Peary caribou and barren-ground caribou COSEWIC assessment and status report: chapter 8

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Biology

General

Peary caribou and Dolphin and Union carbou live in a harsh environment where occasionally severe winters limit the available forage, causing periodic mass starvation and environmentally forced emigration. This latter point is controversial. Large-scale movements beyond a population’s traditional range have not been documented, but genetic evidence suggests that they have occurred (perhaps in the distant past). Recent evidence for such movements is discussed below.

Peary caribou differences from barren-ground caribou (other than Dolphin and Union carbou) are thought to be specific adaptations to their High Arctic environment. The extent of specific adaptations is largely unknown and may include the following:

• The furry face protects from the severe cold. Long legs are not needed in the arctic desert where snow is typically shallow and often packed hard.
• The function of the modified skull shape (shorter and broader, especially in the nasal bones) is unknown, but may pre-heat breath intake to prevent freezing the lungs; or it may be an adaptation for browsing the low height of vegetation.
• The rumen of Peary caribou is proportionately larger than in reindeer (no data were available for other caribou), possibly to accommodate increased amounts of low-nutrition forage (Staaland et al. 1997, cited by Gunn and Dragon 1998).
• The light coat colour is obviously an adaptation to a region where winter lasts from September 1 to May 31 (Miller 1991).

Reproduction and survival

Although healthy Peary caribou cows may breed as yearlings and produce their first calves as 2-year-olds, first calf production at 3 years is more common. Pregnancy rates (especially of yearlings) and calf survival are strongly affected by their nutritional condition (Thomas et al. 1976, Thomas and Broughton 1978, Thomas 1982, Larter and Nagy 2000b). Gunn et al. (1998) suggested that males typically reach breeding age at 4 years, and females at 2 years; both are reproductively capable up to 13 years and live up to 15 years; and up to 80% of adult females may produce calves in a given year. The proportion of females producing calves in any given year varies markedly, depending on prevailing environmental pressures (Thomas and Broughton 1978, Thomas 1982).

Monitoring of calf production and survival has been sporadic except for Banks Island in the 1980s and 1990s. The consistent picture for Peary caribou is high annual variability in both pregnancy or calf production and in calf survival. In severe winters, yearling recruitment can drop to 0. Pregnancy rates vary from nearly 0% to nearly 100% and are associated with physical condition. For example, Thomas and Broughton (1978) found that in 1978, 88% of adult female Peary caribou collected on Melville and Prince Patrick islands were pregnant, up from 6-7% in 1975, 1976, and 1977 – during the first 3 years after the disastrous 1973-1974 winter. At the same time, mean percentage of femur marrow fat increased from 76% to 88%. Likewise, in those same years, the pregnancy rate on Somerset Island and Prince of Wales Island increased from 73% to 100%, while the femur marrow fat increased from 76% to 79%. They concluded that pregnancy rates and fat reserves are closely associated and that partial recovery following the starvation conditions of 1973-1974 took 3 years.

Larter and Nagy (2000b), combining data from 1982 to 1999 for Banks Island, found that the summertime extremes in percentage of calves in the population in July and August after June calving was 12.5% to 32.1%. As ratios of calves per 100 adult females, the variation on Banks Island during those years was from 24.0 to 74.3:100 adult females. Calf production was greater than 50 calves per 100 cows in 8 years. Calves per 100 adult females actually varied from none to 96.7 calves during the 1990s (F.L. Miller, pers. comm. 26 Jan 2004).

Larter and Nagy (2000b) estimated calf production and over-winter calf survival over 7 years (1992-1999), a period when the authors did not rate the winters as severe. The lowest calf production followed the 1 winter with increased snow hardness (1993-94). Calf survival was the lowest during the winter following a severe winter but neither calf survival nor calf production were significantly related to snow hardness or snow depth. Larter and Nagy (2000b) concluded that either their data on calf production and survival had not sampled the full range of winter conditions or that snow depth or snow hardness do not adequately sample the winter condition which affects calf production and winter survival.

Reproductive potential

Peary caribou under ideal conditions have similar rates of increase to other caribou based on breeding in consecutive years, single births, and age of first breeding at 2 years. A potential complication for recovering (and small) populations is the age and sex composition of the ‘starting’ population. A population that has survived a die-off will likely have a high proportion of breeding females. For example, Miller and Gunn (2003b) recorded that 75% of caribou seen on Bathurst Island in 1998 (after the 1994-97 die-off) were breeding age cows compared to 40% before the die-off.

Gunn et al. (2000b) referred to “the accepted maximum” rate of annual increase of 0.3, or 30% (l=1.3), a rate that Bergerud (1978) previously proposed for caribou in general. The Bathurst Island complex local population approached this rate, going from 1103 caribou in 1988 to 2667 in 1993 (Miller 1998), a 19% annual rate of increase (l=1.19) over 5 years. (Caribou on Banks Island and northwestern Victoria Island appear to have increased at higher rates, but those data are suspect, in part because when populations increased on Banks Island they decreased on Victoria Island, and vice versa, suggesting movement between them.) Over longer periods, however, even when conditions remained favourable, Peary and Dolphin and Union caribou experienced lower rates of annual increase:

• Bathurst Island, from 266 caribou in 1974 to 3011 in 1994 (20 years), 13% (l=1.13).
• Dolphin and Union, from 3424 in 1980 to 27 786 in 1997 (17 years), a 13% annual rate of increase (l=1.13).

Miller (1998) also reported a 20-year fixed-rate increase of 13% per year for Peary caribou in the Bathurst Island complex and suggested that it is a reasonable expectation for Peary caribou on the Queen Elizabeth Islands under favourable environmental conditions and without die-off events. Gunn et al. (1998), reporting results of a modelling workshop, also stated that “during particularly favourable periods the population could grow as much as [fixed rates of] 15-20%.”

Physiology

Miller (1991) previously reviewed aspects of physiology that are relevant to assessment of conservation status. Physiology in relation to nutrition was discussed above. Gunn and Dragon (1998) provide a good review of caribou and muskox physiology in relation to nutrition and possibly competition with muskoxen.

Movements/dispersal

Peary caribou have flexible and varied migration strategies: some have relatively small home ranges (e.g., Bathurst Island) that expand during severe winters, while others migrate and have seasonal ranges separated by hundreds of km (e.g., the Melville Island complex). There also seem to be a few Peary caribou in any population that make long, irregular movements outside of their normal home ranges, in both good and bad years. Such movements during severe winters can be attributed to a desperate search for food, but such movements in good years are less easily interpreted. The proportion of Peary caribou willing to make such movements, and the environmental or other factors that prompt them to do so are unknown. Uncertainties as to the frequency and scale of these kinds of movements and the motivation of caribou to make them have generated some controversy among biologists and between biologists and Inuit and Inuvialuit.

Maximum distance travelled (vector sum on horizontal plane) by marked caribou in the Melville Island complex during spring migration was 450 km (Miller et al. 1977b). Miller and Barry (2003) found mean home ranges of 4 of 17 satellite-collared Peary caribou that remained on Bathurst Island over 1 year to be between 1221 and 2429 km² (mean±SE = 1765±160 km², 95% CI=1353 to 2178 km²) during a full year, 1993-1994, a nutritionally and reproductively favourable year. The other 13 caribou used multiple nearby islands, but their home ranges have not yet been reported. Some of the seasonal migrations within the Prince of Wales-Somerset-Boothia complex would have been in the 300-500 km range or longer and within the Bathurst Island complex, 100-200 km. Caribou of the Dolphin and Union herd also seasonally migrate on the scale of 300-500 km (Gunn and Fournier 2000).

Most documented crossings occur in winter over the ice, but crossings have also occurred in summer by swimming at least 2.5 km (Miller 1995).

Peary and Dolphin and Union caribou cows’ fidelity to their calving areas has been shown by aerial surveys (Urquhart 1973, Miller et al. 1977b) and satellite telemetry (Gunn and Fournier 2000, Gunn et al. 2000a). Regular, seasonal movements are reinforced by fidelity of females to calving areas (Gunn and Miller 1986 cited in Miller 1991, Miller and Gunn 2001). Males also make regular use of seasonal home ranges as shown by non-random distribution of antlers (Miller and Barry 1991).

Miller and Barry (2003) also noted that in summer and autumn the caribou used larger sections of their home ranges than in winter and speculated that this reflected the widespread availability of forage, rather than a need to move to find adequate forage. In the same year, Miller (2002) documented a male and a female Peary caribou having home ranges that involved 6 and 5 islands, respectively, with the male making 16 and the female 11 movements among those islands between 1 July 1993 and 30 June 1994. He also reviewed previous studies that showed that, in severe winters, while some caribou remain in their home range on 1 island even to the point of starvation, others will move to other islands.

The basis for reports of inter-island movements varies between direct sightings of caribou and their tracks on the sea-ice to finding caribou on smaller islands (and assuming where they might have come from) to assumptions about movements based on differences in sizes of caribou numbers on neighbouring islands during consecutive surveys. They include:

• among the Melville-Prince Patrick complex of islands (Miller et al. 1977b);
• between and among Bathurst Island and nearby islands less than 10 km distant (Miller 1995, 1998, Miller 2002);
• from Bathurst Island to Baring Island (50 km east); then, Baring to Cornwallis Island; then, Cornwallis to Little Cornwallis Island to Milne Island, back to Bathurst Island (Miller 1995, 1997b, 1998, Miller and Barry 2003);
• from Bathurst Island to Lougheed Island, then to Borden Island (280 km northeast of Bathurst Island, requiring sea ice crossings of at least 48 km to Lougheed Island and 100 km to Borden Island) and dying immediately after arrival on Borden Island (Miller 1997b, 1998);
• among Prince of Wales, Russell Island, Somerset Island and nearby smaller islands, Boothia Peninsula (Miller and Gunn 1978, 1980, Miller and Kiliaan 1980, 1981, Miller et al. 1982); and
• from Bathurst Island to Cornwallis Island at least twice: during the 1973-1974 starvation year Miller et al. (1977a), and again during the 1994-1997 starvation years (Miller 1998, Gunn and Dragon 2002).

In addition to the above documented movements, qaujimajatuqangit suggests that caribou have also moved between the Bathurst Island complex and the Prince of Wales -Somerset complex, and from the Bathurst Island complex to the eastern Queen Elizabeth Islands. Local people believe that such movements are not uncommon and may be prompted by poor range conditions (including those caused by severe winters) or by an inherent need to keep shifting ranges to keep them from becoming overutilized (Ferguson et al. 1998). Also, genetic testing suggests past gene flow between the Prince of Wales-Somerset population and the Bathurst Island complex (Zittlau 2003).

T. Mullen, (Nunavut Wildlife Service Resolute Bay, pers. comm. March 20, 2002) reported having seen caribou tracks heading from Cornwallis Island southwards across Barrow Strait and D. Kaomayok (a hunter from Resolute Bay, pers. comm. cited in Gunn and Dragon 1998) suggested that Prince of Wales caribou may have moved to Bathurst Island in the 1990s, as an explanation for their absence on Prince of Wales, although he had no direct observation. However, Canadian Wildlife Service researchers surveying caribou in the Bathurst Island complex at that time did not see any large-bodied caribou typical of the Prince of Wales-Somerset Island deme during that period (Gunn and Dragon 1998); nor did any Inuit hunter report seeing kingailik (Prince of Wales Island) caribou on Bathurst Island (F.L. Miller, pers. comm. January 26, 2004). Also, T. Mullen (pers. comm. March 20, 2002) and N. Amarualik (Resolute Bay Hunters and Trappers Association, pers. comm., March 20, 2002) said that it is a general understanding in among both Resolute Bay and Grise Fiord hunters that caribou have, in the past, gone from Cornwallis Island to Devon and Ellesmere Island.

During the crash of 1994-1997, caribou from Bathurst Island went east at least as far as Cornwallis Island (Miller and Barry 2003), where about 85±25 were shot, leaving about 315 unaccounted for in Miller’s (1998) and Gunn and Dragon’s (2002) live/dead caribou mass balance for 1995. Inuit from Resolute Bay said that many caribou went to Devon and Ellesmere islands during that period (T. Mullen, Nunavut Wildlife Service, pers. comm. March 20, 2002; N. Amarualik, Resolute Bay Hunters and Trappers Association, pers. comm., March 21-22, 2002).

There is some evidence--tracks on the ice and observations of caribou on small islands in Barrow Strait (Miller and Gunn 1978)--that a few animals may make such movements on a small scale at irregular intervals, but no evidence of regular or mass movements. F. Miller (Canadian Wildlife Service, pers. comm. Dec. 21, 2002) attempted to document such observations but found no one who could verify a first-hand observation of caribou or their tracks crossing between these population areas. He searched for tracks and other evidence of such movements along both sides of Barrow Strait, during the times that they were reported to have occurred, and found none.

The ability to shift ranges by inter-island movements in times of environmental stress may be important to Peary caribou survival, and this same ability is essential to restock previously abandoned ranges (Miller 1990a). Miller and Gunn (1978), for example, speculated that recolonization of decimated Bathurst Island population might occur as movements of caribou from Somerset and Prince of Wales islands across Barrow Strait or Viscount Melville Sound.

Miller and Gunn (2003b) concluded that,

Nutrition and interspecific interactions

A continuing issue since the Bathurst Island population crash of 1973-1974 has been whether depleted forage due to overgrazing, caused by either overpopulation of caribou or by competition with muskoxen, has caused caribou declines. The sudden population crashes in Bathurst Island complex were clearly caused by deep snow and icing events unrelated to forage conditions--a density-independent mechanism (Miller 1991, 1998). Peary caribou population declines or emigration prompted by overpopulation and/or deteriorating forage--a density-dependent mechanism--have been hypothesized (Ferguson et al. 2001), but not documented.

Many authors (see review in Miller 1998:48-54) have found that lichens are often of minor importance in the diets of Peary caribou, relative to caribou on mainland and more southern ranges. Parker (1978), sampling rumen and fecal content on Melville and Axel Heiberg islands, found that willow (Salix arctica) is the most important food item, especially in summer. Winter forage items included forbs, grasses, and some sedges, but caribou maintained better nutritive and reproductive condition on a high willow diet. He showed that in favourable winters there is no interspecific competition with muskoxen on the basis of total forage available, but predicted that, in severe winters, there could be competition as both species sought willows on exposed slopes and ridges. He concluded that deep, prolonged, and dense snow cover is the important climatic factor controlling both muskox and caribou populations. Riewe (1973) also found that willow is “vital” to caribou on Ellesmere Island. Other studies have shown that purple saxifrage (Saxifraga oppositifolia) is very important, especially in summer when caribou select the flowers (see review by Miller 1991).

Thomas et al. (1999), based on their 1974 data, showed that vegetative cover and standing crop in summer and winter were correlated with the density of caribou summer and winter fecal pellet types, demonstrating that caribou seek out these habitat types in both seasons. In summer, caribou pellet densities were greatest in mesic sites where lichens, willow, wood rushes (Luzula spp.), arctic poppy (Papaver radicatum) and chickweed (Stellaria longipes) were abundant. Winter forage sites were typified by high densities of Luzula spp. and lichens. Caribou winter range use on some sites had a significant positive correlation with Cetraria delisei, Thamnolia vermicularis, Juncus bigumis, Alopecurus alpinus and crustose lichens. They also concluded that during the 1973-1974 severe winter when both caribou and muskoxen died, there was no interspecific competition between caribou and muskoxen. The fecal pellet densities of the 2 species were negatively associated and relationships with certain forage species contrasted significantly. Caribou also used predominantly mesic to xeric sites, while muskoxen used primarily wet meadows. They concluded that Luzula spp. are survival foods, used in severe winters when more palatable foods are unavailable. Luzula is only 28% digestible, compared to Thamnola vermicularis (57% to 62%) and Cetraria spp. (61% to 81%).

Peary caribou require about 1 kg DM/day (dry matter per day) for maintenance in winter, or 2.0 kg DM/day for good health (Miller 1998, extrapolated this from Alaskan studies that he cited in consultation with the senior author, Robert G. White). The range produces far more than that, even in poor years. Miller (1998, citing unpublished 1974 data from D.C. Thomas, that is now published as Thomas et al. 1999) calculated the plant biomass (living parts only of all vascular plants plus lichens, excluding moss, crustose lichens and algae) measured as dry matter (DM) available on eastern Melville Island in spring after the 1973-1974 die-off. The total available was 33.5 gDM/m².

Based on the low, 1973 pre-crash density of caribou occupying those ranges on Melville Island that spring, Miller (1998) calculated that they would have required only 0.05 gDM/m²/year, or < 0.2% of the available forage to survive the previous winter. He also calculated that even at maximum historic density for the Bathurst complex (1961: 3608), with only 5 gDM/m², only 2% of the available forage would be required. Therefore, according to Miller (1998), the absolute amount of forage was not limiting and over-utilization of the range was not a factor. His conclusion was that the forage was made unavailable because of snow conditions, particularly those caused by early winter snowfall (1 Sep-30 Nov). This unfavourable condition is particularly severe when associated with rain in late September and/or early October which freezes on, in, or under the snow pack, preventing ungulates from foraging or making their foraging efforts energy-inefficient.

Miller (1998) used the mean home range size for 6 caribou of 1765±160 km² in 1993-1994, a favourable year, to calculate that they needed only 5% to 8% of the available land area on Bathurst Island to obtain their nutritional requirements. He made a distinction between density-dependent forage depletion that could theoretically prompt emigration, but which has not occurred on Bathurst or other western Queen Elizabeth Islands, and density-independent, seasonal forage unavailability that does occur, has caused starvation and may prompt limited emigration.

Many Inuit believe that caribou routinely disperse from their previous home ranges because of deteriorating forage conditions. For example, after the Prince of Wales-Somerset population had declined, local Inuit stated that the decline probably was caused by effects of high caribou densities on their forage (S. Idlout pers. comm. in Ferguson et al. 2001). During the decline of caribou on Banks Island, there was no statistically significant association between any measure of winter severity and either calf production or overwinter survival, nor was there any evidence of a die-off (Larter and Nagy 2000b). Thus, forage availability may have been a factor, and indeed, local Inuvialuit hunters reported that some caribou were in poor condition in the fall prior to onset of severe winter conditions. Likewise, the declines on northwestern Victoria Island (Gunn 1993, Inuvialuit Game Council 2002b) and Prince of Wales-Somerset islands (Gunn and Dragon 1998) were not known to be associated with any large-scale winter mortality as described for the western Queen Elizabeth Islands (Miller et al. 1977a, Miller 1991, Gunn and Dragon 2002). If unfavourable snow conditions and widespread icing on, in, and under the snow cover occurred on northwestern Victoria Island and on Prince of Wales and Somerset islands, they went undetected.

Several instances of range shifts by barren-ground caribou and reindeer on other Arctic islands in response to deteriorating forage supply have been reported (Gates et al. 1986, Adamczewskiet al. 1988, Ouellet et al. 1993, Staaland et al. 1993, Ouellet et al. 1996, Ferguson et al. 2000, Ferguson and Messier 2000). Qaujimajatunqagnit also includes several instances of range shifts away from deteriorating range on Baffin Island (Ferguson and Messier 1997, Ferguson et al. 1998). These situations, however, with higher amounts of non-lichen forage (grasses and sedges) supporting higher densities of reindeer or caribou, are rather different from the High Arctic range and lower Peary caribou densities; moreover, icing events may have been implicated in some of the range shifts (F. Miller, pers. comm., May 21, 2003).

The available evidence for barren-ground caribou suggests that they switch winter ranges for complex (not well documented although much speculated) reasons that sometimes include forage availability. Range shifts before overgrazing can damage the range have often been hypothesized, but would be difficult to demonstrate.

The Dolphin and Union population’s use of fall ranges is a more unusual situation. Since the mid-1980s, the caribou have migrated to the south coast of Victoria Island during the rut and then cross on the newly formed sea-ice to the mainland winter range (Nishi and Gunn 2004). However, in recent years, ice formation is later and the caribou staged along the coast are visibly affecting the vegetation, at least in some areas, while they wait for the ice to form (A. Gunn, Government of the Northwest Territories, pers. comm. July 9, 2003). If global warming were to cause this trend to continue, the consequences could be serious.

Behaviour/adaptability

In structured interviews by Inuktitut speakers, residents of both Resolute Bay (Nunavut Tusaavut Inc. 1997) and Holman Island (Elias 1993) mentioned the propensity of Peary caribou to temporarily abandon ranges in response to environmental conditions, such as extreme snow and ice covering forage, and then to return some years later. In such winters, many caribou that did not leave, starved. One of Elias’s (1993) interviewees also said that Peary caribou do not flee from snow machines, making them unusually vulnerable to hunters. Another area of vulnerability to hunters, mentioned in the Resolute Bay interviews, is when caribou migrate by swimming between islands (caribou migrating through sea-water are termed “sinmiujut”).

Behaviour and adaptability were reviewed by Miller (1991). Peary caribou are found in small group sizes relative to barren-ground caribou. Group sizes increase slightly prior to calving, stabilize or decrease during calving, and then increase into post-calving aggregations as they move inland from coastal areas. Peary caribou crater in the snow pack until threshold hardness (density) is experienced, when they shift to foraging on windswept (hence, snow-free or shallow snow-covered) ridges and hilltops, and in boulder fields where snow is deeper but soft and not crusted by wind. During icing conditions (snow depth >30 cm, temperatures fluctuating above and below 0° C), caribou leave boulder fields and move to snow-free, south-facing ridges, which are also where spring green-up occurs earliest.

The virtual absence of mosquitoes and warble flies allows Peary caribou more uninterrupted foraging leading to accumulation of substantial fat stores, which can amount to 55 mm in subcutaneous deposits on the back. High fat accumulations in the autumn are critical to survival in severe winters. During years of exceptionally severe snow/ice conditions leading to extreme environmental (nutritional) stress, relatively few, or less often, virtually no calves may be produced and/or survive after birth (deaths usually within the first hours, day or < 1 week of life). Some Peary caribou also respond to stress by range shifts on home range or range expansion within a local population’s range and less often by long distance emigration or temporary displacement outside their previous home ranges (Miller 1995, 1997b, 1998, Miller 2002, Miller and Barry 2003) as discussed above.