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By Jennie Liu
Extinct Beaver - The Giant BeaverFuture Beaver - Beaverine
WHAT ARE BEAVERS?



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Fig 1: Photo of a Beaver

Beavers (Castor canadensis) are the second largest rodent in the world. They are nocturnal and semi-aquatic rodents (mammals) of around the same size as a domestic dog. Other semi-aquatic mammals include Otters and Platypuses. There are two species of beavers, the North American Beaver and the Eurasian Beaver. Both species of beavers are well known for their abilities of constructing dams and lodges. Beavers are herbivores which means they are a type of organism that eat plants. Beavers feed mostly on wood but sometimes also on river plants. Beavers were once near extinction but is gradually recovering. Adult beavers do not exceed a weight of 25 kg and have a lifespan of around 24 years. They usually give birth to 2-4 kits at a time. Beavers have been introduced to many countries of the world. Unfortunately, beavers are being hunted for their fur and glands. Humans use their fur to make coats and use their glands to make perfume and medicine.


BEAVERS' HABITAT


Area of the World It Lives in:




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Fig 2: Areas where beavers live in are shaded in red and green.

Beavers live in areas of North America and Eurasia and this is how the two species of beavers are split. The two species of beavers are the North American Beaver and the Eurasian Beaver.

Eurasian Beavers are found in mainland Europe, Northern Europe and Great Britain. The beavers are still recovering from near extinction and in fact beavers went extinct in a many European countries but were reintroduced. 83% of Eurasian beavers live in the area of the Soviet Union, which is now the area of Russian and surrounding countries. The Eurasian Beavers have previously been introduced to many countries and states in mainland Europe including Romania, Bavaria, Netherlands, Biesbosch, Limburg, Gelderse Poort and Oostvaardersplassen. Beavers are found in a few countries of Northern Europe including Sweden, Norway, Finland, Canada, Denmark, Germany and Denmark. Beavers in most of these countries have once been hunted to extinction but have been reintroduced. Eurasian Beavers once became extinct in Great Britain during the sixteenth century but have slowly recovered. They are now found in countries such as England, Wales and Scotland.

The North American Beavers are, of course, found in countries of North America. Most of them are found in Canada and some are found in Alaska, Florida, Mexico and parts of the United States. North American Beavers have been introduced to Europe, Argentina and some areas of Asia. In Europe, the habitat of the North American Beavers is shared with Eurasian Beavers.

During the early 20th century, beavers became very endangered. However with the decline in poaching and products made from Beavers, the size of Beaver populations have grown.


Vegetation of Beaver’s Habitat:
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Fig 3: Diagram of the different types of zones (riparian zone is the zone beavers live in)

Beavers a semi-aquatic animals meaning that his habitat is both underwater and on land. The habitat of the beaver is in rivers, lakes and along the banks of streams or simply the riparian zone. A riparian zone is the area between land and a lake or stream. These areas provide beavers with materials that they use to build dams, lodges and bank dens. Beavers need water of at least 60-90 centimeters deep to store food for the winters, hide from predators and transport logs. If the water is not deep enough or if the beavers want to increase the size of their habitat, they will dig deeper into the riverbed and construct a dam. The dams built by beavers create marshlands that supply them with a great quantity of food that can be easily accessed. Beavers prefer areas of flat land in fertile valleys rather than steep slopes as this helps them create small ponds with their dams.





Predators & Preys:

Beavers have few natural predators as they most of the under protection, either in their lodges or underwater. However they still do have a few predators. The two main predators are, in fact, humans and wolves. Humans damage the habitat of beavers greatly and activities that cause this damage include deforestation and silting of streams while air pollution is also a factor effecting beavers’ habitat. However, people are not especially concerned about this and do nothing to put a stop to the decline of beaver populations. Some other predators include coyotes, bears, lynxes, mountain lions, red foxes, dogs and wolverines.

Beavers are prey themselves, as they do not eat any meat. Beavers eat a combination of bark, roots, leavers, oak, cottonwood, willow, alder, birch trees, water plants and small shrubs. However, due to the activities of humans, marshes become drained and all the food of the beavers will be lost. When humans clear land to build houses, the beavers will not have a sufficient amount of log to build the dams and lodges thus causing destruction in the homes of beavers. Sometimes humans pollute the water beavers live in with chemicals and this also destroys their habitat.

Dams and Lodges:

Beavers build dams to protect themselves from predators, to provide easier access to food and to store food for the winter. The water level of a beaver’s habitat must be at least 6-9 meters deep so the entrance of their lodge will not be blocked by ice during the colder seasons. If a lake is not of this depth, the beaver will construct a dam to increase the water level. A dam will raise the water level of a lake or river as it slows down the river or lake to a flowing stream. This turns the surrounding areas into a meadow with nutrients and moisture. However a beaver’s dam may possibly cause deforestation or flooding.

The type of dam that a beaver builds and how they will build it depends on the speed of the water. Beavers need a lot of materials to build a dam including branches, logs, sticks, bark, rocks, mud, grass, leaves and plants. Beavers work and build dams at night and if a dam is destroyed, it can rebuild it in just one night. A dam consists of two entrances; there is a small hole at the top for ventilation and a center pod.


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Fig 4: Photo of a Beaver's Dam

Lodges are homes of beavers. A lodge is usually around 6 meters in diameter and is cone shaped. It is made from the same materials used to build dams and is built along the shores of lakes. Usually no more than six or eight beaver kits and four adult beavers live in one lodge. Beavers begin to build their lodges around late autumn with fresh mud and when winter arrives, the mud freezes and becomes so hard that predators cannot penetrate it. Beavers enter a lodge through underwater entrances that other animals can’t get in.







beaver-2.jpgmaroon-bells-beaver-dam.jpgFig 5: Illustration of a Beaver's LodgeFig 6: Photo of a Beaver's Lodge

BEAVER'S ADAPTATIONS


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Fig 7: Diagram of a beaver's adaptations
Structural – Water Proof Coat
Beaver’s coats keep the skin of the beaver dry and they act as insulators. The beaver’s scent glands produce water-repellent oil called castoreum or castor oil. Regularly, the beavers will comb this oil through their fur with their grooming claws that have split toenails. Through this method, beavers are able to keep their coats waterproof and keep themselves dry and warm in winter.

Beavers spend most of the winter in cold water in their lodges. Beavers also swim around a lot in the water while building dams and lodges. In order to keep themselves alive during the winter in the freezing water, beavers must have some sort of protection to the cold. This is why beavers have evolved to have thick and waterproof coats.


Structural – Flat, wide tail
Beavers are well known for their flat and wide tails. A beaver’s tail serves multiple purposes. Firstly, the tail stores fat and radiates heat du
Fig 8: Flat & wide tail of beaver
Fig 8: Flat & wide tail of beaver
ring thecolder seasons such as fall and winter and during the warmer seasons the tail releases excess heat from the body. Beavers are good swimmers because of their tail. Their flat tail acts as a rudder while they swim underwater and helps them to steer while in the water. Beavers also use their tail for communication and protection. When they encounter predators such as coyotes and dogs, they will slap their tails on the surface of the water to startle the predators and to war the colony that there is danger. When building dams, beavers would use their tails to haul mud. Sometimes beavers will even carry their young on their tails.

There are explanations to the occurrence of this adaptation during the years of evolution. Firstly, the reason as to why beavers’ tails store fat is that beavers are active during the colder seasons and spend a lot of time under cold icy water in winter. Beavers were alive during the time of the Ice Age, the Pleistocene Era, so this adaptation of storing fats in their tails might have developed during this era and might have helped them survive the Ice Age. Also, beavers use their tails as rudders. This has developed to help them swim faster and better to enhance their abilities in building dams and lodges or to escape from predators.

Structural – Clear Eyelids
Beavers have underwater vision since they have clear eyelids so when they close their eyes they are still able to see. This feature helps them see underwater and to protect their eyes when they are underwater. This improves their ability to swim underwater and increases their water activity making it easier and faster for them to build lodges and dams.

The environmental factor that could have caused this is that there is many particulates inside water and may go inside the beaver’s eyes and hurt them while they are swimming around. Also in order to survive, beavers must build dams and lodges to protect themselves and they must swim a lot underwater to build them. To allow them to swim faster and build the dams and lodges in a shorter amount of they, they evolved to have clear eyelids.


Physiological – Valves in ears and nose
Beavers have valves in their ears and noses that close when they dive underwater. This helps them stay longer in the water and allow them to dive deeper. With this feature, the water activities of beavers are enhanced making it easier for them to travel around in water to build their homes and to run away from predators, as they are more skillful in water than on land.

Two environmental pressure factors caused the development of this feature. Firstly, it is much easier and convenient for beavers to travel around in water and they must build dams and lodges thus this feature was developed. Also bea
vers are clumsier on land and in order to escape from land predators they have to swim underwater.

Physiological – Oxygen Conservation
Beavers have large lungs and large livers for storing oxidized blood. This helps them stay underwater for a very long period of time, in fact they can stay underwater for as long as fifteen minutes.

It is possible and very likely that this feature has developed as a result of the beavers always spending time underwater, once again, to build damns and lodges, to hide from predators or to survive the cold and harsh weathers during winter.


Behavioral – Raising the young
Fig 9: A beaver with its kits
Fig 9: A beaver with its kits

Beavers usually give birth to 2 or 3 beaver kits. After the 106 days of development, the baby beavers stay with their parents for two years learning how to construct dams and lodges, how to eat and how to survive. The purpose is to ensure that the beaver kit will have all the knowledge they need in order to survive by themselves in the future and the two-year period is very important.

The most appropriate explanation for the development of this feature is that when beavers evolved from an aquatic fish to a land and semi-aquatic animal they did not know how to survive on land and in order to allow their offspring to survive on land, they developed this ‘system’ of teaching their young in a two year period the knowledge they need to construct basic lodges using the surrounding materials.




BIBLIOGRAPHY




  1. Beavers: Wetlands & Wildlife. (no date). About Beavers. Retrieved 19th June 2011, from http://www.beaversww.org/beavers-and-wetlands/about-beavers/
  2. Biology at Davidson. (2007). Habitat. Retrieved 23rd June 2011, from http://www.bio.davidson.edu/people/vecase/behavior/Spring2004/Koehrn/habitat.htm
  3. Critter Control. (no date). Beaver. Retrieved 20th June 2011, from http://www.crittercontrol.com/facts/animals/beaver.html
  4. Fohn. (2007). Beaver Pictures & Facts. Retrieved 22nd June 2011, from http://fohn.net/beaver-pictures-facts/beaver-dams.html
  5. How Stuff Works. (2011). Why do beavers build dams? Retrieved 19th June 2011, from http://animals.howstuffworks.com/mammals/beaver-dam.htm
  6. The Daily Puppy. (2011). What Adaptations Do Beavers Have to Survive? Retrieved 18th June 2011, from http://www.dailypuppy.com/articles/what-adaptations-do-beavers-have-to-survive/acd643ad-83a8-9282-be6e-d2e3897b5a51
  7. Wikipedia. (2011). Beavers. Retrieved 20th June 2011, from http://en.wikipedia.org/wiki/Beaver

IMAGE SOURCES


Fig 1: Photo of a beaver - http://nationalzoo.si.edu/publications/zoogoer/2002/2/beaverbasics.cfm
Fig 2: Areas where beavers live - http://en.wikipedia.org/wiki/File:Locations_eurobeaver.png
Fig 3: Diagram of the different types of zones - http://www.co.benton.or.us/parks/facilities/clemens_nfguide.php
Fig 4: Photo of a beaver's dam - http://www.howstuffworks.com/beaver-dam.htm
Fig 5: Illustration of a beaver's lodge - http://animals.howstuffworks.com/mammals/beaver-dam.htm
Fig 6: Photo of a beaver's lodge - http://www.mccullagh.org/image/13/maroon-bells-beaver-dam.html
Fig 7: Diagram of a beaver's adaptations
- Image from http://thestickytongue.org/wp-content/uploads/2011/01/300px-American_Beaver.jpg
- Labels by Jennie Liu
Fig 8: Flat & wide tail of beaver - http://www.infovisual.info/02/photo/beaver.html
Fig 9: A beaver with its kits - http://dale-joanne.com/NearHome/NWTrek/NWTBeaver0192.jpg