Editing Kangaroo (section)

=== Adaptations ===
[[File:Kangaroo and joey03.jpg|thumb|upright=0.7|Kangaroo and joey in pouch]]
[[File:Kaenguru Hinterfuss-drawing.jpg|thumb|upright=0.7|The hind leg of a kangaroo]]
Kangaroos have developed a number of adaptations to a dry, infertile country and highly variable climate. As with all [[marsupial]]s, the young are born at a very early stage of development—after a [[gestation]] of 31–36 days. At this stage, only the forelimbs are somewhat developed, to allow the newborn to climb to the [[Pouch (marsupial)|pouch]] and attach to a [[teat]]. In comparison, a human [[embryo]] at a similar stage of development would be at about 7 weeks gestation (even in a modern intensive care unit, [[Premature birth|premature babies]] born at less than 23 weeks gestation are usually not mature enough to survive). When the joey is born, it is about the size of a [[Phaseolus lunatus|lima bean]]. The joey will usually stay in the pouch for about 9 months (180–320 days for the Western Grey) before starting to leave the pouch for small periods of time. It is usually fed by its mother until reaching 18 months.

The female kangaroo is usually pregnant in permanence, except on the day she gives birth; however, she has the ability to freeze the development of an embryo until the previous [[Joey (marsupial)|joey]] is able to leave the pouch. This is known as [[embryonic diapause]] and will occur in times of drought and in areas with poor food sources. The composition of the milk produced by the mother varies according to the needs of the joey. In addition, the mother is able to produce two different kinds of milk simultaneously for the newborn and the older joey still in the pouch.

Unusually, during a dry period, males will not produce sperm and females will conceive only if enough rain has fallen to produce a large quantity of green vegetation.<ref name="animal">{{cite book | last = Burnie | first = David |author2=Don E. Wilson | title = Animal | publisher=DK | year = 2001 | location = New York NY | pages = 99–101 | isbn = 978-0-7894-7764-4}}</ref>

Kangaroos and wallabies have large, elastic tendons in their hind legs. They store elastic strain energy in the [[tendon]]s of their large hind legs, providing most of the energy required for each hop by the spring action of the tendons rather than by any muscular effort.<ref name="tendons & hopping">{{cite book| last=Dawson| first=Terence J. (Professor UNSW)| title=Kangaroos: Biology of the largest marsupials| year=1998| publisher=Comstock Pub. Associates| location=Ithaca, N.Y.| isbn=9780801482625| pages=7–11| url=https://books.google.com/books?id=KNxIlprcmTYC&q=ancestors+of+kangaroos&pg=PA6| edition=2nd| access-date=3 August 2013| format=Paperback| archive-date=16 May 2021| archive-url=https://web.archive.org/web/20210516185428/https://books.google.com/books?id=KNxIlprcmTYC&q=ancestors+of+kangaroos&pg=PA6| url-status=live}}</ref> This is true in all animal species which have muscles connected to their skeletons through elastic elements such as tendons, but the effect is more pronounced in kangaroos.

There is also a link between the hopping action and breathing: as the feet leave the ground, air is expelled from the lungs; bringing the feet forward ready for landing refills the lungs, providing further energy efficiency. Studies of kangaroos and [[Wallaby|wallabies]] have demonstrated, beyond the minimum energy expenditure required to hop at all, increased speed requires very little extra effort (much less than the same speed increase in, say, a horse, dog or human), and the extra energy is required to carry extra weight. For kangaroos, the key benefit of hopping is not speed to escape predators—the top speed of a kangaroo is no higher than that of a similarly sized quadruped, and the Australian native predators are in any case less fearsome than those of other countries—but economy: in an infertile country with highly variable weather patterns, the ability of a kangaroo to travel long distances at moderately high speed in search of food sources is crucial to survival.

New research has revealed that a kangaroo's tail acts as a third leg rather than just a balancing strut. Kangaroos have a unique three-stage walk where they plant their front legs and tail first, then push off their tail, followed lastly by the back legs. The propulsive force of the tail is equal to that of both the front and hind legs combined and performs as much work as what a human leg walking can at the same speed.<ref>{{cite news|title=Kangaroos use tail like a leg to walk|url=http://www.australiangeographic.com.au/news/2014/07/kangaroos-use-tail-to-walk|access-date=18 November 2014|website=Australian Geographic|date=2 July 2014|archive-date=10 November 2014|archive-url=https://web.archive.org/web/20141110100429/http://www.australiangeographic.com.au/news/2014/07/kangaroos-use-tail-to-walk|url-status=live}}</ref>

A [[DNA sequencing]] project of the [[genome]] of a member of the kangaroo family, the [[tammar wallaby]], was started in 2004. It was a collaboration between Australia (mainly funded by the [[Victoria (Australia)|State of Victoria]]) and the [[National Institutes of Health]] in the US.<ref name="genome">{{Cite press release|author=Spencer, Geoff |title=Kangaroo Hops in Line for Genome Sequencing |url=http://www.nih.gov/news/pr/jun2004/nhgri-08.htm |work=National Human Genome Research Institute |publisher=National Institutes of Health |access-date=2 October 2011 |date=8 June 2004 |url-status=dead |archive-url=https://web.archive.org/web/20111002020057/http://www.nih.gov/news/pr/jun2004/nhgri-08.htm |archive-date=2 October 2011 }}</ref> The tammar's genome was fully sequenced in 2011.<ref name="Renfree/Papenfuss">{{cite journal |author=Renfree M. B. |author2=Papenfuss, A. T. |author3=Deakin, J. E. |display-authors=etal | title=Genome sequence of an Australian kangaroo, ''Macropus eugenii'', provides insight into the evolution of mammalian reproduction and development.| journal=Genome Biology| volume=12| issue=8| pages=R81| pmid=21854559| doi=10.1186/gb-2011-12-8-r81| pmc=3277949 | date=19 August 2011 |doi-access=free }}</ref> The genome of a marsupial such as the kangaroo is of great interest to scientists studying [[comparative genomics]], because marsupials are at an ideal degree of evolutionary divergence from humans: [[mouse|mice]] are too close and have not developed many different functions, while birds are genetically too remote. The dairy industry could also benefit from this project.<ref>{{cite journal|title=The tammar wallaby: A model to examine endocrine and local control of lactation|journal=IUBMB Life|last1=Brennan|first1=A. J.|last2=Sharp|first2=J. A.|last3=Digby|first3=M. R.|last4=Nicholas|first4=K. R.|volume=59|issue=3|pages=146–150|year=2007|doi=10.1080/15216540701335724|doi-access=free|pmid=17487685}}</ref>