Given the study's results, predict which of these chimps is most likely to pee after another one does.
Photo: Dirk M. de Boer/Shutterstock
The radio program Short Wave (on NPR) highlights 3 science-related news stories every week. In this episode, one story is about a study showing that peeing is contagious in chimpanzees. The study is an excellent example of observational research, statistical reasoning, and turning an informal observation into a full-fledged empirical study.
You can listen to the story here--the study is explained in the first 3 minutes of an 8-minute broadcast. I'm going to paste sections of the transcript here, which features a conversation between reporters Regina Barber, Jonathan Lambert, and Ari Shapiro:
BARBER: [...] Ena Onishi, a grad student at Kyoto University in Japan, was studying a group of captive chimpanzees when something similar struck her as odd.
ENA ONISHI: I noticed that they seemed to have a tendency to urinate around the same time. And it kind of reminded me of some human behaviors of going to the bathroom together, for example.
[...]
LAMBERT: Yeah. So this observation got Ena wondering if this behavior might be socially contagious, like yawning. And to see if it was, she spent more than 600 hours watching a group of 20 chimpanzees.
[...]
LAMBERT: [...] she said that it was easier to hear them pee than to see them pee. But so she noted, when each individual chimp peed and where they were relative to each other. And looking at the data altogether, an interesting pattern emerged.
BARBER: So basically, chimps were peeing together a bit more often than you'd expect if they were just peeing at random. And she published that conclusion in the journal Current Biology this week.
SHAPIRO: Could she tell why this was happening?
BARBER: So being closer helped, but proximity wasn't, like, the main factor here. Chimps have a hierarchical society, and it turns out that the lower ranking chimps were more likely to catch the urge to pee from more dominant chimps.
SHAPIRO: So when my boss pees, I'm going to pee?
BARBER: Yeah.
SHAPIRO: Is that what you're telling me?
LAMBERT: Maybe. And they don't know why this is. It could just be that lower ranking chimps are paying closer attention to higher ranking ones. But that's just one possibility.
SHAPIRO: OK, so there's clearly more to study here, but is there any evolutionary reason that contagious urination would be an advantage?
LAMBERT: One idea is that doing the same thing together just kind of helps a group sync up, which could help them operate better as a unit. If this happens in the wild, it might help the chimps avoid predators who get attracted by the smell of pee by concentrating it all in one spot.
BARBER: But we can't rule out that there might be, like, a non-adaptive reason. It could be just that the chimps pee when they hear other chimps pee, sort of like humans get the urge when you hear, like, running water.
In the empirical study, Onishi and her co-authors counted up (for a total of 600 hours) how often an instance of one chimpanzee's pee co-occured within 60 seconds of another one's pee. This was an instance of "pee contagion." Then, they compared their observed number of c0-occuring pees to a simulation model. In the simulation model, a computer estimated the total number of co-occuring pees they'd see in 600 hours if the null hypothesis is true--that is, how often would the pees occur within 60 seconds of each other if there's no contagion? They found that the observed data showed more pee contagion than the simulated model, confirming that there was contagious peeing.
Then they correlated "pee contagion" to two predictors. First, they found that contagion was more likely when two chimpanzees were physically closer to each other: There was more contagion when chimps were arms' reach, compared to 1 meter or more than 3 meters away. Second, they correlated pee contagion with each chimp's position in the hierarchy, and found a positive correlation, such that the higher the first chimp was in the hierarchy, the more likely another chimp would contagious pee after they did.
Questions
a) Reading the transcript (or listening to the story), what was Onishi's informal observation, and how did she turn it into a study? Reflect on how her research started with an informal hunch, and evolved into a full-fledged published article. Do you have any research hunches that you'd like to turn into a published article?
b) The commentators bring up three very different explanations--mechanisms--for the contagious peeing effect. Identify the three explanations in the story.
c) Pick one of these three explanations. How could you collect data in a new study to test that explanation? What variables would you measure, and what result would support the explanation?
The Current Biology article is available here. You can see scatterplots of pee-contagion and dominance, and dot plots of pee-contagion by distance away. (may be paywalled)