Liverpool footballer Luis Suarez has courted controversy yet again, this time by biting an opponent on the arm. How often do adults bite?
Striker Luis Suarez is expected to be banned for biting Chelsea defender Branislav Ivanovic on the arm.
It's behaviour that is expected among unruly toddlers, but not from an adult.
Suarez has done it before. In 2010, while at Ajax in Amsterdam, he was banned for seven matches for biting PSV Eindhoven midfielder Otman Bakkal's shoulder.
And there have been other cases in football. Tottenham striker Jermain Defoe was booked in 2006 and afterwards accused of trying to bite the arm of West Ham midfielder Javier Mascherano. On this occasion the authorities took no further action.
In rugby too, there have been high-profile cases.
Boxer Mike Tyson caused sport's most
infamous biting incident, in Las Vegas
on 28 June 1997.
Always a controversial figure, Tyson was
trying to regain the WBA world heavyweight
title from Evander Holyfield, who had beaten
him seven months earlier.
In the third round, Tyson took a small chunk
out of Holyfield's left ear. Despite this, the
referee ordered the boxers to carry on, but
the match was stopped after Tyson made a
lunge for Holyfield's other ear.
Tyson was fined $3m and his boxing licence
was permanently revoked by the Nevada
State Commission. The ban was later
South Africa prop Johan le Roux was sent home from a tour of New Zealand in 1994 after being found guilty of biting the ear of opposition captain Sean Fitzpatrick. He also received an 18-month ban.
More recently, England rugby player Dylan Hartley was suspended for eight weeks for biting Ireland player Stephen Ferris during a Six Nations match.
But probably the most infamous bite in sport happened during the boxing match now dubbed the "Bite Fight".
Mike Tyson bit off a large chunk of Evander Holyfield's ear and spat it on the ground as the pair fought for the world heavyweight title in 1997. Tyson was later disqualified.
Sports psychologist Dr Thomas Fawcett, of the University of Salford, says biting is rare in most sports - but in contact sports it can be more of an issue.
In rugby rucks and mauls, where players are in close contact and protected from view, or in boxing where the action moves on very quickly - in effect, in situations where people believe they might just get away with it - biting can be more frequent.
But mostly it is a primitive response where the emotion precedes the thought process, and this is true of the Suarez case, Dr Fawcett believes.
"It's not pre-planned - it's a very spontaneous, emotional response. He's doing it on impulse," says the psychologist, who has studied the footage extensively.
Spurs striker Jermain Defoe was accused
of biting Javier Mascherano, of West Ham,
after he was fouled in 2006.
England rugby player Dylan Hartley (above)
received an eight-week ban for biting Ireland's
Stephen Ferris during a Six Nations match
Also in 2012, England rugby league
international James Graham was suspended
for 12 matches after biting an opponent in an
Australian cup final.
Most often biting is a sign of frustration. A negative response when tensions reach boiling point, he says.
In this particular case, there were "niggles" between Suarez and Ivanovic throughout the match. "A lot of the game was played in sheer frustration from Suarez's point of view," he says.
The player had just given away a penalty and that was the final straw. "He took it out on the arm of the Chelsea player."
But when else might an adult bite?
It is hard to estimate how common bites are, as records are usually only kept of bites that were serious enough to require hospital treatment. Many are not.
The most recent figures for England suggest there are about 6,000 cases of dog bites a year, and 2,500 cases due to other bites, including human.
Prof David Wilson, a criminologist at Birmingham City University, says biting can be used in crime - by a victim trying to fend off an attacker or by the perpetrator who wants to mark the victim symbolically.
Biting is classed as a common assault and any punishment would depend on the specifics of the case, he says.
"[Biting is carried out] to mark a victim. But I think what's more important to the perpetrator is that it reveals something about how they viewed that particular victim," he says. Often it is a hallmark of very violent or sexual crime.
There is absolutely no way of knowing how common its use as a weapon is, says Prof Wilson, because it is most often carried out with other behaviour, such as strangulation, stabbing or sex assault.
It is generally unusual in other crimes, for example a fight in a pub, he says.
"You have to get very close to your victim to bite them and if you get very close they might harm you - so you might [prefer to] throw a punch," he says.
He adds: "[Biting] is quite a niche thing."
A baby will learn quickly that biting is wrong, for example during breast feeding, and this will be reinforced at the language stage, says Wilson.
Police say they will not take any action against Suarez, with Ivanovic not reporting any physical injury and informing police he does not want to take the matter further.
But Suarez has been offered anger management therapy.
Dr Fawcett thinks this is likely to have a minimal effect. "It's in the man," he says. "I would think that in five years' time if there was a certain nerve hit or chord rung with Suarez in a different situation he would react in the same way."