Social Organization

Beavers live in family units that consist of a monogamous pair, kits, yearlings, and sometimes two-year-olds (Sun and Mueller-Schwarze, 2003; Rosell et al., 2000). Family sizes range from an adult pair up to families consisting of ten members (Sun and Mueller-Schwarze, 2003). Smaller families may be able to reside in just one lodge, but larger families usually build additional lodges to live in (Sun and Mueller-Schwarze, 2003). However, in the northern latitudes even a large beaver family may live in one lodge because it is closest to the food cache (Sun and Mueller-Schwarze, 2003).

Within the family unit beavers exhibit helping behavior driven by natural selection rather than kin selection; both yearlings and two-year-olds partake in this helping behavior (Sun and Mueller-Schwarze, 2003). For the most part, yearlings help their parents for selfish reasons. They have evolved to be extremely reliant on their parents at a young age. Not only are they dependent for food, but also for lessons in life skills (Sun and Mueller-Schwarze, 2003). Beaver parents teach their young by example, exhibiting the life skills that their offspring will need when they are living on their own (Sun and Mueller-Schwarze, 2003). Yearlings learn by helping adults build food caches in the fall, and to a lesser extent by helping to repair lodges and dams (Sun and Mueller-Schwarze, 2003). However, adults continue to do the majority of this work (Sun and Mueller-Schwarze, 2003). For the most part yearlings will explore, play, and copy parent’s behavior (Sun and Mueller-Schwarze, 2003). In their 2003 publication Sun and Mueller-Schwarze reported observing a yearling trying to help repair a dam. It dove to the bottom of the pond to fetch mud, but it did so too far from the dam. By the time it reached the dam, after leaving a trail of muddy water behind it, it had no mud with which to repair. This example of copying adult behavior shows that yearlings are not always able to help their parents sufficiently. Copying adults’ behavior is a form of imprinting, and is meant to teach the young life skills. There is little immediate benefit for parents.

Even older offspring, who are more able to help, do so for selfish reasons. Two-year-olds help in feeding, grooming, and guarding newborn kits; in addition they help parents build food caches and repair lodges (Sun and Mueller-Schwarze, 2003). However, it is not essential for a family to have these helpers. Although the presence of helpers increases the chance of survival for the young kits, two-year-olds will only stay to help when there is a shortage of resources preventing their own dispersal (Sun and Mueller-Schwarze, 2003). For instance, dispersal will be postponed only in times of food shortage, high population density, or drought, all of which limit the possibility of successfully establishing a territory (Sun and Mueller-Schwarze, 2003). The motivation behind helping for the two-year-olds is a need for protection. If a beaver disperses without the hope of a territory, they are likely to perish for lack of protection from predators (Sun and Mueller-Schwarze, 2003). Therefore they are temporarily dependent on their parents for a territory and protection if other territories are not available (Sun and Mueller-Schwarze, 2003). Because because two-year-olds are not always around to help, and an adult pair is able to successfully raise offspring on their own, helping behavior in beavers is non-critical.

Beaver dams and lodges are extremely important for beavers' survival. Offspring will postpone their own dispersal and reproduction if there are no available territories. It is better to wait and have the ability to reproduce later, than to risk the chance of perishing without a territory. (Sun and Mueller-Schwarze, 2003)

Image courtesy of Jörg Salzer from http://www.biologie.uni-ulm.de/bio3/biber/bib_project.html

When beaver offspring finally disperse, they do not usually settle far from their birth territory (Rosell and Bjorkoyli, 2002). Kin are able to recognize each other using their acute sense of smell to detect differences in anal gland secretion (AGS) composition (Sun and Mueller-Schwarze, 1998; Sun and Mueller-Schwarze, 2003). This kind of identification is a kind of phenotype recognition (Sun and Mueller-Schwarze, 1998; Sun and Mueller-Schwarze, 2003). AGS is made of varying concentrations of substances, probably governed strongly by genetic inheritance, as neither temporal nor spatial factors seem to affect it (Sun and Mueller-Schwarze, 1998). First-degree relatives, therefore, have more similar AGS compositions than beavers not related, and so relatives and non-relatives can be distinguished (Sun and Mueller-Schwarze, 1998). As mentioned in the social organization section, scents of related neighbors evoke a more tolerant response from neighboring beavers, so the ability to recognize kin can be very important in beaver social behavior (Rosell and Bjorkoyli, 2002). It has been proposed that AGS recognition is also used to categorize individuals into groups of relatedness (Sun and Mueller-Schwarze, 1998). In addition, AGS is very important for mother-offspring recognition (Sun and Mueller-Schwarze, 1998).