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Paper presented at The First Conference, Nuuk, , 3-8 September, 1991.

Comparative ecological and behavioral adaptations of Ovibos moschatus and Rangifer tarandus

David R. Klein Cooperative Fish and Wildlife Research Unit, 209 Irving Bldg., University of Alaska, Fairbanks, AK 99775, USA

Abstract: Caribou/reindeer and muskoxen are the only two ungulate species that have successfully occupied arctic . Although confronted with similar environmental constraints, their morphological dissi• milarities have enabled them to develop unique behavioral and ecological adaptations that under most circum• stances result in minimal overlap in use of forage resources. The large body and gut capacity of muskoxen have enabled them to adopt a strategy maximizing rate of forage intake and energy conservation, whereas ca• ribou/reindeer of substantially smaller body size must pursue selective feeding, requiring high mobility and high energy expenditure. Responses to predators and insects by the two species show similar contrasts in as• sociated energy costs. When confronted with environmental extremes that limit forage availability, competi• tion for food may occur and the resulting differential success is a reflection of their divergent evolutionary routes. Keywords: caribou, reindeer, , ecology, behaviour, morphology Rangifer, 12 (2): 47-55

Introduction Small in relation to body size in musk- Muskoxen and caribou/reindeer are the only oxen result in a much greater foot loading than two extant ungulate species that successfully in Rangifer that have broad hooves and promi• adapted to life in the Arctic. Derived from the nent dew claws. Weight in muskoxen is concen• divergent evolutionary lines of the and trated over the large forehooves in contrast to a Cervidae, they each responded to unique more nearly equal weight distribution in Rangi• aspects of their morphology to follow markedly fer. The differences between the two species different routes of adaptation for existence un• presumably account for greater efficiency in der the environmental constraints of high latitu• digging through snow to obtain forage by des. Rangifer (Klein et ai, 1987). Leg length is di• rectly correlated with locomotive efficiency and barren-ground caribou have the lowest net cost Morphological characteristics of locomotion of any terrestrial speci• A comparison of the morphology of the two es studied (Fancy and White, 1985). The longer species (Table 1) shows that the muskox has a legs, coupled with more equal distribution of much larger body and shorter legs than cari• weight on legs and hooves, account for greater bou/reindeer, although shortening of the legs running speed of Rangifer (Schmidt-Nielsen, has been a product of of reindeer. 1984).

Rangifer, 12 (2), 1992 47 Table 1. Comparative morpho ogical measurements of muskoxen and caribou/reindeer.

Measurements Muskox Source Caribou/reindeer Source Total weight (kg) 218-266 1 87-99 2 Chest height (mm) 484 3 730 4 % body length 25 41

Hoof size (mm2) Fore Male 125 5 185 6 Female 97.5 5 146 6 Hind Male 98.3 5 170 6 Female 92.4 5 138 6 Ratio of fore/hindfoot Male 1.60 1.09 Female 1.25 1.06

Foot loading (g/cm2) Hard surface 770 1 184 4 Soft snow 570 1 125 4 Rumen-reticulum wt. (kg) Summer 28.9 + 5.1 1 14.8 + 0.8 2 Winter 23.2 + 2.1 1 8.2 + 0.1 2 % body weight Summer 14.1 +2.5 1 16.5 + 1.1 2 Winter 11.3 +1.0 1 20.1 ±1.4 2

1. D. R. Klein and H. Thing, unpub. data 2. Staaland ct al, 1979 3. McDonald and Freeman, 1984 4. Telfer and Kelsall, 1984 5. D.R. Klein, unpub. data 6. Thing, 1977

Differences in mouth and gut morphology be• With the possible exception of the immediate tween muskoxen and Rangifer are pronounced. postpartum period when adult females may use The muskox has a wider muzzle than caribou¬ their retained to defend their newborn /reindeer and greater rumen-reticulum size, calves from small predators, antlers on females which is scaled to body size (Klein, 1985). seem to have their greatest function in provi• Horns in muskoxen serve the dual function ding hierarchical superiority over conspecifics of social display, conspecific challenging and during winter foraging and calving (Henshaw, fighting among adult males, and weapons for 1968; Espmark, 1971). defense from predators. The upturned, sharp- Pelage differences between muskoxen and ca• tipped horns, of nearly equal length in males ribou/reindeer are also distinct. The dense, fine and females, of these relatively large-bodied ani• underwool and extremely long guard hairs of mals are apparently the result of selection for the muskox are unparalleled in insulative value. defense against predators (Gray, 1987). Rangifer When lying with the short legs folded beneath are unique among the Cervidae because both se• it and the long guard hairs of the back and si• xes grow antlers, but they use their antlers pri• des forming a blanket to the ground or snow marily in social interaction. The large antlers of surface, insultative efficiency is extremely high. adult males are hardened and potentially useful The extreme insultative efficiency of the pelage as weapons against predators only for a relative• and short extremities of the muskox are effecti• ly brief period in-autumn and early winter as• ve in limiting heat loss under conditions of ex• sociated with the breeding season (Pruitt, 1960). treme cold, but may be a detriment during

48 Rangifer, 12 (2), 1992 ^varm weather and when body heat must be Foraging dynamics lost after running. Caribou/reindeer have short, The foraging dynamics and diets of Ovibos and hollow guard hairs as the primary insulation Rangifer are products of the morphological dif• and only a low density of finer underwool in ferences between the two species discussed abo• winter. Through piloerective control of the ve. The larger rumen-reticulum capacity in mu• stiff, hollow guard hairs, .they can vary the in- skoxen allows longer rumen retention time and sulative effectiveness of their pelage, in contrast therefore greater digestive efficiency (White et to muskoxen with their dense fleece of under• ai, 1981). Although large rumen size requires . Thus, caribou/reindeer with relatively large amounts of forage, the slower rate of long and less thickly haired legs and the capabi• turnover of rumen contents limits forage intake lity of altering the thickness of their pelage accordingly. This and the broad muzzle allow have more flexible thermoregulatory capacity muskoxen to maintain rumen fill with forage of than muskoxen. relatively low digestibility by rapid bulk fee• Muskoxen and caribou/reindeer have made ding in areas of high forage biomass. By con• use of their distinct morphological characteris• trast, caribou/reindeer must be more selective tics in their adaptation to life at high latitudes feeders to obtain plant material of high digesti• (Table 2); thus as a consequence of their mor• bility for a rapid rate of passage of the digesta phological differences, their ecology and pat• to meet nutrient requirements. The consequen• terns of behaviour are also uniquely distinct. ces of these two divergent dietary strategies of

Table 2. Adaptive advantages of morphological features of muskox and caribou/reindeer. Morphological Adaptive Advantages Features Muskox Caribou/Reindeer Body size Large body/low surface area = Smaller body = lower daily conservation of body heat; large body maintenance cost; lower cost of = predator defense locomotion Leg length Short legs = energy conservation, and Long legs= greater speed for predator foraging efficiency in absence of snow avoidance; efficient locomotion; efficient foraging through snow size & Larger forehooves = stability in Broad hooves = buoyancy in soft loading rutting bouts and stationary predator substrate; efficiency in cratering & defense swimming. Equal size of hooves = speed of locomotion Pelage Dense and thick underwool + long Straight hollow hair, minimal guard hairs = maximum insulation, underwool = high thermal regulatory especially when lying, and defense efficiency, increased buoyancy in from insects water Muzzle width Broad mouth = efficient bulk feeder Narrower muzzle = efficiency in selective feeding / Permanent, upturned, sharp-tipped Antlers provide hierarchical feeding characteristics horns of equal length, in sexes advantages through winter and during effective for defense against predators. calving to productive females. Englarged boss in males absorbs shock Massive antlers of males provide of rutting clashes social status in the rut, are shed prior to full winter Rumen/reticulum Large rumen scaled to body size = Smaller rumen = more rapid rate of size slow rate of passage of digesta and passage, quick turnover of nutrients, efficient digestion of low-quality and lower energy cost of movement forage, favors bulk feeding

49 60 Comparison of time spent feeding during the 800 post-calving period is complicated by insects, CARIBOU (Central Arctic Herd Roby, 1978)- against which caribou/reindeer are less well de• E 600 fended than muskoxen. The greater daily move•

UJ 40 - ments rates of caribou/reindeer at this time are accounted for by movements associated with in• 400 - sect avoidance in addition to those required for MUSKOX I (Sadlerochit Jingfors, i960) feeding (Fig. 2). Migratory caribou travel over a BULL 20 g linear distance of up to several thousand kilo• 200 cow meters annually in their seasonal tracking of fo• < rage of high digestibility. Muskoxen, in contrast Q UJ to caribou, are for the most part relatively se• o S dentary, although seasonal movements may oc• OBSERVATION PERIOD cur in some areas (Jingfors, 1980; Thing et aL, Fie 1. Comparative feeding time of muskoxen and 1987; P. Reynolds, viva voce). caribou in northern Alaska. dominate the winter diet of migrato• ry caribou whereas vascular plants, including forbs, leaves of (Salix), and sedges, are the two species is marked. The narrow muzzle the major constituents of the summer diet. of caribou/reindeer is better equipped for selec• Considerable variation in the winter diet of ca• tive foraging but requires greater mobility on a ribou/reindeer exists along a latitudinal gradient daily and a seasonal basis. This can be compen• from south to north (Klein, 1985). Woodland- sated for through their longer legs, broader dwelling caribou and the large migratory cari• hooves, and more appropriate body conforma• bou herds are characterized by winter diets do• tion. Caribou spend more daily time feeding minated by lichens, but in the High Arctic than muskoxen in comparable habitats, with where biomass is low, (R. the exception of the post-calving perion when t. pearyi) and reindeer (R. t. platyrhync- daily feeding times are nearly equal and mu- hus) make heavy use of and select some skox feeding times are at a maximum (Fig. 1). mosses (Fig. 3).

MUSKOX CARIBOU (Sadlerochit River) (Prudhoe Bay) Summer 1972 60 (Jingfors 1980) (White, et al. 1975) LU 50 - IE V- 40 l±_

50 Rangifer, 12 (2), 1992 CARIBOU/REINDEER MUSKOXEN

Fig. 3. Generalized winter diets of muskoxen and caribou/reindeer in relation to latitudinal and geographic location. Data from Tener (1965), Parker (1978), Boertje (1981), White et ai (1981), Thomas and Ed• monds (1983), Thing et al. (1987), and Klein and Bay (1990). Snow and their differential adaptability to it In doing so they forego opportunities for selec• are important factors influencing foraging dyna• tive foraging and their winter diets is generally mics of the two species. Caribou are well equip• dominated by graminoids of relatively low di• ped for locomotion in snow and for digging gestibility (Klein, 1985) (Fig. 4). Relative to cari• through it to obtain forage and are capable of bou, availability of forage seems to be more im• migrating long dinstances over snow-covered portant than forage type for muskoxen in win• terrain to optimize opportunities to secure diets ter. In some of the more southern areas of their of high digestibility in both summer and winter distribution, willows that assume shrub form (Fig. 4). Muskoxen, by contrast, select areas may be more readily available and therefore with low snow accumulation and restrict their constitute a major portion of the muskox diet movements, especially in winter (Klein, 1985). (O'Brien, 1988). Tener (1965) also found that willows were important for muskoxen in win• ter where they were available. The consequences for the forage resource of a sedentary versus a wide-ranging foraging strate• gy of herbivores using it are potentially quite different. A wide-ranging foraging strategy wo• uld seem less likely to have detrimental conse• quences for the continued production of forage than one that is localized. Density of the herbi• vores is, however, a major determining factor as is the forage type involved. Lichens are much less resilient to the effects of grazing than most vascular plants and among the latter, grami• noids are highly resilient (Klein, 1968). There• fore, muskoxen that have a relatively small home range and make heavy use of graminoids exist on a very resilient forage base. Henry et al. (1986) demonstrated that muskoxen may en• hance productivity of sedge meadows in the High Arctic through grazing by accelerating the turnover of nutrients and through removal of

Graminoids Shrubs Lichens Forbs Other the shading effect of accumulating leaf biomass. Fig. 4. Comparison of generalized summer and win• Willows, however, may not withstand continu• ter diets of muskoxen and caribou/reindeer in ed heavy browsing by muskoxen, and in areas mid-arctic areas. Data sources as in Fig. 3. where muskoxen have been introduced to unoc-

Rangifer, 12 (2), 1992 51 cupied habitats, the extent of riparian willows NORTHEAST declined (H. Thing, viva voce and D. Klein, ALASKA (Jingfors 1980) per. obs.). By contrast, those caribou/reindeer JAMESON LAND, with extremely large home ranges and that are N.E. GREENLAND seasonally migratory are usually dependent on DOVRE, (Olesen 1987) lichens as a major winter dietary item. Overgra• (Olesen 1987) PEARY LAND, zing of the slow-growing lichens is limited by N. GREENLAND (Klein and Bay 1990) their restricted availability through the snow cover and the wide-ranging foraging habits of caribou/reindeer. Arboreal lichens are available as forage only where they can be reached by LU O the , who are often aided by the sup• Q port of accumulated hard-packed snow or wind Z) m that fells trees or blows lichens from their crowns. Ground lichens are often protected by snow cover through which caribou/reindeer > must dig to obtain them, with the result that snow adjacent to feeding craters that is distur• < bed as a consequence of its displacement from the craters or by trampling is often so dense that the energetic cost of digging adjacent to < old feeding craters becomes excessive (Pruitt, 1959). 62° 70° 71° 82° NORTH LATITUDE Fig. 5. Latitudinal gradient in daily feeding times in Activity budgets muskoxen. Daily and seasonal activity patterns of musko- xen and caribou/reindeer are a function of their unique morphological and associated physiologi• daily activity of caribou exceeds that of musko• cal and dietary differences. Muskoxen pursue an xen in the same general area. A possible excep• energy-conservative life style that is compatible tion is the post-calving period when feeding ti• with large body size, high insulative value of mes may be similar. Activity patterns of both their winter pelage with limited thermoregula• species, however, may vary with latitude. For tory plasticity, dependence on low-quality fora• example, time spent feeding increases with in• ge in winter, resistance to insect harassment, creasing latitude during summer in muskoxen low locomotive efficiency, and ability to defend (Fig. 5), apparently as a function of the lower themselves from predators without running. density of available forage biomass and the need Caribou/reindeer, by contrast, lead an active to maximize intake of high-quality forage du• life style requiring a relatively high energy turn• ring the brief summer growth period (Klein over. They are morphologically and physiologi• and Bay, 1990). Comparison of caribou activity cally adapted for efficient locomotion over va• budgets from Alaska and caribou/reindeer at rying substrates, including speed in running, higher latitudes also reflects the need for high which is their most effective predator avoidance arctic to maximize dietary intake du• strategy. They are vulnerable to insect haras• ring the brief arctic summer when both forage sment and respond by increasing activity. To quality and available biomass are high and phy• maintain an energetically costly life style they siological activity is greatest (White et aL, 1981). must be selective feeders that track forage of The Central Arctic Herd at 70°N in Alaska high quality and digestibility seasonally and spent about 40% of daily activity feeding during through annual migratory movements. summer (Roby, 1978) whereas Svalbard wild A comparison of activity budgets as reflected reindeer at 78°N spent about 60% (Reimers, in feeding time (Fig. 1) and movement rates 1980). Reduced insect harassment at high latitu• (Fig. 2) of muskoxen and caribou in northern des and absence of predators on Svalbard likely Alaska showed that throughout the entire year contributed to these differences.

52 Rangifer, 12 (2), 1992 Differences in predator avoidance strategies ribou where they coexist, but have not between Ovibos and Rangifer (grouped stationa• been able to sustain their populations for long ry defense in the former versus alertness and periods in northern Greenland where musko• swift running speed in the latter) presumably xen have been their only large herbivore prey contribute to the divergence in energetic expen• species (Dawes et al, 1986). ditures. An exeption is the , Insects have a much more dynamic effect on which in the absence of predators and serious the energy balance of caribou/reindeer than on insect harassment, evolved toward the muskox muskoxen (White et al, 1975; Jingfors, 1980), ecological niche with shortened legs, capability although the more northern distribution of to use low-quality forage and to accumulate most muskoxen coincides with decreasing fre• large fat reserves, and an energy-conservative quency and intensity of potential insect haras• life style (Klein and Staaland, 1984). sment. Insects, primarily mosquitoes (Culicidae) Timing of parturition in the two species re• and biting flies (Simuliidae), may account for flects differences in their seasonal energetic stra• pronounced mortality of neonates in woodland tegies and predator avoidance behaviour. Cari• caribou (R. t. caribou) (Kelsall, 1968), and in• bou/reindeer, with closely synchronized timing creased winter mortality of calves of barren-gro• of birth within a population, minimize preda¬ und caribou (R. t. grand) has been associated tion on neonates by the swamping effect (the with heavy infestation by larvae of parasitic fli• concentration of newborn calves in time and es (Oedemagena tarandi and Cephenomyia trom¬ space, thus reducing the risk of predation per pe) following a warm summer favorable to in• ). Their migratory behaviour also locates sects (Davis et al, 1980). calving away from predators concentrated on Overgrazing of forage resources as a density- wintering grounds. This strategy, however, re• dependent limiting factor would be expected quires extended precalving movements with as• more frequently among sedentary than migrato• sociated energy costs (Fancy, 1986). Calving in ry species. This generalization for muskoxen muskox populations usually extends over seve• and caribou/reindeer must be adjusted to acco• ral weeks. Apparently selective pressure for unt for the relative resistance to grazing pressu• synchrony of calving has not been strong. re of the forage types within their habitats. Their predator defense and sedentary nature Graminoids, which dominate the diet of musk- give equal protection to neonates independent oxen, are much more resilient after grazing of birth date. The increased energy expenditure than lichens (Klein, 1968). Furthermore, lichens of migration associated with timing of birth or throughout much of their distribution in the separation from predators in caribou/reindeer is forests of and the avoided by muskoxen. of Eurasia are subject to catastrophic destruc• tion by forest fires, which temporarily remove vast areas as potential sources of winter forage for caribou and reindeer. Population limiting factors Availability and quality of forage and climatic Differences in behaviour and ecology of Ovibos extremes seem to be primary limiting factors and Rangifer predispose them to differences in for both muskoxen and caribou/reindeer living the importance of population limiting factors. at similar latitudes. The of R. T. eo- Predators can be more important in limiting groenlandicus in Northeast Greenland was appa• Rangifer populations than appears to be the rently associated with extreme snow and icing case for muskoxen. Muskoxen often live at conditions brought about by the reduction of much lower population densities that caribou sea ice off the coast (Vibe, 1967). Muskoxen or wild reindeer and are frequently remote survived, although populations were greatly re• from other large herbivores; under these condi• duced, apparently through their ability to sub• tions their populations may not be sufficient to sist on extremely poor quality forage available support resident populations of wolves (Cams in areas of low snow accumulation. In the Ca• lupus). Migratory caribou/reindeer, however, nadian High Arctic, muskox and Peary caribou may expose themselves to several discrete preda• populations have fluctuated widely in response tor populations in their seasonally separate eco• to stochastic climatic extremes (Parker et al., systems. Wolves often depend primarily on ca• 1975).

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54 Rangifer, 12 (2), 1992 White, R. G., Thomson, B. R., Skogland, T., Per• White, R. G., Bunnel, F. L., Gaare, E., Skogland, son, S. J., Russell, D. E., Holleman, D. F. & Lu• T. & Hubert, B. 1981. Ungulates on arctic ranges. kk, J. R. 1975. Ecology of caribou at Prudhoe - In: Bliss, L. C, Heal, O. W. & Moore, J. J. Bay, Alaska. - In: Brown, J. (ed.). Ecological in• (eds.). Tundra ecosystems: A comparative analysis. vestigations of the tundra biome in the Prudhoe Cambridge University Press, Cambridge, England, Bay Region, Alaska. Biol. Pap. Univ. Alaska Spec. pp. 397-483. Rep. 2: 151-187. Manuscript accepted 2 April, 1992.

Rangifer, 12 (2), 1992 55