Northern Bog Lemming, Synaptomys borealis (Richardson) Southern Bog Lemming, Synaptomys cooperi ( Baird)
Common Names: Bog lemmings,lemming mice
Species: Synaptomys borealis, Synaptomys cooperi
General Description The northern and southern bog lemming are small, round thick- set animals, closely related to meadow mice and voles, with long, mixed grey and brown pelage sprinkled with black, giving it a grizzled appearance. Hair is a lighter buffy, grey or ashy colour underneath often with silver tips. Both species have four carpels and five tarsals, short tails and ears, and a robust, angular skull, thought to be an example of morphological convergence with other arvicolids (Courant, 1997). S.borealis is 110- 150 mm long, with a bicoloured tail (19- 27 mm) only slightly larger than its hind foot (17- 22 mm). Its skull is 22- 27mm long and 13.5- 18.5 mm in width. S. cooperi has similar measurements at 99- 134 mm in length, with a tail 15- 24mm and a hind foot of 16- 20mm. The skull is 24- 27mm long and 14.0- 16.8mm width. Although at birth both species weigh about 2.5g, an adult southern bog lemming weighs less on average at 14- 40g than their northern counterparts at 18- 60g. This is agreeable with a study by Wilson (1997) which found an increase in size within S. cooperi from south to north. Superficially, the northern bog lemming is similar to its relative, S. cooperi, however strong differences do exist. S. borealis lacks the closed triangle pattern of enamel found on the outer margins of the mandibular molars of the southern bog lemming. The incisors are paler, more slender with the upper incisors molded into sharp splints, due to unworn outer corners. A notable spinous projection is located on the posterior edge if the palate, and eight mammae are present in S. borealis rather than the average six found in S. cooperi . The two middle claws of the northern species have been found to enlarge in the winter months, an interesting resemblance to the bifid claws of the collared lemmings of North American and Asian tundra (Dicrostomyx). A final distinction between the two species is the developed hip glands marked by conspicuous white fur on adult S. borealis males. Little piles of grasses of equal length and of varying degrees of freshness found in burrows or swamps are prominent signs of their presence in the area. Their droppings- small, oval and green- found scattered or in masses throughout the tunnels are also indicative of their inhabitance.
Geographical Distribution and Habitat Requirements S.borealis is found in northern, western and southern New Brunswick. Several of its subspecies range throughout Alaska and the entirety of Northern Canada to the sub- arctic. They also extend south of the St. Lawrence River in areas of Quebec, Nova Scotia, Maine and New Hampshire, descending as far as the Smoky Mountains. In accordance with their wide distribution, their habitats include dry hillsides of bluegrass, fields of weeds, dense hemlock and beech forests, and coastal swampy sphagnum bogs. S. cooperi is found throughout all three Maritime provinces, northern Quebec, west through Ontario to Manitoba and south to North Carolina. It is restricted to low, damp bogs or meadows, however, they can be found in higher, drier, but usually deeply littered areas in their most northern reaches.
General Biology These lemmings build small, compact nests of dead grasses and leaves, some with moss or fur linings, either in a chamber several inches below ground, in a slight depression or in a clump of grass above ground. In fields they construct runways of bluegrass which they trim with their teeth to keep clear. In forests, they tunnel through the black soil (leaf mold) or use the burrows of other small mammals. They eat grass, roots, sprouts, some berries and fungi, such as Endogone. Owls, harriers (Walk, 1998), hawks, predatory mammals and snakes all prey upon this animal. They are rather sociable, and live in colonies of a few to several dozen. They are prolific breeders during their one- year life span. The breeding season varies but is always approximately 10 months, from spring to fall with a short gestation period of 19- 21 days. The litters are from one to four, and the young, hairless and blind at birth, are ready to disperse and mate by their third week. All of these factors combined contribute to a 3-4 year cyclical pattern in lemming populations, and the false legend that lemmings deliberately join a death march to the sea to drown. Actually, when a lemming population exceeds its food supply they migrate by swimming across streams and rivers to find land with food. Although lemmings can swim for up to a kilometer, they sometimes try to swim bodies of water too vast or deep, and they drown in great numbers. Breeding occurs less often and over a shorter season and there is greater infant mortality when lemming populations reach critical highs.
Conservation Synaptomys borealis is one of the rarest in collections of any eastern Canadian rodent, however it not uncommon and has a secure conservation status. However, a recent study claims the invasion of the meadow vole in deforested areas of Southeastern Kentucky (Microtus pennsylvanicus) may be causing declines in Synaptomys cooperi. It is thought that extensive deforestation in the area coupled with climatic cooling may have lead to the expansion of the vole, and the competitive exclusion of the southern bog lemming (Krupa, 1996). Also, some subspecies of S.cooperi have not been reported in the literature since the 1940Õs and 1960Õs and may be extinct (Wilson, 1997). In fact, S. cooperi paludis , the Kansas bog lemming, and S. cooperi relictus , the Nebraska bog lemming, are listed as extinct on the IUCN redlist.