Editing Beaver (section)

==Characteristics==

[[File:Beaver skeleton.jpg|thumb|left|alt=see caption|Mounted North American beaver skeleton.]]

Beavers are the second-largest living rodents, after [[Hydrochoerus|capybara]]s. They have a head–body length of {{convert|80|–|120|cm|abbr=on}}, with a {{convert|25|–|50|cm|0|abbr=on}} tail, a shoulder height of {{convert|30|–|60|cm|abbr=on}}, and generally weigh {{convert|11|–|30|kg|abbr=on}},<ref name="MacDonald">{{Cite book |last1=Busher |first1=P. |url=https://archive.org/details/encyclopediaofma0000unse_m0x7/page/590/mode/2up |title=The Encyclopedia of Mammals |last2=Hartman |first2=G. |publisher=[[Oxford University Press]] |year=2001 |isbn=978-0760719695 |editor=MacDonald |editor-first=D. W. |edition=2nd |pages=590–593 |contribution=Beavers}}</ref> but can be as heavy as {{convert|50|kg|lb|abbr=on}}. Males and females are almost identical externally.{{sfn|Müller-Schwarze|Sun|2003|pp=10, 14}} Their bodies are streamlined like [[marine mammal]]s and their robust build allows them to pull heavy loads.<ref name=JohnHopkins/>{{sfn|Runtz|2015|p=73}} A beaver [[Animal coat|coat]] has 12,000–23,000 hairs/cm<sup>2</sup> (77,000–148,000 hairs/in<sup>2</sup>) and functions to keep the animal warm, to help it float in water, and to protect it against predators. [[Guard hairs]] are {{convert|5|–|6|cm|abbr=on}} long and typically reddish brown, but can range from yellowish brown to nearly black. The [[underfur]] is {{convert|2|–|3|cm|1|abbr=on}} long and dark gray. Beavers [[molt]] every summer.<ref name="MacDonald"/>{{sfn|Müller-Schwarze|Sun|2003|pp=12–13}}

Beavers have large skulls with powerful [[Muscles of mastication|chewing muscles]]. They have four chisel-shaped incisors that continue to grow throughout their lives. The incisors are covered in a thick [[tooth enamel|enamel]] that is colored orange or reddish-brown by iron compounds.{{sfn|Müller-Schwarze|Sun|2003|pp=11–12}}<ref>{{cite journal |last1=Gordon |first1=L. M. |last2=Cohn |first2=M. J.|last3=MacRenaris |first3=K. W. |last4=Pasteris |first4=J. D. |last5=Seda |first5=T. |last6=Joester |first6=D. |year=2015 |title=Amorphous intergranular phases control the properties of rodent tooth enamel|journal=[[Science (journal)|Science]] |volume=347 |issue=6223 |pages=746–750 |doi=10.1126/science.1258950 |pmid=25678658 |bibcode=2015Sci...347..746G |s2cid=8762487|doi-access=free }}</ref> The lower incisors have roots that are almost as long as the entire lower jaw. Beavers have one premolar and three molars on all four sides of the jaws, adding up to 20 teeth. The molars have meandering ridges for grinding woody material.{{sfn|Müller-Schwarze|Sun|2003|p=12}} The eyes, ears and nostrils are arranged so that they can remain above water while the rest of the body is submerged. The nostrils and ears have valves that close underwater, while [[nictitating membranes]] cover the eyes. To protect the [[larynx]] and [[trachea]] from water flow, the [[epiglottis]] is contained within the nasal cavity instead of the throat. In addition, the back of the tongue can rise and create a waterproof seal. A beaver's lips can close behind the incisors, preventing water from entering their mouths as they cut and bite onto things while submerged.{{sfn|Runtz|2015|p=55}}<ref name=Pelagic/>

[[File:Picture Natural History - No 40 41 42 - Beaver feet and tail.png|thumb|right|alt=Illustration of a fore foot, a hind foot showing webbing, and the tail of a beaver|The fore foot, hind foot, and tail of a beaver.]]
[[File:Beaver tail print on snow.jpg|thumb|Beaver tail and footprints on snow.]]

The beaver's front feet are dexterous, allowing them to grasp and manipulate objects and food, as well as dig. The hind feet are larger and have [[webbed foot|webbing between the toes]], and the second innermost toe has a "double nail" used for grooming.<ref name=Pelagic/>{{sfn|Runtz|2015|p=71}} Beavers can swim at {{cvt|8|km/h|0|abbr=on}};{{sfn|Runtz|2015|p=73}} only their webbed hind feet are used to swim, while the front feet fold under the chest.<ref name=Pelagic>{{cite book|last1=Campbell-Palmer|first1=Róisín|last2=Gow|first2=Derek|last3=Needham|first3=Robert|last4=Jones|first4=Simon|last5=Rosell|first5=Frank|date=2015|title=The Eurasian Beaver|publisher=Pelagic Publishing Ltd|pages=7–12|isbn=978-1784270407}}</ref> On the surface, the hind limbs thrust one after the other; while underwater, they move at the same time.<ref>{{cite journal|last1=Allers|first1=D.|last2=Culik|first2=B. M.|year=1997|title=Energy Requirements of Beavers (''Castor canadensis'') Swimming Underwater |journal=[[Physiological Zoology]]|volume=70|issue=4|pages=456–463|doi=10.1086/515852|pmid=9237306|s2cid=21784970}}</ref> Beavers are awkward on land but can move quickly when they feel threatened. They can carry objects while walking on their hind legs.<ref name=JohnHopkins>{{cite book |last1=Baker |first1=B. W. |last2=Hill|first2=E. P.|contribution=Beaver ''Castor canadensis'' |title=Wild Mammals of North America: Biology, Management, and Conservation|editor1-last=Feldhamer |editor1-first=G. A.|editor2-last=Thompson|editor2-first=B. C.|editor3-last=Chapman|editor3-first=J. A.|publisher=[[Johns Hopkins University Press]]|edition=2|year=2003|pages=289–297 |isbn=978-0801874161|oclc=51969059}}</ref><ref name=Pelagic/>

The beaver's distinctive tail has a conical, muscular, hairy base; the remaining two-thirds of the appendage is flat and scaly. The tail has multiple functions: it provides support for the animal when it is upright (such as when chewing down a tree), acts as a rudder when it is swimming, and stores fat for winter. It also has a [[countercurrent exchange|countercurrent]] blood vessel system which allows the animal to lose heat in warm temperatures and retain heat in cold temperatures.{{sfn|Runtz|2015|pp=55, 63–67}}

The beaver's sex organs are inside the body, and the male's penis has a cartilaginous [[baculum]]. They have only one opening, a [[cloaca]], which is used for reproduction, scent-marking, defecation, and urination. The cloaca evolved secondarily, as most mammals have lost this feature, and may reduce the area vulnerable to infection in dirty water. The beaver's intestine is six times longer than its body, and the [[caecum]] is double the volume of its stomach.{{sfn|Müller-Schwarze|Sun|2003|pp=13–14, 17, 44}} Microorganisms in the caecum allow them to process around 30 percent of the [[cellulose]] they eat.<ref name=JohnHopkins/> A beaver defecates in the water, leaving behind balls of sawdust. Female beavers have four [[mammary glands]]; these produce milk with 19 percent fat, a higher fat content than other rodents. Beavers have two pairs of glands: castor sacs, which are part of the urethra, and [[anal gland]]s. The castor sacs secrete [[castoreum]], a liquid substance used mainly for marking territory. Anal glands produce an oily substance which the beaver uses as a waterproof ointment for its coat. The substance plays a role in individual and family recognition. Anal secretions are darker in females than males among Eurasian beavers, while the reverse is true for the North American species.{{sfn|Müller-Schwarze|Sun|2003|pp=6, 13–14, 41–45}}
[[File:Castor fiber vistulanus3.jpg|thumb|left|Eurasian beaver swimming.|alt=Beaver swimming]]
Compared to many other rodents, a beaver's brain has a [[hypothalamus]] that is much smaller than the [[cerebrum]]; this indicates a relatively advanced brain with higher intelligence. The [[cerebellum]] is large, allowing the animal to move within a three-dimensional space (such as underwater) similar to tree-climbing squirrels. The [[neocortex]] is devoted mainly to [[Somatosensory system|touch]] and hearing. Touch is more advanced in the lips and hands than the [[whiskers]] and tail. Vision in the beaver is relatively poor; the beaver eye cannot see as well underwater as an [[otter]]. Beavers have a good sense of smell, which they use for detecting land predators and for inspecting scent marks, food, and other individuals.{{sfn|Müller-Schwarze|Sun|2003|pp=11, 14–15}}

Beavers can hold their breath for as long as 15 minutes but typically remain underwater for no more than five or six minutes.{{sfn|Runtz|2015|p=74}} Dives typically last less than 30 seconds and are usually no more than {{convert|1|m|abbr=on}} deep.<ref>{{cite journal|last1=Graf|first1=P. M.|last2=Wilson|first2=R. P.|last3=Sanchez|first3=L. C.|last4=Hacklӓnder|first4=K.|last5=Rosell|first5=F.|year=2017|title=Diving behavior in a free-living, semi-aquatic herbivore, the Eurasian beaver ''Castor fiber''|journal=Ecology and Evolution|volume=8|issue=2|pages=997–1008|doi=10.1002/ece3.3726|pmid=29375773|pmc=5773300|doi-access=free}}</ref> When diving, their heart rate decreases to 60 beats per minute, half its normal pace, and blood flow is directed more towards the brain. A beaver's body also has a high tolerance for carbon dioxide. When surfacing, the animal can replace 75 percent of the air in its lungs in one breath, compared to 15 percent for a human.<ref name=Pelagic/>{{sfn|Runtz|2015|p=74}}