Editing Flying squirrel (section)

===Evolutionary history===
Prior to the 21st century, the evolutionary history of the flying squirrel was frequently debated.<ref>{{cite journal|last=Arbogast|first=B.S.|title=A brief history of the new world flying squirrels: Phylogeny, biogeography, and conservation genetics|journal=Journal of Mammalogy|year=2007|volume=88|issue=4|pages=840–849|doi=10.1644/06-MAMM-S-322R1.1|doi-access=free}}</ref> This debate was clarified greatly as a result of two molecular studies.<ref>{{cite journal|last=Mercer|first=J.M.|author2=V.L. Roth|s2cid=40366357|title=The effects of cenozoic global change on squirrel phylogeny|journal=Science|year=2003|volume=299|issue=5612|pages=1568–1572|doi=10.1126/science.1079705|pmid=12595609|bibcode=2003Sci...299.1568M}}</ref><ref>{{cite journal|last=Steppan|first=S.J. |author2=B.L. Storz |author3=R.S. Hoffmann|title=Nuclear DNA phylogeny of the squirrels (Mammalia : Rodentia) and the evolution of arboreality from c-myc and RAG1|journal=Molecular Phylogenetics and Evolution|year=2004|volume=30|issue=3|pages=703–719|doi=10.1016/S1055-7903(03)00204-5|pmid=15012949|bibcode=2004MolPE..30..703S }}</ref> These studies found support that flying squirrels originated 18–20 million years ago, are monophyletic, and have a sister relationship with tree squirrels. Due to their close ancestry, the morphological differences between flying squirrels and tree squirrels reveal insight into the formation of the gliding mechanism. Compared to squirrels of similar size, flying squirrels, northern and southern flying squirrels show lengthening in bones of the lumbar vertebrae and forearm, whereas bones of the feet, hands, and distal vertebrae are reduced in length. Such differences in body proportions reveal the flying squirrels' adaptation to minimize wing loading and to increase maneuverability while gliding. The consequence for these differences is that unlike regular squirrels, flying squirrels are not well adapted for quadrupedal locomotion and therefore must rely more heavily on their gliding abilities.<ref>{{Cite journal|title = How to make a flying squirrel: Glaucomys anatomy in phylogenetic perspective|last1 = Thorington|first1 = Richard W.|date = 2007|journal = Journal of Mammalogy|doi = 10.1644/06-mamm-s-325r2.1|first2 = Erica M.|last2 = Santana|volume = 88|issue = 4|pages = 882–896|doi-access = free}}</ref>

Several hypotheses have attempted to explain the evolution of gliding in flying squirrels.<ref>{{cite journal|last=Flaherty|first=E.A. |author2=M. Ben-David |author3=W.P. Smith|title=Quadrupedal locomotor performance in two species of arboreal squirrels: predicting energy savings of gliding|journal=Journal of Comparative Physiology B|year=2010|volume=180|issue=7|pages=1067–1078|doi=10.1007/s00360-010-0470-1|pmid=20361193 |s2cid=240833 }}</ref> One possible explanation is related to energy efficiency and foraging.<ref name=":0">{{Cite journal|title = Evolution of vertebrate flight: an aerodynamic model for the transition from gliding to active flight|last = Norberg|first = Ulla M.|date = 1985|journal = American Naturalist|doi = 10.1086/284419|pages = 303–327|volume = 126|issue = 3|s2cid = 85306259}}</ref><ref name=":1">{{Cite journal|title = Take-off and landing forces and the evolution of controlled gliding in northern flying squirrels Glaucomys sabrinus|last1 = Paskins|first1 = Keith E.|date = 2007|journal = The Journal of Experimental Biology|doi = 10.1242/jeb.02747|pmid = 17401124|last2 = Bowyer|first2 = Adrian|last3 = Megill|last4 = Scheibe|first3 = William M.|first4 = John S.|pages = 1413–1423|volume = 210|issue = Pt 8|doi-access = free}}</ref> Gliding is an energetically efficient way to progress from one tree to another while foraging, as opposed to climbing down trees and maneuvering on the ground floor or executing dangerous leaps in the air.<ref name=":0" /> By gliding at high speeds, flying squirrels can rummage through a greater area of forest more quickly than tree squirrels. Flying squirrels can glide long distances by increasing their aerial speed and increasing their lift.<ref name=":1" />

Other hypotheses state that the mechanism evolved to avoid nearby predators and prevent injuries. If a dangerous situation arises on a specific tree, flying squirrels can glide to another, and thereby typically escape the previous danger.<ref name=":1" /><ref>{{Cite journal|title = Morphological attributes of gliding rodents: a preliminary analysis|last1 = Scheibe|first1 = John S.|date = 1990|journal = Transactions of the Missouri Academy of Science|last2 = Figgs|first2 = Daylan|first3 = Jeff|last3 = Heiland|volume = 24|pages = 49–56}}</ref> Furthermore, take-off and landing procedures during leaps, implemented for safety purposes, may explain the gliding mechanism. While leaps at high speeds are important to escape danger, the high-force impact of landing on a new tree could be detrimental to a squirrel's health.<ref name=":1" /> Yet the gliding mechanism of flying squirrels involves structures and techniques during flight that allow for great stability and control. If a leap is miscalculated, a flying squirrel may easily steer back onto the original course by using its gliding ability.<ref name=":1" /> A flying squirrel also creates a large glide angle when approaching its target tree, decreasing its velocity due to an increase in air resistance and allowing all four limbs to absorb the impact of the target.<ref name=":1" /><ref>{{Cite journal|title = Ecological and Biomechanical Insights into the Evolution of Gliding in Mammals|last1 = Byrnes|first1 = Greg|date = 2011|journal = Integrative and Comparative Biology|doi = 10.1093/icb/icr069|pmid = 21719434|last2 = Spence|first2 = Andrew J.|pages = 991–1001|volume = 51|issue = 6|doi-access = free}}</ref>