth (Pearson correlation: r37 ¼ 0:36, P ¼ 0:02). The lack of independence between water depth and watercourse width complicated the interpretation of statistical results such as assessment of the relative importance of predictor variables. Including both depth and watercourse width in a binary logistic regression (model: c22 ¼ 12:5, P ¼ 0:002) suggests that only watercourse depth was significantly related to cache occurrence (log-likelihood ratio test, log depth: c21 ¼ 11:6, P ¼ 0:0007; log width: c21 ¼ 0:38, P ¼ 0:54). Initiation of food cache-building was evenly distributed from the end of September to the end of November (weeks 38e48). Time of initiation was significantly negatively
Study Area The study was carried out in the vicinity of the town Deje (59(36#N, 13(28#E) in the province of Va¨rmland in Sweden. The area is in the southern part of the boreal zone and is dominated by coniferous forest. The most frequent tree species are Scots pine, Pinus sylvestris, Norway spruce, Picea abies, birch, Betula pubescens and B. pendula, willows, Salix spp., and black alder, Alnus glutinosa. Average temperature is below freezing for 4 months of the year (DecembereMarch). Beavers were exterminated in Va¨rmland during the 19th century and reintroduced in the 1920s (Fries 1940). Average population density in 1992 was estimated at 0.2 beaver settlements/km2 (total county area; Hartman 1994), and at 0.3 beaver settlements/km2 in 1999 (G. Hartman, unpublished data).
Procedure We surveyed the study area on foot during the summer of 1996. We measured water depth by each known beaver settlement 2 m in front of the lodge entrance, three to four times during summer and once in November. Watercourse width at each settlement was measured with a thin rope or calculated from a map if the watercourse was too wide to throw a rope across it. To estimate the time when caching was initiated, we visited all settlements at least once a week from September to freezeup in the last week of November (week 48). We estimated an index of finite food cache volume by measuring maximum breadth, total length, water depth and cache height above the surface at the time of freezeup (breadth ! length ! depth C height above surface). Data were analysed using binary logistic regression and multiple regressions. Skewed data were log-transformed. Relationships were checked for linearity. RESULTS We identified 39 beaver settlements. At 26 of these, beavers built a food cache. Water depth was 0.5e2.3 m by
Table 1. Winter food cache size (m3), water depth outside the lodge (m), watercourse width (m) and week of initiation of food cachebuilding, by beaver settlements in Va¨rmland, Sweden Lodge no. Cache size (m3) Depth (m) Width (m) Week 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
d d d d d d d d d d d d d 4.0 1.2 0.6 9.2 10.5 6.8 1.2 40.5 10.8 * 12.0 20.6 30.9 47.2 45.0 17.6 6.6 49.5 1.2 * 4.7 93.6 * 3.6 36.0 55.2
Asterisks indicate missing values.
0.3 0.4 0.4 0.4 0.5 0.5 0.6 0.6 0.6 0.7 1.0 1.2 1.5 0.5 0.6 0.6 0.6 0.7 0.7 0.8 0.9 0.9 0.9 1.0 1.0 1.0 1.0 1.0 1.1 1.1 1.1 1.2 1.2 1.2 1.3 1.4 1.8 2.0 2.3
3 4 4 7 45 95 4 40 255 12 12 95 95 5 5 12 105 4 4 10 10 70 100 4 91 100 500 590 12 95 170 3 95 100 770 100 3 100 12
d d d d d d d d d d d d d 44 48 48 45 42 43 46 38 39 38 44 39 39 38 39 46 44 40 44 45 45 42 44 48 43 42
HARTMAN & AXELSSON: FOOD CACHING BY BEAVER
related to watercourse width in a multiple linear regression (t ¼ 2:78, P ¼ 0:01), but was not related to depth outside the lodge (t ¼ 0:19, P ¼ 0:85). The fit of the overall regression was poor (R2 ¼ 0:19) but significant (log time ¼ 46:1 2:16 log width þ 0:70 log depth, F2;23 ¼ 3:96, P ¼ 0:033). Thick ice prevented estimation of finite food cache size by three settlements which had to be excluded. Estimates of finite food cache size ranged from 0.6 to 94 m3 (XGSD ¼ 22G5:0 m3 , N ¼ 23). In a multiple linear regression, finite size was significantly, positively related to both watercourse width (t ¼ 3:34, P ¼ 0:003) and water depth (t ¼ 2:14, P ¼ 0:045). The overall model was consequently also significant (log cache size ¼ 0:409 C 0:424 log widthC1:29 log depth, R2 ¼ 0:44, F2;20 ¼ 9:79, P ¼ 0:001). Finite size of the food cache was also significantly related to time of initiation (linear regression: log cache size ¼ 7:20 0:144 time, R2 ¼ 0:54, F1;21 ¼ 27:13, P ¼ 0:0001). DISCUSSION Our results indicate that occurrence, time of initiation and finite size of food caches were related to watercourse characteristics in the population of European beavers that we studied. Water depth in front of the lodge was a strong predictor of occurrence of food caches but a weak predictor of finite cache size. A possible explanation is that sufficient depth will be one of several factors triggering collecting behaviour. However, in those cases where beavers built a food cache, the onset of this behaviour was connected more closely to watercourse width than to depth. One explanation as to why beavers on wide waters seemed to begin constructing food caches earlier and have larger caches may be that wider watercourses (lakes and still water areas in streams) usually have a slower current and thereby freeze sooner, and wider watercourses will generally be ice covered longer than narrower watercourses. However, we did not measure water current in this study. These suggested differences in caching behaviour in relation to water depth and width support Lavsund’s (1989) hypothesis that, if water volume is insufficient to hold a functional food cache, beavers might not even attempt to establish one and may instead rely on other means of winter survival. An alternative explanation is that settlements without food caches may have been temporary settlements of dispersing subadults. However, more than one beaver was observed at some settlements during field work, and several lodges were active in the following years, contradicting this explanation. Our results suggest that, in our study area and among the variables we studied, the occurrence of food caches is related mainly to water depth outside the lodge, but the relative time of initiation and thereby the finite size of food caches is determined mainly by the width of the watercourse. The relation between time of initiation and finite cache size was also observed in a study of North American beaver by Osmundsen & Buskirk (1993). If there is a relation between colony size and size of food cache (Djoshkin & Safonov 1972; Easter-Pilcher 1990;
Vasin 2001), and the results of this study are valid, then one would also predict a relation between colony size and watercourse characteristics. Furthermore, the results imply that beavers may have to move to new sites as colony size increases. This hypothesis has yet to be tested, but it could explain some of the movements of entire families that have been observed in both C. fiber (Wilsson 1971) and C. canadensis (Nordstrom 1972; Svendsen 1989). Although the two beaver species are similar in appearance and behaviour and are therefore often referred to as if they were identical, they may differ in more than chromosome number. In Karelia, where both species occur, North American beavers built lodges and dams more frequently than did European beavers (Danilov & Kan’shiev 1983). This difference in building behaviour might also apply to cache building. Some support for this difference is that beavers in Massachusetts, which has a climate similar to that of Va¨rmland, always build food caches (Busher 1996). There is consensus that beavers in colder climates build food caches more often than beavers in mild climates. Consequently, there is also consensus that food caches are more important and possibly necessary for beavers’ chances of surviving winter in harsh climates. The influence of water characteristics on habitat selection by beavers has been reported (Beier & Barret 1987; Dieter & McCabe 1989; Hartman 1996), but it has not been shown whether the effect of water characteristics on habitat selection depends on latitude. If this were true, one would also expect relatively lower population densities in colder climates simply because the necessity of food caches would make fewer sites suitable. Beavers inhabit a wide climatic range and a variety of habitats. A similar ecological plasticity could be expected for winter strategies in particular, expressed as behavioural differences at the population level as well as at the individual level. The relations suggested by our results should be viewed in the context of all other factors determining winter behaviour of beavers. Several of our cases also deviate remarkably from the pattern. We cannot expect one simple, general description of the winter strategies of beavers. To understand the strategic possibilities and constraints of how beavers cope with winter survival, one should study the question as a multivariate problem, including factors such as climate, species, colony size and composition, fat deposits, inactivity and metabolic rate, availability of food species suitable for caching, availability of alternative food, over-ice activity, and site characteristics including water depth and watercourse width. The ecological plasticity of beavers may arguably show that they are flexible in their behaviour and can alter their strategy at the individual level, depending on these factors.
Acknowledgments We thank Peter Busher, Andrew Kitchener and two anonymous referees for their comments on the manuscript.
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