"Friendships can deteriorate very quickly if you don't invest in them - it probably only takes about three months," says evolutionary psychologist Prof Robin Dunbar.
So the social strain of lockdown, while hopefully short-term, could have some long-term effects on some friendships, he says.
In a paper in the Royal Society journal, Proceedings A, Prof Dunbar has delved into the ways in which our social connections will be changed by lockdown.
The University of Oxford academic's insight into those effects comes from a social world far from Zoom quizzes and Whatsapp groups. The roots of our friendships, he says, lie in the social lives of non-human primates.
For many of those primates, strong social bonds - being part of a "stable group" - means protection from predators and rivals.
That goes some way to revealing why many of us treasure our closest friends as though our lives depend on them. In our evolutionary history, they did.
And those bonds require a great deal of maintenance.
In both monkeys and humans, research shows that the quality of a relationship - measured by how likely a fellow monkey, ape or human is to step up and defend you - depends directly on the time invested in it.
"We have to see people surprisingly often to maintain a friendship," explains Prof Dunbar, from the University of Oxford. And, because nurturing friendships requires all that time and cognitive capacity, we can only keep up a limited number of social connections.
"In lockdown, many people are forming new friendships with people on their street and in their community for the first time," says Prof Dunbar.
"So when we emerge from lockdown, some of our more marginal friendships might be replaced by some of these new ones."
One impact of this is something that has been called "relationship funnelling" - an effect picked up by a large survey that social scientists carried out in France during the highly restrictive lockdown there.
Put simply, while some friendships were prioritised and even strengthened through care and increased communication, other more marginal connections just "fizzled out".
One major problem resulting from this "fizzling" is any lasting impact on older people's friendships.
"When we're older, we generally find it more difficult to make new friends," says Prof Dunbar.
"And the biggest single factor affecting health, wellbeing, happiness - even the ability to survive surgery or illness - is the number of high-quality friendships you have."
Needing a hug
So long as it is temporary, our closer, more valued friendships should survive intact through lockdown - reinforced at least in some part, by the time we are still able to spend with our friends online.
Dr Jenny Groarke from Queen's University, Belfast, has been studying loneliness during the pandemic.
"People are using digital modes of communication to meet their social needs, but they're less satisfied with the quality of this form relative to face-to-face contact," she says.
"[This] lower satisfaction with the quality of digital social contact, we found, was associated with higher loneliness."
This concurs with the findings of Prof Dunbar's research into social behaviour. There's no substitute, he says, for close, face-to-face encounters.
Part of that is the human need for touch.
"People [in our surveys] also spoke about missing physical touch, and finding it 'bizarre' and 'not normal' to go so long without touching people," says Dr Groarke.
And looking to our closest primate relatives - the chimpanzees - touch is not only "normal", it's socially vital.
Chimps often spend hours each day grooming one another. This close, strictly one-to-one, stroking and parasite-picking is not just about hygiene. Research shows it reinforces social bonds and triggers the brain to release innate, pain-relieving and pleasure-boosting chemicals called endorphins.
However, as a large number of our modern human interactions move online, our own brains are still wired to respond to a similar gentle touch (providing, of course, that it is wholly invited and appropriate).
We, like our primate cousins, have a specialised system of nerve fibres that pick up and transmit the sensation of touch from our skin to those endorphin-releasing bundles of brain cells.
Scientists studying this touch-triggered system of pleasure have even carried out experiments revealing that the more "human-like" the sensation of being stroked on our forearm is, the "more pleasant" it feels.
As researchers reported in a recently published study in the Journal of Experimental Psychology: "Perceiving gentle touch as human appears to promote pleasure possibly because this serves to reinforce interpersonal contact as a means for creating and maintaining social bonds."
That gives new physiological meaning to the feeling of needing a hug from a friend.
"We make physical contact all the time," says Prof Dunbar. "There are strict natural rules about who we can touch, but with close friends and family, we pat on the back, we touch a shoulder…
"Because it's below the horizon of consciousness, we don't appreciate how important it is to us."
Fortunately though, for humans, there are other social activities that activate the brain's pleasure centres - many of which can be done at a social distance or online. Laughing, singing, dancing and eating and drinking alcohol together have all been found to release endorphins and play a role in the upkeep of our all-important social bonds.
For most of us, Prof Dunbar says reassuringly, this time of social distance will be a sad but temporary frustration. But we will have to put in the time to repair locked-down relationships.Source: BBC Health