Each morning when psychologist Drew Rendall, PhD, and his wife Karen set out to follow a band of baboons around the wilds of Botswana, they often located the animals by listening for the loud, whooping barks that represent what researchers have dubbed their "contact calls."
The baboons give these calls in what seems like a call-and-response pattern with voices echoing out from one area and being returned from another. And for decades, researchers have assumed that, indeed, the vocalizations represent calls and answers. Two groups of animals may whoop back and forth so they can meet up and regroup. Or individuals separated from their group may bark and receive a reply from a baboon in the main group as a way to reunite with the clan.
But a growing body of research, including a new study by Rendall and his colleagues, suggests that caution may be warranted before drawing too close a parallel between animal and human behavior. In fact, adult baboons rarely call in response to others' calls, they find, and baboon mothers don't even return the calls of their lost infants. The mothers may look around and head in the direction of the call, but they do not call back the way a human mother would to reassure her infant she's on the way.
These findings, published this month in APA's Journal of Comparative Psychology (Vol. 114, No. 1), call into question the degree to which the mechanisms underlying baboons' and other nonhuman primates' communication and human speech are similar, and also provide another piece to a growing body of evidence that monkeys lack what psychologists call a "theory of mind"--the ability to understand that others have knowledge, thoughts and feelings that may be different from one's own, which develops in human children around age 3.
"The findings are quite consistent that baboons don't appear to respond to each others' calls," says Richard Byrne, PhD, professor of evolutionary psychology at the University of St. Andrews in Scotland, who also studies baboon cognitive behavior. "And that could imply they don't understand the plight of the other."
Along with language, researchers have raised theory of mind as a potential cognitive dividing line between humans and nonhuman primates. The term is used to capture a range of knowledge humans regularly assume about others--for example, that what people know is based on their experiences and perceptions and that this knowledge can be manipulated to deceive. Such knowledge allows us to understand the behavior of others in terms of what we believe they think, know and intend.
Over the decades researchers have landed on both sides of the theoretical fence with some arguing that apes and even some monkeys certainly have access to both language and theory of mind, and others convinced that none of the nonhuman primates possess either skill.
The results of the new study by Rendall, Robert Seyfarth, PhD, and Dorothy Cheney, PhD, raise more doubt that monkeys share either cognitive skill with humans. Baboons, the findings suggest, do not use their calls to "answer" others. And from that the researchers postulate that they likely don't understand that their vocalizations can be used to influence the mental state or the behavior of other animals, says Rendall, the study's lead author and an assistant professor at the University of Lethbridge in Canada.
A cry ignored
Rendall has long been interested in animal communication, so working as a postdoc with Seyfarth and Cheney at the University of Pennsylvania from 1996 to 1998, it seemed natural that he extend their recent research on contact calls between adult female baboons.
Both studies took place at a research field site Cheney and Seyfarth maintain in northern Botswana. In the earlier study, Cheney, Seyfarth and Ryne Palombit, PhD, examined whether adult female baboons would answer the calls of related females in preference to those of unrelated females. They didn't. Indeed, the baboons rarely returned others' calls regardless of whether the caller was kin.
This finding contradicted what researchers in the field had long assumed about contact calls. But it is, in fact, consistent with findings from lab work indicating that monkeys have no theory of mind, says Rendall.
"When you see animals separated from the group giving calls back and forth it's easy to assume they're trying to locate one another and actively answering one another to do so," says Rendall. "One animal lost would get a response from others in the group and thereby find his or her way back to the group. But such an assumption has some serious cognitive implications."
Indeed, it assumes the animals can understand others' perspectives or needs and tailor their behavior accordingly.
Perhaps, however, Cheney's and Seyfarth's original study in adults simply didn't tap a situation where the animals were motivated to display their understanding of another's plight, thought Rendall. In contrast, mothers might be highly motivated to return calls from their defenseless infants.
He and his wife spent 14 months in Botswana testing this idea. To start, they simply observed females and found that they were most likely to give contact calls when on the periphery of their group and out of sight of other adults--in other words, when they were at risk of getting lost. However, they also sometimes called from the thick of the group if they lost sight of their infants.
In the second part of the study, the researchers watched to see whether mothers responded when their infants called. They also conducted an experiment, playing the calls of infant baboons over concealed loud speakers. Sometimes they played the calls of a female's own infant, other times they played the calls from another unrelated infant from the group. During the experiment, the females themselves were either in the midst of the group or separated but they were always out of visual contact with their own infants.
"If mom has a true theory of mind, she ought to respond regardless of her own condition," says Seyfarth.
In contrast, if she doesn't, she should call only when she herself is separated. In that case, what appears to researchers to be calling and answering may simply be simultaneous calling by multiple individuals who are feeling agitated about the risk of getting separated from the group.
"That's exactly what we found," says Seyfarth. "If mom is in the center of the group, she didn't respond. When she was on the periphery, she did respond. You can explain this by saying that animals bark when they themselves think they are getting lost."
Although mothers didn't call to their infants, they did tend to look around for them and often rushed toward the sound.
"They certainly recognized the infants' cries and were very motivated to find their kids," says Rendall. "But somehow they don't make the connection that 'If I call, it will affect what my kid knows.'"
Of course, there could be other ecological reasons why mothers don't return their infants' calls, says Rendall. It could be that calling in reply might put the infant at grave risk from predators or other folly if the infant then tries to locate the mother.
"It could be more adaptive to go collect the infant," says Rendall. "So the test isn't a lock solid test of whether or not mothers understand."
But it doesn't explain why adult baboons also fail to respond to each others' calls. And, adds Rendall, allowing an infant to continue crying while the mother searches probably isn't the safest thing for the infant, either.
The conclusion that baboons lack a theory of mind has several implications. First, it may in part explain the fundamental difference between the communication of monkeys and apes and human language, says Seyfarth. Most researchers have ascribed the major difference to syntax. But equally important may be the nature of the information conveyed when animals communicate.
There are three levels at which organisms can respond to other organisms: purely automatically, without thought; with some thought about what the other organism is doing; and with thought about what the other organism is thinking. It's this third level that requires theory of mind.
"When you say a sentence to me," explains Seyfarth, "I get a certain amount of information about the outside world, but I also treat it as a representation of what you think about the outside world. I extrapolate information about your mental state."
If baboons have no theory of mind, they may well extract information from others' vocalizations, but they likely don't interpret anything about the internal state of the caller. And, by extension, they might not understand that by producing a vocalization themselves they can change the mental states of companions.
Not having a theory of mind doesn't imply that monkeys live any less complex lives, says Daniel Povinelli, PhD, who studies theory of mind in chimpanzees. In fact, their abilities allow them to live in highly complex societies and form complex social relationships and long-term bonds.
Such a finding has implications for research into human afflictions, such as autism, he says, which may be one reason why the National Institutes of Health funded part of this research, along with the National Science Foundation, the National Geographic Society, the University of Pennsylvania and the National Sciences and Engineering Research Council of Canada. One theory is that people with autism lack a theory of mind. But if monkeys and apes truly lack this cognitive skill, yet are able to maintain complex social relationships, then something more must be amiss in autism.
"We're drawn to make romantic interpretations of animal behavior in the wild," says Povinelli. "The power of a study like Drew's is in helping us understand that these are species in their own right that have minds of their own. It provides part of the answer to the question, how do monkeys view the world? Profoundly differently from us."
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