Editing Gorilla (section)

== Behaviour ==

=== Social structure {{anchor |Group life}} ===
{{redirect2 |Blackback |Silverback ||Blackback (disambiguation) |and |Silverback (disambiguation)}}
[[File:Silverback.JPG|thumb|Silverback with female]]
Gorillas live in groups called troops. Troops tend to be made of one adult male or silverback, with a harem of multiple adult females and their offspring.<ref name="Watts 1996">{{cite book |last=Watts |first=D. P. |year=1996 |chapter=Comparative socio-ecology of gorillas |editor1-last=McGrew |editor1-first=W. C. |editor2-last=Marchant |editor2-first=L. F. |editor3-last=Nishida |editor3-first=T. |title=Great ape societies |location=Cambridge, UK |publisher=Cambridge University Press |pages=16–28}}</ref><ref name="Yamagiwa 2003">{{cite journal |last1=Yamagiwa |first1=J. |last2=Kahekwa |first2=J. |last3=Kanyunyi Basabose |first3=A. |year=2003 |title=Intra-specific variation in social organization of gorillas: implications for their social evolution |journal=Primates |volume=44 |pages= 359–369 |pmid=12942370 |doi=10.1007/s10329-003-0049-5 |issue=4 |s2cid=21216499}}</ref>{{sfn|Robbins|2001|pp=29–58}} However, multiple-male troops also exist.<ref name="Yamagiwa 2003" /> A silverback is typically more than 12 years of age, and is named for the distinctive patch of silver hair on his back, which comes with maturity. Silverbacks have large canine teeth that also come with maturity. Both males and females tend to emigrate from their natal groups. For mountain gorillas, females disperse from their natal troops more than males.<ref name="Watts 1996" /><ref name="Stokes 2003" /> Mountain gorillas and western lowland gorillas also commonly transfer to second new groups.<ref name="Watts 1996" />

Mature males also tend to leave their groups and establish their own troops by attracting emigrating females. However, male mountain gorillas sometimes stay in their natal troops and become subordinate to the silverback. If the silverback dies, these males may be able to become dominant or mate with the females. This behaviour has not been observed in eastern lowland gorillas. In a single male group, when the silverback dies, the females and their offspring disperse and find a new troop.<ref name="Stokes 2003">{{cite journal |last1=Stokes |first1=E. J. |last2=Parnell |first2=R. J. |last3=Olejniczak |first3=C. |year=2003 |title=Female dispersal and reproductive success in wild western lowland gorillas (''Gorilla gorilla gorilla'') |journal=Behavioral Ecology and Sociobiology |volume=54 |pages=329–339 |jstor=25063274 |doi=10.1007/s00265-003-0630-3 |issue=4 |bibcode=2003BEcoS..54..329S |s2cid=21995743}}</ref>{{sfn|Yamagiwa|Kahekwa|2001|pp=89–122}} Without a silverback to protect them, the infants will likely fall victim to [[Infanticide in primates|infanticide]]. Joining a new group is likely to be a tactic against this.<ref name="Stokes 2003" /><ref name="Watts 1989">{{cite journal |last=Watts |first=D. P. |year=1989 |title=Infanticide in mountain gorillas: new cases and a reconsideration of the evidence |journal=Ethology |volume=81 |issue=1 |pages=1–18 |doi=10.1111/j.1439-0310.1989.tb00754.x|bibcode=1989Ethol..81....1W }}</ref> However, while gorilla troops usually disband after the silverback dies, female eastern lowlands gorillas and their offspring have been recorded staying together until a new silverback transfers into the group. This likely serves as protection from leopards.{{sfn|Yamagiwa|Kahekwa|2001|pp=89–122}}
[[File:Gorilla gorilla11.jpg|thumb|left |Silverback gorilla]]
The silverback is the centre of the troop's attention, making all the decisions, mediating conflicts, determining the movements of the group, leading the others to feeding sites, and taking responsibility for the safety and well-being of the troop. Younger males subordinate to the silverback, known as blackbacks, may serve as backup protection. Blackbacks are aged between 8 and 12 years{{sfn|Robbins|2001|pp=29–58}} and lack the silver back hair. The bond that a silverback has with his females forms the core of gorilla social life. Bonds between them are maintained by grooming and staying close together.<ref name="Watts 2003">{{cite book |last=Watts |first=D. P. |year=2003 |chapter=Gorilla social relationships: a comparative review |editor1-last=Taylor |editor1-first=A. B. |editor2-last=Goldsmith |editor2-first=M. L. |title=Gorilla biology: a multidisciplinary perspective |location=Cambridge, UK |publisher=Cambridge University Press |pages=302–327}}</ref> Females form strong relationships with males to gain mating opportunities and protection from predators and infanticidal outside males.{{sfn|Watts|2001|pp=216–240}} However, aggressive behaviours between males and females do occur, but rarely lead to serious injury. Relationships between females may vary. Maternally related females in a troop tend to be friendly towards each other and associate closely. Otherwise, females have few friendly encounters and commonly act aggressively towards each other.<ref name="Watts 1996"/>

Females may fight for social access to males and a male may intervene.<ref name="Watts 2003"/> Male gorillas have weak social bonds, particularly in multiple-male groups with apparent dominance hierarchies and strong competition for mates. Males in all-male groups, though, tend to have friendly interactions and socialise through play, grooming, and staying together,{{sfn|Robbins|2001|pp=29–58}} and occasionally they even engage in homosexual interactions.<ref>{{cite journal |first=Juichi |last=Yamagiwa |title=Intra- and inter-group interactions of an all-male group of Virunga mountain gorillas (''Gorilla gorilla beringei'' ) |journal=Primates |volume=28 |issue=1 |year=1987 |doi=10.1007/BF02382180 |pages=1–30 |s2cid=24667667}}</ref> Severe aggression is rare in stable groups, but when two mountain gorilla groups meet the two silverbacks can sometimes engage in a fight to the death, using their canines to cause deep, gaping injuries.<ref name="Fossey1983" />

=== Reproduction and parenting ===
[[File:Gorillas in Uganda-3, by Fiver Löcker.jpg|thumb|right |Young gorilla riding on mother]]
Females mature at 10–12 years (earlier in captivity), and males at 11–13 years. A female's first ovulatory cycle occurs when she is six years of age, and is followed by a two-year period of adolescent infertility.{{sfn|Czekala|Robbins|2001|pp=317–339}} The estrous cycle lasts 30–33 days, with outward ovulation signs subtle compared to those of chimpanzees. The gestation period lasts 8.5 months. Female mountain gorillas first give birth at 10 years of age and have four-year interbirth intervals.{{sfn|Czekala|Robbins|2001|pp=317–339}} Males can be fertile before reaching adulthood. Gorillas mate year round.<ref name="Watts 1991">{{cite journal |last=Watts |first=D. P. |year=1991 |doi=10.1002/ajp.1350240307 |title=Mountain gorilla reproduction and sexual behavior |journal=American Journal of Primatology |volume=24 |issue=3–4 |pages=211–225 |pmid=31952383 |s2cid=85023681}}</ref>

Females will purse their lips and slowly approach a male while making eye contact. This serves to urge the male to mount her. If the male does not respond, then she will try to attract his attention by reaching towards him or slapping the ground.{{sfn|Sicotte|2001|pp=59–87}} In multiple-male groups, solicitation indicates female preference, but females can be forced to mate with multiple males.{{sfn|Sicotte|2001|pp=59–87}} Males incite copulation by approaching a female and displaying at her or touching her and giving a "train grunt".<ref name="Watts 1991"/> Recently, gorillas have been observed engaging in [[missionary position|face-to-face sex]], a trait once considered unique to humans and [[bonobo]]s.<ref>{{cite web |url=https://www.nbcnews.com/id/wbna23148063 |title=Caught in the act! Gorillas mate face to face |first=T. C. |last=Nguyen |work=NBC News|date=13 February 2008 }}</ref>

[[File:Moka with baby gorilla at Pittsburgh Zoo 8, 2012-02-17.jpg|thumb|left|Mother gorilla with 10-day-old infant]]

Gorilla infants are vulnerable and dependent, thus mothers, their primary caregivers, are important to their survival.<ref name="Watts 1989"/> Male gorillas are not active in caring for the young, but they do play a role in socialising them to other youngsters.{{sfn|Stewart|2001|pp=183–213}} The silverback has a largely supportive relationship with the infants in his troop and shields them from aggression within the group.{{sfn|Stewart|2001|pp=183–213}} Infants remain in contact with their mothers for the first five months and mothers stay near the silverback for protection.{{sfn|Stewart|2001|pp=183–213}} Infants suck at least once per hour and sleep with their mothers in the same nest.<ref name="Stewart 1988">{{cite journal |last=Stewart |first=K. J. |year=1988 |title=Suckling and lactational anoestrus in wild gorillas (''Gorilla gorilla'') |journal=Journal of Reproduction and Fertility |pmid=3411555 |volume=83 |issue=2 |pages=627–634 |doi=10.1530/jrf.0.0830627 |doi-access=free}}</ref>

Infants begin to break contact with their mothers after five months, but only for a brief period each time. By 12 months old, infants move up to {{cvt|5|m|ft|sp=us |spell=in}} from their mothers. At around 18–21 months, the distance between mother and offspring increases and they regularly spend time away from each other.{{sfn|Fletcher|2001|pp=153–182}} In addition, nursing decreases to once every two hours.<ref name="Stewart 1988"/> Infants spend only half of their time with their mothers by 30 months. They enter their juvenile period at their third year, and this lasts until their sixth year. At this time, gorillas are weaned and they sleep in a separate nest from their mothers.{{sfn|Stewart|2001|pp=183–213}} After their offspring are weaned, females begin to ovulate and soon become pregnant again.{{sfn|Stewart|2001|pp=183–213}}<ref name="Stewart 1988"/> The presence of play partners, including the silverback, minimizes conflicts in weaning between mother and offspring.{{sfn|Fletcher|2001|pp=153–182}}

=== Communication ===
{{redirect-distinguish |Gorilla communication |Guerrilla communication}}
Twenty-five distinct [[animal communication|vocalisation]]s are recognised, many of which are used primarily for group communication within dense vegetation. Sounds classified as grunts and barks are heard most frequently while traveling, and indicate the whereabouts of individual group members.<ref name=HarcourtSH>{{cite journal |last1=Harcourt |first1=A. H. |last2=Stewart |first2=K. J. |last3=Hauser |first3=M. |year=1993 |title=Functions of wild gorilla 'close' calls. I. Repertoire, context, and interspecific comparison |journal=Behaviour |volume=124 |issue=1–2 |page=89 |doi=10.1163/156853993X00524}}</ref> They may also be used during social interactions when discipline is required. Screams and [[roar]]s signal alarm or warning, and are produced most often by silverbacks. Deep, rumbling belches suggest contentment and are heard frequently during feeding and resting periods. They are the most common form of intragroup communication.<ref name= Fossey1983>{{cite book |last=Fossey |first=D. |year=1983 |title=Gorillas in the mist |location=Boston |publisher=Houghton Mifflin Company |isbn=0395282179}}</ref>

For this reason, conflicts are most often resolved by displays and other threat behaviours that are intended to intimidate without becoming physical. As a result, fights do not occur very frequently. The [[ritual]]ized charge display is unique to gorillas. The entire sequence has nine steps: (1) progressively quickening hooting, (2) symbolic feeding, (3) rising bipedally, (4) throwing vegetation, (5) chest-beating with cupped hands, (6) one leg kick, (7) sideways running, two-legged to four-legged, (8) slapping and tearing vegetation, and (9) thumping the ground with palms to end display.<ref name=MapleHoff>{{cite book |last1=Maple |first1=T. L. |last2=Hoff |first2=M.P. |year=1982 |title=Gorilla behavior |location=New York |publisher=Van Nostrand Reinhold Company}}</ref>

A gorilla's chest-beat may vary in frequency depending on its size. Smaller ones tend to have higher frequencies, while larger ones tend to be lower. They also do it the most when females are ready to mate.<ref>{{Cite journal |last=Wright |first=E. |display-authors=etal |year=2021 |title=Chest beats as an honest signal of body size in male mountain gorillas (''Gorilla beringei beringei'') |journal=Scientific Reports |volume=11 |issue=1 |page=6879 |doi=10.1038/s41598-021-86261-8 |pmid=33833252 |pmc=8032651 |bibcode=2021NatSR..11.6879W |doi-access=free}}</ref>