How Monkeys Pleasure Themselves and Horses Use Tools

I recently learned about two eye-opening studies of nonhuman animal (animal) behavior. The first, by Camilla Cenni and her colleagues, is called Do Monkeys Use Sex Toys? Evidence of stone tool-assisted masturbation in free-ranging long-tailed macaques. The second, by Konstanze Krüger and her colleagues, is titled Tool Use in Horses.¹ Both of these studies have received widespread coverage in popular media.²

What was of particular interest to me about these studies was that I recalled two conversations with students about these two topics in the courses I taught in animal behavior. Wendy told me about her dog Veronica who used to rub her genitals on various toys. Wendy wondered if Veronica was using them as sex toys because Veronica really seemed to enjoy them and "displayed bliss." Miguel told me that when he worked with horses during many summers, he was sure they made and used tools such as sticks to move things around. These were not the only stories about these two behaviors.

Tool-assisted masturbation by monkeys

To a limited extent, the data collected by Cenni and her research team are convincing that some female and male monkeys engage in "self-directed tool-assisted masturbation." The researchers predicted that using stones to tap the genital and inguinal areas would be highly associated with female estrus and male penile erection and that the stones used to masturbate would show less variation than stones used during play.¹ They observed these patterns of behavior in only one population of monkeys and there was variation among different groups of monkeys.They were not as common as some reports make them out to be.

In contrast to some hyped-up headlines in popular media, the researchers write, "Overall, our data partly supported the sex toy hypothesis indicating that stone-directed tapping and rubbing onto the genital and inguinal areas are sexually motivated behaviors" (my emphasis).² While these observations are very important, more data are needed to see how widespread this sort of masturbation is. It also would be valuable to know why some monkeys do it and others don't.

Tool use by horses

Needless to say, horses making and using tools aren't as "sexy" as masturbating macaques. Still, Konstanze Krüger and her colleagues used crowd-sourcing, a form of citizen science, and asked people who owned horses and others who cared for them for examples of tool use. They also searched YouTube and Facebook for videos of equids showing tool use.³ Analyzing 635 reports of 1014 behaviors, the researchers learned about 20 cases of tool use, "13 of which were unambiguous in that it was clear that the behaviour was not trained, caused by reduced welfare, incidental or accidental." Tools were used to gather hay, forage, or to groom others, activities that could be pleasurable.

The data collected show that on very few occasions horses used tools and the researchers aptly write, "we stress our findings should be regarded as provisional and suggestive rather than definitive." I agree. The reports they received form the beginning of a valuable database and as do citizen scientists' reports of tool use in dogs that show canids do indeed use tools.

Where to from here?

Clearly, more data are needed for both of these very interesting behaviors and the researchers realize this.

I don't want to be a party pooper. I want to be clear that both of the above studies are very important and well done. But we need more data before making sweeping statements about how common these behavior patterns actually are. Learning that some animals do these sorts of things will hopefully stimulate more research.

Citizen science and more systematic studies will show just how widespread these behaviors are. These studies open the door for people who have observed these patterns of behavior to report them to researchers who study them.