Editing Virginia opossum (section)

==Description==
[[File:Opossum Skull.jpg|thumb|Skull of a Virginia opossum|alt=]]
[[File:Virginia opossum opposable thumb.jpg|alt=|thumb|The back foot has an opposable "thumb".]]
Virginia opossums can vary considerably in size, with larger specimens found to the north of the opossum's range and smaller specimens in the tropics. They measure {{cvt|33–55|cm}} long from their snout to the base of the tail, with the tail adding another {{cvt|25|–|54|cm}}. Males are slightly larger, with an average body length of {{cvt|40.8|cm}} with an average tail length of {{cvt|29.4|cm}}, while females are {{cvt|40.6|cm}} long with a {{cvt|28.1|cm}} tail. Weight for males ranges from {{cvt|2.1–2.8|kg}} and for females from {{cvt|1.9|–|2.1|kg}}.<ref>{{Cite web|url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Didelphis_virginiana.html|title=ADW: Didelphis virginiana: Information (1974-05-02)|website=animaldiversity.ummz.umich.edu|access-date=2011-09-15}}</ref>

Their coats are a dull grayish brown, other than on their faces, which are white. Opossums have long, hairless, [[prehensile]] tails, which can be used to grab branches and carry small objects. They also have hairless ears and a long, flat nose. Opossums have [[dentition|50 teeth]], more than any other North American land mammal,<ref>[http://web.extension.illinois.edu/wildlife/directory_show.cfm?species=opossum Wildlife Directory: Virginia Opossum — Living with Wildlife — University of Illinois Extension] {{Webarchive|url=https://web.archive.org/web/20180513144116/http://web.extension.illinois.edu/wildlife/directory_show.cfm?species=opossum |date=2018-05-13 }}. Web.extension.illinois.edu. Retrieved on 2011-09-15.</ref> and opposable, clawless thumbs on their rear limbs. Opossums have 13 [[nipple]]s, arranged in a circle of 12 with one in the middle.<ref>[http://digitalcollections.fiu.edu/wild/transcripts/possums1.htm With the Wild Things - Transcripts] {{Webarchive|url=https://web.archive.org/web/20130323001102/http://digitalcollections.fiu.edu/wild/transcripts/possums1.htm |date=2013-03-23 }}. Digitalcollections.fiu.edu. Retrieved on 2011-09-15.</ref><ref>Mary Stockard, AWRC Mammal Supervisor (2001) [https://web.archive.org/web/20041223190821/http://www.awrc.org/Baby%20Opossums.htm Raising Orphaned Baby Opossums]. AWRC.org</ref>

The dental formula of an opossum is {{DentalFormula|upper=5.1.3.4|lower=4.1.3.4}}.<ref>{{Cite web|url=http://www.nsrl.ttu.edu/tmot1/didevirg.htm|title=Virginia Opossum (Didelphis virginiana)|website=www.nsrl.ttu.edu|access-date=2018-11-11}}</ref> No other mammal in North America has more than 6 upper [[incisor]]s, but the Virginia opossum has 10.

Perhaps surprisingly for such a widespread and successful species, the Virginia opossum has one of the lowest [[encephalization quotient]]s of any marsupial.<ref>{{cite journal
  | last = Ashwell
  | first = K.w.s. 
  | title = Encephalization of Australian and New Guinean Marsupials
  | journal = Brain, Behavior and Evolution
  | volume = 71
  | issue = 3
  | pages = 181–199
  | date = April 2008
  | issn = 0006-8977
  | doi = 10.1159/000114406
  | pmid = 18230970| s2cid = 7544968 
 }}
</ref>
Its brain is one-fifth the size of a raccoon's.<ref name=Audubon>{{cite web|url=http://www.massaudubon.org/Nature_Connection/wildlife/index.php?subject=Mammals&id=71|title=Virginia Opossum|publisher=Mass Audubon|access-date=May 11, 2011|quote=Opossums are frequently encountered as corpses along highways. Some biologists believe that many die as they feed on road-killed animals – a favorite food. Others believe that the opossums’ small brain (5 times smaller than that of a [[raccoon]][sic - erroneous logic]) suggests that they may just be too dumb to get out of the way of vehicles!|archive-date=December 29, 2010|archive-url=https://web.archive.org/web/20101229120106/http://www.massaudubon.org/Nature_Connection/wildlife/index.php?subject=Mammals&id=71|url-status=dead}}</ref>

===Tracks===
{{more citations needed section|date=March 2018}}
{{Multiple image
| image1            = Opossum and vole tracks in mud.JPG
| caption1          = Opossum tracks (photo center) in mud: Left-fore print appears on left center of photo, right-hind print appears right center. The small, circular tracks at bottom center of photo were made by a meadow vole. The yellow ruler (top) is in inches.
| image2            = Pacing diagram opossum.png
| caption2          = Pacing diagram for Virginia opossum - key: rectangles represent hind tracks, ellipses are fore tracks, left tracks are red, right are green. (a) the position of the four feet frozen in mid-pace. (b) the opossum brings right fore and hind feet forward. (c) the opossum brings left fore and hind feet forward. One grid square represents one square inch.
| total_width       = 350
}}
Virginia opossum tracks generally show five finger-like toes in both the fore and hind prints.<ref name=Krause>Krause, William J.; Krause, Winifred A. (2006).[https://web.missouri.edu/~krausew/Histology/Home_files/opossum.pdf  ''The Opossum: Its Amazing Story''] {{Webarchive|url=https://web.archive.org/web/20121211214644/https://web.missouri.edu/~krausew/Histology/Home_files/opossum.pdf |date=2012-12-11 }}. Department of Pathology and Anatomical Sciences, School of Medicine, University of Missouri, Columbia, Missouri. 80 pages.</ref> The hind tracks are unusual and distinctive due to the opossum's opposable [[thumb]], which generally prints at an angle of 90° or greater to the other fingers (sometimes near 180°). Individual adult tracks generally measure 1.9 in long by 2.0&nbsp;in wide (4.8 × 5.1&nbsp;cm) for the fore prints and 2.5 in long by 2.3 in wide (6.4 × 5.7&nbsp;cm) for the hind prints. Opossums have claws on all fingers fore and hind except on the two thumbs (in the photograph, claw marks show as small holes just beyond the tip of each finger); these generally show in the tracks. In a soft medium, such as the mud in this photograph, the foot pads clearly show (these are the deep, darker areas where the fingers and toes meet the rest of the hand or foot, which have been filled with plant debris by wind due to the advanced age of the tracks).

The tracks in the photograph were made while the opossum was walking with its typical pacing [[gait]]. The four aligned toes on the hind print show the approximate direction of travel.

In a pacing gait, the limbs on one side of the body are moved simultaneously, just prior to moving both limbs on the other side of the body. This is illustrated in the pacing diagram, which explains why the left-fore and right-hind tracks are generally found together (and vice versa). If the opossum was not walking (perhaps running), the prints would fall in a different pattern. Other animals that generally employ a pacing gait are [[raccoon]]s, [[bear]]s, [[skunk]]s, [[badger]]s, [[woodchuck]]s, [[porcupine]]s, and [[beaver]]s.

When pacing, the opossum's 'stride' generally measures from 7 to 10&nbsp;in, or 18 to 25&nbsp;cm (in the pacing diagram the stride is 8.5&nbsp;in, where one grid square is equal to 1 in<sup>2</sup>). To determine the stride of a pacing gait, measure from the tip (just beyond the fingers or toes in the direction of travel, disregarding claw marks) of one set of fore/hind tracks to the tip of the next set. By taking careful stride and track-size measurements, one can usually determine what species of animal created a set of tracks, even when individual track details are vague or obscured.