Nothing can break your heart quite as efficiently as your sociable, boisterous 5-year-old coming home from her new school and saying, “Mama, no one wants to be my friend.” Standing outside the school gate, new myself, I had the exact same experience. None of the mummies would talk to me.

Then I saw M on the sidelines. We made awkward eye contact, started talking. Turned out she was new, too. The next day our girls were given each others’ names, some common interests, and in a week, they were best friends. M and I formed a newbie cell. It would be a year before the other mothers acknowledged us, but at least for now, we were not alone. We stood a fighting chance against a marauding tribe. Because making friends, connections outside of your relatives, is probably a primeval survival tactic—for all species.

Research on other animals including Australian sharks and macauque monkeys in Thailand showed not just tendencies to form smaller, mutually-beneficial groups within the larger tribe, but also significantly, unexplained dislike or avoidance of some individuals outside the small group.

Human social psychology breaks down the components of friendship into a few basic factors: proximity, reciprocity, self-disclosure, similarity and the acceptance of the relationship by peers and family. You see this all the time: Younger children’s best friends are usually the people who sit closest to them in school and who will share their erasers and crayons.

Attractiveness and social competence also play an important part in making friends, according to the study. (However, as someone who always sat with the awkward, sociopathic nerds, I can’t say I have anecdotal evidence of this.)

What is not so clear is why we outgrow these friendships. M and I continue to be pals. There was no escalation into soul-sister territory, but we laugh at the same things, have complementary views on education and child-rearing, and share conspiratorial eyebrow-waggling at PTA-Mom-In-Leopard-Print-Jeggings. By the time they were 12 however, our daughters had drifted apart quite categorically.

It’s interesting to observe a floundering friendship being treated with the emotional CPR of nostalgia and guilt. The acknowledgement that ‘this friendship’ is on the wane is rarely simultaneous. Like in all relationships, there is the sense that one person is moving on, one is being left behind.

You see the hurt feelings. Some of those manifest as other things – mean-girling, passive-aggression; the teens I know who are falling out manoeuver around each other warily. But they are very clear about moving on and out of the friendship. M and I discussed and ended up comfortably acquiescing that our children would never be best friends. It didn’t change our relationship at all.

At an age when their personalities are beginning to coalesce, when they are beginning to be more independently involved in their environment, their friends start to define Teen Muchness. A year and some before they do their big tenth grade exams, I see the distance growing between peers who are maturing, developing a healthy competitiveness in academics, and those who are still in last year’s beauty-contest phase. The new bonds are less about where they sit in class and more about a shared sense of humour, interest in maths and sport, a love for doughnuts and distaste for Taylor Swift. Hilariously, among my daughter’s classmates, there is also a new circle of friends that only formed when it was discovered that one girl’s neighbourhood is absolutely teeming with handsome teen boys.

Hanging out with people who want the same things as you is an instinctive social strategy. You form support systems, egg each other on, reinforce your own values and ideals. At this emotionally fraught time in their lives, it’s where teens get their validation. So when a friendship starts to dissolve, it is usually because there is a failure at reciprocity. “It’s all about her, mum.” “I don’t think I get her anymore.” “She’s suddenly turned mean to me.”

As adults, we’ve all made that joke: ‘There are fifty ways to leave your lover’ – but no one tells you how to break-up with a best friend. Whatever the final straw is, looking back you will see all the tell-tale signs of the toxicity that crept in, the one-sided emotional bolstering, the forced breath of nostalgia. The closer the friend, the heavier it can be.

Growing up is hard no matter what age you are. Some friendships I am happy to have outgrown, but I mourn at least one old, close friendship lost. Still, I find that even in my 40s, I have been lucky to forge new bonds with wonderful, simpatico strangers and rediscover old acquaintances minus the respective baggage of our headstrong, naïve years. Real life is easier because of them.

In a study on bonding in dolphins, two former rivals, Pod A and Pod B made a strategic alliance when Pod A found itself suddenly under attack. I didn’t think I’d find as easy a metaphor when I was researching friendship for this piece.

I’ve had friendships survive despite the breakdown of the scientific components: We grew older, less similar, moved far away from each other geographically. What keeps us together is mutuality, respect and a loyalty to each other. You need a solid pod if you’re going swimming with sharks. Our pups know this instinctively.