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One of our cats has discovered a new way to annoy me. There is a ledge above our kitchen window, which she can use to reach my bedroom window. If I leave the window open to let in the cool night air, she comes in unexpectedly in the middle of the night and jumps on my head. If I leave it mostly shut to prevent this, she sits on the ledge and sings the song of her people until I wake up and open the window. I could almost admire her commitment to the bit, if I wasnât so sleep-deprived.
Those of you who have pets will no doubt have similar stories: incidents and patterns of behaviour that lead you to question your own wisdom in inviting these creatures into your homes. Then they do something nice, like sit quietly on your lap when youâre feeling ropy, and you forget about the stupidity until the next time.
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Keeping pets seems to be a fairly modern behaviour. I recently compiled a timeline of human evolution, and domestic animals were the penultimate item. The earliest domestication seems to have been dogs, beginning perhaps 40,000 years ago and giving rise to recognisably âmodernâ dogs by about 14,000 years ago. On a similar timeline, we also picked up synanthropic animals like foxes and pigeons, which benefit from living alongside us but have never become domesticated. Millennia later, once some societies had started farming and living in dense settlements, other domestications occurred â including wheat, rice and cats.
In the grand scheme of human evolution, which spans the millions of years since our ancestors split from the ancestors of chimpanzees and bonobos, the past 40,000 years is minuscule. For most of prehistory, humans and hominins did not have pets.
But our ancestors may have engaged in similar behaviours: spending time with animals of other species, even cuddling them and playing with them. Such affiliative behaviours, which relate to forming social bonds, may be the precursors to pet-keeping.
This is not speculation: we now know that modern primates do this â a lot.
Itâs so cute
Today, is a biological anthropologist at the University of Oxford. But about 20 years ago, while still a student, he visited a zoo in Shanghai, China. âI saw a gibbon interacting affiliatively with a little mouse that had wandered into its enclosure,â he says. âI saw the gibbon caressing the mouse, holding the mouse in its hand.â
Later, Grueter was doing fieldwork on black-and-white snub-nosed monkeys in China. âI saw how juveniles were jumping on domestic pigs,â he says. âThey were trying to play with domestic pigs.â This led him to become fascinated by how primates, the group of animals to which we belong, interact with members of other species. At least sometimes, primates seem to play with members of other species, even make friends with them.
At this point Iâm going to introduce a bit of jargon, simply because if I write âmembers of other speciesâ one more time Iâm going to scream. An animal that belongs to the same species is a âconspecificâ, and an animal that belongs to a different species is a âheterospecificâ. When the gibbon picked up the mouse, it was interacting with a heterospecific.
Over many years, Grueter and his colleagues have compiled all the examples they could find of primates interacting with heterospecifics. They now have 427 cases: some from the academic literature, others from media sources like YouTube videos and a global survey of 37 primatologists. in the journal Primates on 21 July.
Grueter emphasises that it is very much a first pass. âThere wasnât really much in the mainstream scientific literature,â he says. Most observations were anecdotal, and the editors of scientific journals rarely accept anecdotes. âThereâs so much missing informationâ, because these interactions have been largely ignored by researchers. âWeâre hoping that this study is also a wake-up call for primatologists, or zoologists more generally, to look for similar behaviours in both the wild and in captivity.â
Despite these struggles, Grueterâs team managed to document 88 primate species interacting socially with either other primates or one of 127 non-primate species. This was enough to see some basic patterns.
Most simply, âthis is quite a common behaviourâ, says Grueter. Itâs also not an artefact of captivity, which can sometimes cause animals to behave oddly: the team found 311 instances from the wild and just 111 from captivity. We can be confident that wild primates sometimes socialise with heterospecifics.
âInitially, we thought heterospecific social behaviour would be more common among closely related species, because theyâre probably better at reading each otherâs signals, at communicating with each other,â says Grueter. Seemingly in line with that, the most common interactions were between Old World (African and Eurasian) monkey species: there were 89 such interactions. âBut then we also found interactions between very distantly related species.â The second most common pairing was Old World monkeys and dogs: 24 such interactions.
The team classified the interactions into 17 types. They included âcarryâ, âhuddleâ, âride on backâ and (sorry, there had to be a dark bit) âtreat abusivelyâ. However, the most frequently reported were play (139) and grooming (136).
Letâs have some examples. An adult female white-faced capuchin groomed an adult female black-handed spider monkey in Santa Rosa, Costa Rica. A tufted gray langur stroked a grizzled giant squirrel in Yala National Park, Sri Lanka. Stump-tailed macaques at Khao Tao Mo temple in Thailand groomed a domestic dog. A Japanese macaque mounted a sika deer in Yakushima, Japan, then rode on its back.
Before pets
What is going on here? Why are all these primates socialising and playing with heterospecifics?
âOne of the major things we donât really know is what the benefits of these interactions are,â says Grueter. When a primate plays with a heterospecific, that may offer the same benefits as play with a conspecific: they learn motor skills and social skills. But what about when, say, a langur touches a squirrel? âThereâs no functional benefits that we can think ofâ, says Grueter, other than the simple and direct fact that it might feel nice and offer comfort.
Often, primates may simply be curious about the heterospecifics they encounter. âI think itâs exploratory behaviour,â says Grueter.
This may explain the nastier side of these interactions. âThe males were usually the ones who handled the heterospecific individual quite roughly,â says Grueter. The team confirmed 13 deaths of heterospecifics: seven birds, three primates, two pangolins and one domestic cat.
Male primates were also less likely to interact with heterospecifics at all. In contrast, juveniles and females were more likely to do so.
One possibility is that the primates are attracted to cute-looking heterospecifics. For humans, that often means juveniles: we find baby animals cuter than adults and are drawn to them. âItâs possible that non-human primates respond to similar stimuli in a similar way,â says Grueter, âbut we did not find any evidence for that.â Primates did not selectively seek out juvenile heterospecifics for play and cuddles, based on the data available.
Itâs more likely that primates reach out to heterospecifics when there are no conspecifics around. Primates are highly social, and it may be that they will take social interactions where they can get them.
Does this mean wild primates have pets? Grueter says the evidence doesnât support that, if only because pet-keeping implies a long-term relationship, which the data doesnât show. âFrom what we could glean from the literature, [the interactions] lasted anywhere from a minute to maybe an hour, or a day, or a couple of days,â he says. We wouldnât call that pet-keeping.
However, there are a few cases where heterospecifics seem to have formed longer-lasting friendships. Grueter flags a 2006 report of . The marmoset stuck around for at least 10 months and integrated into the group: two adoptive mothers nurtured it and the other capuchins tolerated it.
âI would say this is a case where you could probably use the term pet-keeping,â says Grueter. âAt least, itâs a rudimentary form of pet-keeping behaviour.â
I suspect that, if primatologists do start looking for heterospecific interactions, they will find more examples. Something similar happened with grief behaviours: a few isolated anecdotes of primates seemingly mourning dead conspecifics eventually led to systematic evidence of mortality-related behaviours in primates and other social animals. Thatâs certainly what Grueter hopes will happen: he is planning a more extensive follow-up survey.
I also think these findings fit a broader pattern about human uniqueness. Everything that is supposedly unique about humans turns out to be a matter of degree. Yes, our languages are more complicated and versatile than the communication systems used by other animals, but animals can still convey a lot to one another. Likewise, those of us who keep pets may be more committed to it than wild primates, but wild primates are still capable of forming relationships with heterospecifics.
Finally, these primate-heterospecific interactions are yet another reminder that we are, and always have been, part of the biosphere. We are inextricably linked to the ecosystems in which we live. Our ancestors no doubt hunted animals for food â but they may also have cuddled them and thrown sticks for them to chase.
