Editing Walrus (section)

==Life history==
[[File:Noaa-walrus12.jpg|thumb|Walruses fighting|alt=Photo of five walruses on rocky shore]]

===Reproduction===
Walruses live to about 20–30 years old in the wild.<ref>{{cite journal|vauthors = Fay FH|year = 1960|title = Carnivorous walrus and some arctic zoonoses|url = http://pubs.aina.ucalgary.ca/arctic/Arctic13-2-111.pdf|journal = Arctic|volume = 13|issue = 2|pages = 111–122|doi = 10.14430/arctic3691|access-date = 3 December 2010|archive-date = 9 August 2017|archive-url = https://web.archive.org/web/20170809040102/http://pubs.aina.ucalgary.ca/arctic/Arctic13-2-111.pdf|url-status = dead}} [http://pubs.aina.ucalgary.ca/arctic/Arctic14-1-76.pdf Comment] {{Webarchive|url=https://web.archive.org/web/20080409082148/http://pubs.aina.ucalgary.ca/arctic/Arctic14-1-76.pdf |date=9 April 2008 }}</ref> The males reach [[sexual maturity]] as early as seven years, but do not typically mate until fully developed at around 15 years of age.<ref name="Fay85"/> They [[Rut (mammalian reproduction)|rut]] from January through April, decreasing their food intake dramatically. The females begin ovulating as soon as four to six years old.<ref name="Fay85"/> The females are [[oestrus|diestrous]], coming into heat in late summer and around February, yet the males are fertile only around February; the potential fertility of this second period is unknown. Breeding occurs from January to March, peaking in February. Males aggregate in the water around ice-bound groups of estrous females and engage in competitive vocal displays.<ref>{{cite journal| vauthors = Nowicki SN, Stirling I, Sjare B |year = 1997|title = Duration of stereotypes underwater vocal displays by make Atlantic walruses in relation to aerobic dive limit|journal = Marine Mammal Science|volume = 13|issue = 4|pages = 566–575|doi = 10.1111/j.1748-7692.1997.tb00084.x| bibcode=1997MMamS..13..566N }}</ref> The females join them and [[Copulation (zoology)|copulate]] in the water.<ref name="Fay82"/>

[[File:Walrus - Kamogawa Seaworld - pup -1.jpg|thumb|A walrus pup at [[Kamogawa Sea World|Kamogawa Seaworld]], Japan]]
[[Gestation]] lasts 15 to 16 months. The first three to four months are spent with the [[blastula]] in suspended development before it implants itself in the uterus. This strategy of [[delayed implantation]], common among pinnipeds, presumably evolved to optimize both the mating season and the birthing season, determined by ecological conditions that promote newborn survival.<ref>{{cite journal | vauthors = Sandell M | title = The evolution of seasonal delayed implantation | journal = The Quarterly Review of Biology | volume = 65 | issue = 1 | pages = 23–42 | date = March 1990 | pmid = 2186428 | doi = 10.1086/416583 | s2cid = 35615292 }}</ref> Calves are born during the spring migration, from April to June. They weigh {{convert|45|to|75|kg|lb|abbr=on}} at birth and are able to swim. The mothers nurse for over a year before weaning, but the young can spend up to five years with the mothers.<ref name="Fay82"/> Walrus milk contains higher amounts of fats and protein compared to land animals but lower compared to [[Pinniped|phocid seal]]s.<ref>{{cite book|last1=Riedman|first1=Marianne | name-list-style = vanc |title=The pinnipeds : seals, sea lions, and walruses|date=1990|publisher=University of California Press|location=Berkeley|isbn=978-0520064973|url={{google books |plainurl=y |id=McNEUgU8Q58C|page=281}}|pages=281–282}}</ref> This lower fat content in turn causes a slower growth rate among calves and a longer nursing investment for their mothers.<ref>{{cite book|editor-last1=Marsden|editor-first1=Terry|editor-last2=Murdoch|editor-first2=Jonathan | name-list-style = vanc |title=Current Topics in Developmental Biology, Volume 72.|date=2006|publisher=Elsevier|location=Burlington|isbn=978-0080463414|url={{google books |plainurl=y |id=rrRNK-i5yI0C|page=239}}|pages=277|edition=1st}}</ref> Young may be suckled at sea as well as during long haul-outs, making walrus the only pinnipeds that exhibit aquatic suckling.<ref name="davis">{{Cite book |last=Davis |first=Randall W. |title=Marine Mammals. Adaptations for an Aquatic Life |publisher=Springer |year=2019 |isbn=978-3319982786 |location=Berlin/Heidelberg |chapter=8.3 Lactation}}</ref> Because ovulation is suppressed until the calf is weaned, females give birth at most every two years, leaving the walrus with the lowest reproductive rate of any pinniped.<ref>{{cite book| veditors = Evans PG, Raga JA |year = 2001|title = Marine mammals: biology and conservation|location = London & New York|publisher = Springer|url={{google books |plainurl=y |id=riDDOeRd9JAC}}|isbn=978-0-306-46573-4}}</ref>

===Migration===
The rest of the year (late summer and fall), walruses tend to form massive aggregations of tens of thousands of individuals on rocky beaches or outcrops. The migration between the ice and the beach can be long-distance and dramatic. In late spring and summer, for example, several hundred thousand Pacific walruses migrate from the [[Bering Sea]] into the [[Chukchi Sea]] through the relatively narrow [[Bering Strait]].<ref name="Fay82"/><ref>{{cite book|last1=Fischbach|first1=Anthony S.|last2=Kochnev|first2=Anatoly A.|last3=Garlich-Miller|first3=Joel L.|last4=Jay|first4=Chadwick V. | name-list-style = vanc |title=Pacific Walrus Coastal Haulout Database, 1852–2016—Background Report|date=2016|publisher=U.S. Department of the Interior, U.S. Geological Survey|location=Reston, VA|url=https://purl.fdlp.gov/GPO/gpo70240|access-date=29 July 2016}}</ref>