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Facial and Masticatory Muscles |
Natural History of the Beaver (Castor canadensis)
Classification: Common name: North American beaver Evolution and Phylogeny: The first beaver-like rodents appeared during the early Eocene, approximately 50 million years ago [1]. The family Castoridae itself seems to have first appeared in North America during the late Oligocene, approximately 30 million years ago, and later spread to Asia and Europe during the Miocene [2, 3]. These basal species were small, burrowing rodents that were solely land dwelling. The semi-aquatic lifestyle common to modern beavers is a more recent ecological adaptation, though in some situations, beavers today still function as burrowing rodents. The genus Castor originated much later in Europe and Asia during the Pliocene, and then migrated from Eurasia to North America during the same epoch [1, 4]. Although modern semiaquatic beavers seem to have coexisted with now-extinct genera of giant beavers, Trogontherium (Europe) and Castoroides (Eurasia and North America), there remain only two species in existence: C. canadensis (North American beaver) and C. fiber (European beaver) [3]. Appearance: Among the largest living rodents, adult individuals of C. canadensis range in length from 1 to 2m and range in weight from 11 to 26kg [4]. Their most distinctive morphological trait is a flat and scaly tail, which serves multiple behavioral (e.g., warning signals) and biological (e.g., fat storage) functions [1]. Having large, webbed hind feet, C. canadensis is well adapted to aquatic locomotion, and is able to effectively and rapidly paddle in water [5]. On land, the front feet are extremely dexterous and are used to grasp and manipulate food, dig, and groom the fur [1]. C. canadensis has an unusually thick coat of fur, or pelage, consisting of a soft, lead-gray underfur overlaid by coarse guard hairs [2, 4]. The ears and nose close automatically underwater, the lips seal behind the incisors for underwater chewing, and a nictitating membrane protects the open eyes [3, 4, 6]. The dental formula of C. canadensis is 1-0-1-3 [4]. The enlarged incisors are ever-growing and adapted to gnawing (a feature common to all rodents), while the premolar and molar cusps are characterized by thick enamel and distinctive ridges designed to act as grating surfaces during chewing [1]. Geography and Habitat: C. canadensis has a wide range, spanning the continental United States, Alaska, Canada, and extreme North Mexico [2, 6]. In areas where rivers, streams, or other bodies of water are available, the beavers typically live a semi-aquatic lifestyle. In drier areas such as the deserts of the Southwestern United States, they assume the burrowing lifestyles similar to their ancestors. Due to their ability to function both semi-aquatically and fully terrestrially, the beavers appear to inhabit almost any temperate locations where water is available to live in and trees available to eat [3]. As a semiaquatic species, C. canadensis prefers areas where streams and small rivers are located. Once a suitable body of running water has been located, beavers use their unique tree-cutting ability (aided by the size and sturdiness of enlarged incisors) to construct watertight dams. This allows the beaver to create small ponds in which to build complex burrows and small mud and wood "lodges" [4]. These lodges serve as their principle shelter, providing protection from cold, heat, and predation [1]. Groups of related individuals, called colonies, tend to build dams consecutively to impound water along streams, thus keeping the lodge entrances under water and permitting the floating of logs and branches, diving to safety, and travel to feeding areas [1]. As mentioned previously, beavers are also known to live terrestrially. In these instances, burrowing serves numerous ecological functions: burrowing for shelter, to escape predators, etc. Thus, on land, burrowing serves many of the same functions that underwater locomotion and construction of lodges serve for the semi-aquatic beavers.
C. canadensis is primarily nocturnal, although work may begin in the late afternoon one to two hours before dark [2, 6]. Colonies are typically comprised of anywhere from four to eight related individuals, including a mated pair of adults and young offspring, known as kits, up to two years of age [2, 6]. Beavers are somewhat unique among mammals due to their generally monogamous lifestyle [6]. The female generally selects the lodge construction site, and if her mate dies, she may take in a new male while remaining in the same territory [3]. In most situations, maintenance of the territory depends on the female, and if she dies, the territory may be abandoned [3]. Aquatic and Terrestrial Locomotion While on land, C. canadensis tends to move in a slow, quadrupedal fashion, though it has been known to gallop when frightened [2, 4]. Burrowing is also an important form of terrestrial locomotion. The muscular, flattened tail serves to prop the individual in an upright position for cutting and gnawing on trees, in addition to its aquatic roles [3]. The webbed hind feet of C. canadensis serve as the primary force for locomotion in the water, while the front feet are generally held tight against its body [4]. The tail acts as an efficient rudder, and may also serve to make fine adjustments during slow movements [5]. C. canadensis has a large lung capacity, and although most underwater periods last approximately five minutes, much longer periods have been recorded [2]. Diet C. canadensis has enlarged, ever-growing incisors that aid in gnawing and cutting down trees. In certain situations, the incisors may also serve an important role in burrowing (e.g., for the fully terrestrial, burrowing beavers). During the winter months, consumption of the leaves and bark of woody plants that grow near water, especially aspen and willow trees, is most common [4]. During the summer months, the diet consists of mainly herbaceous and aquatic vegetation, thus less tree cutting is observed [3, 4]. Generally, individuals travel only approximately 90-150m from the lodge for harvesting food [1]. C. canadensis has an enlarged caecum containing symbiotic bacteria, which serves as an adaptation to the ingestion of large amounts of cellulose [1, 6]. The practice of coprophagy is also an important behavioral adaptation [6]. Reproduction and Ontogeny C. canadensis is generally a monogamous species, but both
sexes may mate with multiple partners throughout the lifespan if the other
mate dies [1, 2]. Both males and females typically reach sexual maturity
by 1.5-2 years of age, although this age can be influenced by colony composition
and availability of mates elsewhere [4, 6]. These beavers reproduce once
a year during the winter, and birth can occur anywhere from April to June
depending on latitude [6]. Average gestation time is around 104-111 days,
with mean litter sizes between three and four given adequate food resources
[1, 6]. The young are born fully furred and with all of their teeth, and
the eyes open at birth or several hours later [1]. Birth weight is inversely
related to litter size, with a reported average of 340-360g [1, 4]. The
newborn beavers explore their surroundings and may enter the water the
day they are born [1]. The average lifespan of C. canadensis
is approximately 10 years, although the longevity record for the species
is greater than 20 years [4]. References: 1. Müller-Schwarze, D., & Sun, L. 2003. The Beaver: Natural History of a Wetlands Engineer. Comstock Publishing Associates, Ithaca. 2. Nowak, R.M., &Paradiso, J.L. 1983. Walker's Mammals of the World, 4th Ed. The Johns Hopkins University Press, Baltimore. 3. Forsyth, A. 1999. Mammals of North America: Temperate and Arctic Regions. Firefly Books, Buffalo. 4. Jenkins, S.H., & Busher, P.E. 1979. Castor canadensis. Mammalian Species. 120: 1-8. 5. Cahalane, V.H. 1947. Mammals of North America. The Macmillan Company, New York. 6. Jenkins, S.H., & Smith, D.W. 1999. "American Beaver - Castor
canadensis," pp. 548-552 in
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Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
