Monday, April 24, 2017

The American Beaver

Paul Fischer
4/24/2017
Professor Alice Daniels



The American Beaver


Before engineering, the first things everyone thinks of when a beaver is mentioned are the unique bright orange teeth that mark one of the mammal’s favorite past-times: forest felling. In fact, they are not completely orange, but instead this appearance is from a layer of outer enamel that is harder than the rest of the dentin in the tooth (Holland, 353). As an herbivore, the largest rodent in North America actually uses the sharp, beveled edge created by movement of the incisors against lower teeth to fell trees, shorten twigs, and sharpen their teeth as a recreational activity as seen in figure 2.
Such dams are necessary for breeding as they give beavers access to the water where the mating occurs almost exclusively as well as to the surrounding environment in the bitter cold of the months January to March (407). The escape of beavers during the occasional January thaw to retrieve fresh food can be seen in tracks such as that in figure (368). Females have a short period of time when they are receptive to the attention of the male, around 22-24 hours, and a successful incident will result in between three and six kits, or young beavers, being born between May and July.
Young beavers will frequently spend several weeks of their life inside the dams, or lodges one of which can be viewed in figure 1 (84). This is because of another fascinating feature of beavers, the oil and castoreum used to grease and waterproof fur, that develops a little later. They will be driven from the home after a few years and before little siblings are born, meaning brother and sister are terms without meaning for beavers.
Oil to waterproof the beavers are not the only fascinating aspect of the beaver that adapts the prolific mammal to life in water and on land alike. In fact, the adaptations that allow the beaver to exist on land and underwater are numerous and relatively unique among rodent species (321-3). From webbed feet to a nictitating membrane, or transparent third eylid, the top of the beaver to the tail has been dramatically altered for survival in a variety of warm and cold, dry and wet environments. Even the respiratory system of the beaver is fundamentally altered from that of other species to allow use of five times as much of the oxygen inhaled as humans and to voluntarily increase bloodflow to the brain allowing toleration of higher levels of CO2. To put that in perspective, some beavers might be able to survive, for a period at least, without a suit on parts of Mars, where oxygen levels run at under 1% the level found on Earth, though humans have permanent brain damage as levels of oxygen decrease to under 5 or even the 12% found at higher altitudes (Williams).
A high distribution of predators means that stealth is not only limited to ease of transport in multiple environments and naturally nocturnal behavior, but even the communication of beavers has been altered. Rather than using calls or acrobatic body language, their primary means of communication is through scent mounds of mud and vegetation mixed with pungent gland residue.

Figures:



Figure 1: Beaver dam in a wetland
Centennial Woods Natural Preserve, Vermont, 2017


Figure 2: Evidence of beaver lodge-building activity
Centennial Woods Natural Preserve, Vermont, 2017










References:


Holland, M. (2010). Naturally Curious. Trafalgar Square Books.

Williams, D. R. (2016). Mars fact sheet. NASA Goddard Space Flight Center: Greenbelt, MD Retrieved from http://nssdc. gsfc. nasa. gov/planetary/factsheet/marsfact  (April 24, 2017).

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