Woodland caribou (Rangifer tarandus caribou) COSEWIC assessment and status report: chapter 9
Biology
For more detailed background information on the general biology of caribou, consult (Banfield 1974), the first COSEWIC status report (Kelsall 1984) reviews by Miller (1982) and Bergerud (2000), and recent provincial status reports (Rock 1992, Godwin and Thorpe 2000, Dzus 2001).
Compared with barren-ground caribou, woodland caribou tend to breed younger and have higher calf mortality. Pregnancy was detected in high proportions of yearling females (Rettie and Messier 1998, Dzus 2001) though sample sizes are small. That compares with 12% in the Beverly population of barren-ground caribou (Thomas et al. 1998). The difference between ecotypes can be explained by earlier calving in woodland caribou east of the Cordilleran Mountains, an adaptation to earlier growth of green forage. Green-up is as much as 2 months earlier in the boreal forest and southern mountains compared with barren-ground caribou that migrate to northern calving grounds. Pregnancy rate in Peary and barren-ground caribou is correlated with body weight and degree of fatness (Dauphiné 1976, Thomas 1982, Cameron et al. 1993).
Pregnant females travel to isolated, relatively predator-free areas to calve. Examples are islands in lakes or peatlands, lakeshores, forests, and tundra. Only one calf is born in May or early June. Caribou in the Cordilleran Mountains appear to calve in the first 2 weeks of June (Edmonds 1991) plus the last week of May (Brown et al. 1994). Such timing is comparable to that of barren-ground caribou. Most calves were born mid-May in Saskatchewan (Rettie and Messier pers. comm.) as in eastern Alberta (Fuller and Keith 1981). Peak calving was 17-21 May in the forest-tundra Pen Island population (Abraham and Thompson 1998) similar to caribou in the Boreal population. Therefore, calving is 2-3 weeks earlier in the boreal NEA than in the Cordilleran Mountains. Calves of barren-ground caribou are able to travel within a day or two of birth but there is some evidence that calves of woodland caribou are cached (Chubbs 1993).
Most breeding is by large males with large antlers, which indicates strong sexual selection among males (Butler 1986). Woodland caribou form harems like wapiti (Cowan and Guiguet 1965, Banfield 1974, Geist 1991), whereas barren-ground caribou bulls form loose-tended groups of cows. The mating behaviour of caribou is dictated by the social system, which is considered an adaptive response to the environment and particularly to predators (Butler 1986). Group size is lowest at calving and in summer, increases before the rut, and may decline or increase during the winter. Peak group size may occur at the rut or in early and late winter. Group size at all seasons is much larger in forest-tundra caribou than in forest-dwelling caribou.
On average only 30-50% of calves survive their first year. Typically, about 70 to 74 calves are produced by 100 adult females (>1 year). Population stability occurs when about 30 calves per 100 adult females survive to autumn (Yukon Renewable Resources 1996), which is survival of about 42%. Survival of calves can vary from almost none to 100%, depending on the abundance of predators and forage accessibility during pregnancy and the first year of life (e.g., Bergerud 1983). High rates of survival occur when populations are irrupting after low numbers or after caribou are introduced to new range with few or no predators. Predation is the major cause of death of radio-collared calves and adults (e.g., Bergerud 2000). Much of the behaviour of caribou is related to reducing risk of predation (Bergerud 2000).
High calf mortality implicates poor winter energetics or predation or a combination of both. Terrestrial lichen cover and standing crop is low on some winter ranges relative to taiga ranges of barren-ground caribou (Edmonds and Bloomfield 1984, Thomas and Armbruster 1996a). Caribou in the Southern Mountain population may be nutritionally stressed late in winter before they are able to obtain new-growth vegetation. In B.C., they consume arboreal lichens during mid- to late winter and little or no green forage. In the mountains of Alberta, sparse forage is difficult to access in some winters. Some populations in Saskatchewan rely on arboreal lichens in and around peatlands to supplement food under snow (Thomas and Armbruster 1996b). Caribou that are nutritionally stressed in late winter-early spring are likely to produce weak calves. Such calves are more likely to die from predators, pneumonia, and other causes, which points to an interaction of factors. There are few data on the nutritional status of forest-dwelling caribou. Adult female caribou captured in Jasper National Park in October had little back fat (mean = 7 mm, median = 2 mm) (Brown et al. 1994) compared with caribou sampled in the Qamanirjuaq (mean = 15 mm) (Dauphiné 1976) and Beverly (mean = 13 mm) populations of forest-tundra caribou (Thomas et al.1998).
The adult (>1 year) mortality rate of female caribou usually varies between 5% and 15%, with half to two thirds attributed to wolf predation (Bergerud and Elliott 1998, Yukon Renewable Resources 1996, Dzus 2001). Average annual mortality of adults in Alberta averages 14.5% and 11.0% (females only) in the SMP and BP, respectively (Dzus 2001). In Saskatchewan, average annual mortality of adult females was 14% (Rettie and Messier 1998). The mortality of male barren-ground caribou (38%) was more than double that of females (17%) after sexual maturity at age 4 and 3 years, respectively (Miller 1974).
Population ‘stability’ occurs when recruitment (addition of calves 1 year old) and annual mortality of adult females is about equal and typically is 10-16%. Demographics and ‘equilibria’ are dynamic in forest-dwelling populations. Stability cannot be assumed if there is no change in estimated numbers between surveys spaced several years apart. Wide confidence intervals for most surveys almost guarantee that no statistical difference in population size will be found between successive surveys. Similarly, stability cannot be assumed between surveys spaced several years apart even if they yield similar population estimates. Long-term stability is unlikely in caribou populations because major limiting factors such as weather, predation, and hunting are highly variable.
The probability of survival of caribou is age dependent. For example in the Beverly population, the expected 1-year survival of females 3 and 10 years old was 0.89 and 0.78, respectively (Thomas and Barry 1990). The age structure of captured caribou tends to be older than the entire population because cows with calves and cows with large antlers tend to be selected. For example, the average age of females at capture in Jasper National Park was 7.7 years (Brown et al. 1994). By the time of death, the age structure has advanced 1-4 years. Expected survival of those relatively old caribou is lower than in the general population. For example, two of five deaths among 24 collared caribou in Alberta were old females (Edmonds 1991), which inflated the mortality rate to 22%. Similarly, in Jasper National Park average annual mortality of adult females and males was 31% and 34%, respectively (Brown et al. 1994). However, 5 of 11 females that died were 12 years+ and two of the three males that died were approaching age 10 years. Wolves and a bear accounted for all deaths of those seven old caribou, which were in poor condition when killed in late winter or early spring. Only 2-3% of 2+ year old female and male forest-tundra caribou survive past 12 and 10 years, respectively (Miller 1974, Messier et al. 1988, Thomas and Barry 1990). Therefore, the survival of radio-collared caribou should be adjusted by comparing expected survival of each caribou to actual survival.
Most of what is known about caribou function comes from literature on reindeer. Through inductive logic and analogy, some of the results can be extrapolated to woodland caribou. Caribou have adapted to feed on lichens, though they also consume a wide array of plant species. They can withstand severe cold because their thick winter coat contains semi-hollow hair but they are susceptible to over heating because cows do not shed their winter coat until after calving. The dark summer coat absorbs all wavelengths and points to the importance of shade and cool forest cover types. Adaptations to snow include large feet and a furred muzzle. Caribou are the most energetically efficient walkers of ungulates tested (Fancy and White 1987).
Caribou are almost always on the move. Consequently, predators and parasitic insects cannot predict where caribou will occur and lichen ranges are not overused and trampled. A negative result of movement is that caribou can travel into areas occupied by wolves. Some movements are local or elevational, whereas others are migratory (traditional, twice annual movements between two distinct seasonal ranges). Those movements can be explained as responses to specific seasonal habitat requirements, predators, food quantity and quality, snow conditions, insects, thermal extremes, or combinations of those factors. Their mobility and responses to changing environments means that dispersal is most likely when environmental conditions are extremely unfavourable and during range expansion at population ‘highs.’ Therefore, travel corridors need to be maintained to facilitate inter-population movements and occupation of former range. Forest-dwelling caribou are least mobile in summer and winter with most movement occurring in spring before calving and before the rut and winter (e.g., Brown et al. 1986, Edmonds 1988, Racey et al. 1991, Brown et al. 1994, J. Rettie pers. comm. 2001).
Movement between large fen complexes in northeastern Alberta was only about 5% per year (Stuart-Smith et al. 1997). Local populations in Alberta, with one exception, were discrete based on short-term data mostly for adult females (Dzus 2001). Similarly, in Saskatchewan there was no movement of adult females among five contiguous local populations (Rettie and Messier 1998). Strong fidelity of individuals to their home ranges was also noted in Yukon (Farnell and McDonald 1986), B.C. (Hatler 1986, Seip 1992), Manitoba (Brown et al. 2000b), and Quebec (Brown et al. 1986).
Several authors have noted that radio-collared females will return to the same general area to calve in successive years (Paré and Huot 1985, Brown et al. 1986, Edmonds 1988, Seip 1992, Brown et al. 1994, Brown et al. 2000b). Others have found no fidelity to specific calving sites but definitions of fidelity vary (Rettie and Messier pers. comm.).
Caribou are ruminants that have specialized bacteria and protozoa in their rumens that efficiently digest lichens. They are also able to recycle urea to preserve nitrogen, of which preferred lichens are deficient. During winter, they use fat reserves and catabolize protein from muscle when their diet is nitrogen deficient. Caribou are described as catholic, fine feeders. That is, they can eat a wide variety of plants and they select small parts of plants with high nutritional value that are readily digested. In summer they eat sedges, grasses, forbs, lichens, fungi, and the leaves of shrubs, particularly willow. In winter, they eat large amounts of lichens if they are available. Included are lichens that grow on the ground, when snow depth less than 50-100 cm, and arboreal lichens when the availability of surface lichens is poor.
Most interspecific interactions are with predators, particularly the wolf. There is no evidence that caribou actively avoid other ungulates yet separation usually is the case because their use of habitat differs from that of other ungulates.
Caribou have innate behaviour that varies little among populations throughout the world. They often are described as “curious” and will stop and watch a human at a ‘safe’ distance. They often circle a person or even approach to within 50 metres. Distances triggering flight (escape) vary with sex and age, physical condition, and population. Caribou can be enticed to approach humans by unusual activities such as periodically waving an arm. Such behaviour may relay an image of a fit and fearless individual to a predator but it makes caribou highly susceptible to hunters. Caribou have not adapted to hunters with modern rifles and are vulnerable because they aggregate in open areas such as frozen lakes and are easily overtaken by snowmobiles.
Caribou have considerable genetic variability and are highly adaptable. They occupy diverse range and habitat types, from coniferous forests to tundra. Some populations migrate for several reasons whereas others remain year round in a small area, such as fens. For sedentary populations, there is no real escape from predators, insects, or heat except for specific cover types where some relief is obtained.
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