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ON 9 MARCH, Mark Rutte, the Dutch prime minister, called a press conference to discuss his country鈥檚 response to the covid-19 pandemic. 鈥淔rom now on, we stop shaking hands,鈥� he declared 鈥� before promptly .

Many of us can empathise. Social distancing sounds innocuous, but this year we have discovered how hard it can be in practice. Touchy-feely greetings, such as handshakes, hugs, kisses and nose rubbing, are deeply embedded in many cultures. These gestures aren鈥檛 merely learned, however. Look to the animal kingdom and you will see that many species 鈥� especially highly social ones 鈥� perform physical rituals when they approach each other. If our urges to touch one another in greeting seem instinctual, it is because they are.

Greetings adopted by animals can be very different to our own 鈥� they include eye poking and other gestures that might make you squirm 鈥� but understanding these behaviours can give us an insight into human salutations. Examining the evolution of greetings throws light on the subtle ways they lubricate social interactions and also helps to explain why they are so diverse. As we are a super-social species, it isn鈥檛 surprising that many of us are struggling to adjust to the new normal. But the good news is that we are proven masters at adapting our greetings to fit new situations.

Animal encounters

Mammals tend to use scents to suss each other out, which explains why their greetings are so intimate. A new encounter often entails sniffing another individual鈥檚 face, flanks and genitals for volatile chemicals that reflect its hormonal state. , allowing animals to size up potential opponents and mates.

Like our own greetings, the duration and intimacy of such exchanges reflect the nature of the relationship. Subordinate rats, for example, submit to prolonged sniffing from more dominant individuals but . They seem to be offering some kind of 鈥渁ppeasement signal鈥� that de-escalates tension 鈥� and it鈥檚 not just about the chemical signals in their body odour. Daniel Wesson at Case Western Reserve University in Ohio has found that rats that have lost their sense of smell still sniff one another, suggesting that the behaviour itself is also important in establishing the social hierarchy.

Cats and dogs behave similarly, with a characteristic rub of the head to exchange smells on greeting. There is evidence that such gestures can even . A study of cats and dogs living in the same household found that 75 per cent regularly engage in nose-to-nose sniffing, a gesture that appears to help them live side by side. This can be accompanied by other physical signals: , while a dog may crouch and look upward to show that it has no intention of fighting.

For more elaborate greetings, however, we need to look to our fellow primates, many of which appear to use ritualised signals as a way to navigate social relationships. In certain species of baboon, for example, greetings range from lip-smacking and head-bobbing to touching buttocks or genitals. . 鈥淭hey have this very peculiar greeting, during which one male approaches the other one, and he takes the hind quarter of the other and then they start walking,鈥� says Federica Dal Pesco at the German Primate Center in Gottingen.

鈥淕reetings help individuals test their trust in each other and build alliances鈥�

Many of these behaviours 鈥� particularly those involving genitals 鈥� include an element of risk and vulnerability. 鈥淚t鈥檚 really important that they don鈥檛 get injured there because of their future reproductive success,鈥� says Dal Pesco. From an evolutionary point of view, such behaviours should be a serious taboo, unless there is a significant benefit to outweigh the danger. But what could it be?

The answer, Dal Pesco believes, lies in the structures of baboon societies. She points out that riskier and more intimate behaviours are more common among highly cooperative species like Guinea baboons, in which males form closely bonded coalitions. Greetings, she suggests, help individuals that could improve their chances of survival in the future. 鈥淭he greetings allow you to touch base with many individuals in your society, and in different ways depending on the relationship you have,鈥� says Dal Pesco.

Similarly intricate behaviours are found among white-faced capuchins, whose , embracing and the toe-curling practice of sticking a finger into one another鈥檚 eye sockets, sometimes up to the first knuckle. Like baboons, capuchins have a complex society in which groups of males band together 鈥� again suggesting that risky interactions allow the monkeys to test and affirm their relationships. 鈥淚t clearly can鈥檛 feel that good to take somebody鈥檚 dirty claw and finger and stick it in your eye socket,鈥� says Susan Perry at the University of California, Los Angeles, who has spent thousands of hours observing these interactions. 鈥淭he fact that they can pull this off really says a lot about their trust.鈥�

So what can these animal interactions teach us about ourselves? We too live in complex societies in which trust and cooperation are paramount. Of course, our greetings are imbued with a deeper symbolism and meaning and they have enormous variety, from the namaste bow in India to the hongi of the Maori, in which two people press their noses together, as well as handshakes and cheek kisses (see 鈥�A very brief history of greetings鈥�). Yet however much more sophisticated we may seem, the latest research suggests that many of our gestures serve the same functions as those in other animals.

Sniffing you out

Any greetings that involve bodily contact may offer us a way to pick up chemical cues. Although the existence of human pheromones is controversial, research suggests that we may be able to from compounds in their saliva 鈥� a possible rationale for the strange phenomenon of romantic kissing. What鈥檚 more, there is evidence that body odour can communicate someone鈥檚 and even their . Our greetings can allow us to sample this aroma without overtly sniffing someone鈥檚 body, according to , then at the Weizmann Institute of Science in Israel.

Frumin was inspired when, at a conference, he noticed that people often ran their hand under their nose after shaking hands with someone else. He wondered whether the purpose of this 鈥渨eird ritual鈥� was to get a whiff of the other person鈥檚 natural perfume. To test the idea, his team covertly filmed visitors to the lab as they waited to take part in an experiment. About 3 minutes into the wait, a scientist of the same gender entered the room, introduced themselves and then left. The footage revealed that people were more likely to bring their hand to their face if the scientist had shaken it; many directly touched their noses after the encounter. To make sure that people were actually sniffing their hand, Frumin fitted another group of volunteers with a sensor that measured nasal airflow. This doubled when the participant鈥檚 shaken hand was near their face.

This supports the idea that formalised greetings offer a covert means of checking someone鈥檚 scent. 鈥淲e鈥檙e not very different from rats, in that sense,鈥� says Frumin. If he is right, the idea can also explain why we find it so hard to stop touching our faces 鈥� another no-no in today鈥檚 pandemic.

Like the baboon and capuchin rituals, our tactile greetings also allow us to assess someone鈥檚 character and to establish our trust in them. One study found that the strength and duration of a handshake offers a including extraversion, neuroticism and open-mindedness. Francesca Gino at Harvard University, meanwhile, has found that students engaged in a simulated real-estate negotiation if they had been encouraged to shake hands before the task.

Signals of trust

Although these studies looked solely at the handshake, many other physical gestures could serve a similar purpose. An analysis of players in the US National Basketball Association, for example, found that teams that regularly engaged in fist bumps, high fives, hugs and huddles at the start of the 2008鈥�2009 season tended to . Such open body language renders us physically vulnerable, signalling our willingness to work with the other party in good faith.

Unfortunately, as we are now discovering, there is a downside to tactile contact. No matter how much we trust someone, a close-up interaction runs the risk of transferring pathogens. In fact, the threat of adopted by many cultures long before covid-19 came along.

Damian Murray at Tulane University in New Orleans and his colleagues combed an ethnographic database for records of greeting etiquette in 186 cultures, which they then rated for physical contact, from 0 to 5. A handshake was rated as 2, a hug was 3 and exchange of saliva in a kiss or 鈥渟pit shake鈥� scored 5. They found a small but significant correlation between these ratings and the local pathogen prevalence. Where the risk is high, people have evolved distanced greetings that symbolically convey the wish for cooperation and trust, says Murray. Similarly, the researchers found that romantic kissing 鈥� previously considered to be universal 鈥� was absent from 58 per cent of the societies that had above-average pathogen prevalence. 鈥淭hey balance the costs of physical contact versus the benefits,鈥� says Murray.

These findings chime with other research on the 鈥渂ehavioural immune system鈥� 鈥� the idea that part of our defence against infection is an evolved tendency to adapt our actions in response to a threat. Studies reveal that the mere thought of disease can . For example, people are when they are more conscious of the risk of infection. 鈥淛ust like physical contact, extended social contact is associated with a higher threat of contracting disease,鈥� says Murray.

The behavioural immune system may also explain why we find it especially difficult to avoid hugging or kissing our friends and family. Research indicates that we are primed to assume that people in our 鈥渋n group鈥� are less likely to transmit diseases to us than outsiders. In our evolutionary past, after all, . This means that our intuitive sense of the risk of bodily contact is much lower when we are among friends and family members 鈥� even though they are just as likely to be carrying the virus responsible for covid-19 as anybody else. The problem is compounded because these are the people with whom we crave the most physical contact to reaffirm our bonds.

Given the competition between our behavioural immune system and our desire for physical contact, it is little wonder that many of us 鈥� including our leaders and heads of state 鈥� have found it hard to navigate the new social norms. Of course, human behaviour is flexible and we are learning fast.

鈥淭he threat of disease seems to have influenced the greetings adopted by many cultures鈥�

Whether these habits will stick, once the immediate threat is over, remains an open question. 鈥淚t鈥檚 something that we鈥檒l have to watch in real time,鈥� says Murray. But he suspects that they could linger. 鈥淭ime and time again we see that institutional requirements on behaviours have changed underlying cultural norms,鈥� he says. He points out that it is now considered morally objectionable to smoke on a plane or to drive without wearing a seat belt, for example 鈥� behaviours that were once widely accepted. Likewise, handshaking and hugging may come to seem as inappropriate as poking someone in the eye or grabbing their buttocks.