But the vast use of the practice surprised Brindle. She says she had expected examples of kissing among apes and humans, but was surprised to see the gentle behaviour shared between bugs, albatrosses and polar bears.

“For some reason, I didn’t expect this many of them to kiss,” Brindle says.

Among their research groups were Neanderthals, which, despite their differences, shared microbes with modern humans. It leaves open the chance, the study said, that the two swapped spit in not-so-distant history.

Brindle says she hopes the study can be a foundation for further studies on kissing, and determine – as the study itself notes – whether it is more than Ingrid Bergman’s preferred pastime. Other scientists, she hopes, might start recording their observations of these behaviours while in the field.

“If we had more data on this,” she says, “then we could really start to kind of unpack the potential kind of adaptive advantages of kissing.”