Social SpacingBeavers defend and maintain classical territories, which include areas for feeding, mating, and nesting (Sun and Mueller-Schwarze, 2003). Beavers' territories are extremely important to them, first of all because of the amount of energy invested in the territory, building lodges and dams, and becoming familiar with the area, and second of all because lodges and dams provide the protection beavers need from predators (“Beaver: Canadian Domain,” 2004; Rosell and Nolet, 1997). Beavers mark their territories to protect them from other beavers. Because their sense of smell is very acute, beavers use scent marking to broadcast to other beavers that they are the owner of a territory (Rosell et al., 1997; Rosell et al., 2000). They construct scent mounds which consist of mud, debris, and an anal gland secretion called castoreum (Rosell et al., 2000). Castoreum is a urine based substance that is excreted by beavers' castor sacs located between the pelvis and the base of the tail (Rosell et al., 1997). Scent mounds are usually distributed at territorial boarders where intruders will most likely see them (Rosell et al., 2000). In the presence of an unfamiliar scent, a beaver will investigate the scent before anything else (Rosell et al., 2000). Finding an intruder takes priority over finding food (Rosell et al., 2000). Beavers' Territories
Because their survival depends on keeping their territories, encounters between beavers when an intruder is discovered can be very violent, often times resulting in serious injury and even death (Rosell and Bjorkoyli, 2002). Therefore, it is important for beavers to construct scent mounds as a warning to others of their presence, hopefully preventing confrontation.“[T]he scent marks in a territory act as a cue to potential fighting ability and willingness to fight in an asymmetric contest between resident and intruder” (Rosell and Nolet 1997). Increasing the number of scent mounds in a territory will let other beavers know that the owner has enough energy to maintain its territory well, and therefore has enough energy to put up a good fight in its defense. A territory with more scent mounds may be avoided more frequently than one with fewer, because fewer scent mounds indicates an easier fight for the intruder (Rosell and Nolet 1997). In this way, scent mounds provide a way of broadcasting beavers' resource holding power (Case, 28 Jan. 2004). Owners of a territory are more likely to escalate an aggressive encounter than an intruder, because it has invested so much energy in its territory, creating scent mounds, becoming familiar with the area, and making and repairing lodges and dams (Rosell and Nolet 1997). It cannot afford to lose its investment (Rosell and Nolet 1997. Migrating beavers will often avoid marked territories, because of the threat of an encounter (Ulevicius and Balciauskas, 2000). Dispersing young who challenge an owner of a territory are more likely to suffer in encounters than the owner is (Ulevicius and Balciauskas, 2000). Factors that affect frequency of scent marking in beavers include seasonal change, and population density. Rosell and Bergan reported in their 2000 study that male beavers will increase scent marking during the breeding season in January and February. They do so to keep other males out of their territory and away from their mate while she is in estrous. Scent marking is also increased during the month of August when the dispersal of yearlings occurs (Rosell and Nolet, 1997). This increase prevents dispersing young from intruding on occupied territories (Rosell and Nolet, 1997). Population density also affects the frequency of scent marking. In 1997 Rosell and Nolet concluded in their study that “[t]he number of scent markings increased significantly with the number of neighboring territories and individuals.” Another way beavers avoid violent encounters
is by exhibiting the dear enemy phenomenon towards neighbors. In a
study conducted by Rosell and Bjorkoyli in 2002
beavers were found to
spend significantly more time investigating unfamiliar scents in their
territories than familiar ones. Beavers also showed stronger agnostic
behaviors toward unfamiliar
smells than familiar ones. This behavior seems to indicate that the
beavers either learn to recognize their neighbors’ scents over
time and became tolerant, or their neighbors were kin and they had
an innate ability to recognize similar phenotype through
scent. Practicing the dear enemy phenomenon allows beavers to avoid
unnecessary violent encounters by being able to recognize neighbors
who will most likely not invade
their territory.
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