The Semantics of Vervet Monkey Alarm Calls: Part I

Anti-predatory alarm calls are important  for social animals to alert others of approaching predators. Without the presence of "language", some non-human primates are known to give out different predator-specific alarm calls to alert conspecific. These non-human primates include ring-tailed lemurs (Zuberbühler et al., 1999), white-faced capuchin monkeys (Fichtel et al., 2005), Diana monkeys (Zuberbühler, 1999), Campbell's monkeys (Ouattara et al., 2009) and vervet monkeys (Seyfarth et al., 1980).

Alarm calls are typically high frequency sounds because these calls are hard to localized by predators. On the other hand, low frequency sounds are easier to localized by predators. Calls that are hard to localized by predators are selected for because conspecific can pick up on the warning but predators cannot identify the location of the caller. If an individual successfully alert its social group of approaching predator yet does not reveal its location, it will significantly decrease the chance of the caller to be detected and increase the chance of its social group to avoid predation.

Vervet monkeys. Photo from Wikipedia.

Here, I will focus on the study of predatory alarm calls in vervet monkey (Chlorocebus pygerythrus) by Seyfarth et al. (1980) in the Amboseli National Park, Kenya. Vervet monkeys are Old World monkeys that range between Eastern and Southern Africa. These monkeys are diurnal and live in closely-knit social groups. They are quadrupedal and are both terrestrial and arboreal. Like all Old World monkeys, vervet monkeys have the characteristic cheek pouches that enables them to forage and store food to be eaten later. Male vervet monkeys have blue scrotal area and a red penis. Males and females are sexually dimorphic, with males slightly larger than females.

Male vervet monkey with blue scrotal area and red penis. Photo from Something Up Her Sleeve.

Vervet monkeys are known to elicit predator-specific alarm calls. Three well-documented vervet monkey alarm calls are those for leopard, martial eagle and python. Leopard alarm calls are short tonal calls produced in a series of inhalations and exhalations. Eagle alarm calls are low pitched grunt while python alarm calls are high pitched "chutters". Different alarm calls seem to evoke different responses to individuals that heard the alarm calls. However, the first reaction of a vervet monkey upon hearing an alarm call is to look at the direction of the caller. Looking at the direction of the caller gives them clues as to why the alarm calls were made and also where the caller is facing reveals the direction of the approaching predator. You can listen to these different alarm calls on this site.

As we said before, different alarm calls evoke different responses. Leopard alarm calls would make the monkeys run up into the tree to avoid being ambushed by the leopard. Also, these monkeys would sit on the branches further away from the tree because, even though leopards can climb trees, the branches could not support the leopard's weight. When an eagle alarm call is given, vervet monkeys would make them look up, run for the nearest bush or both to avoid an approaching aerial attack. Python alarm calls would the monkeys stand bipedally and look down on the ground.

A martial eagle. Photo by Jacques S G from Flickr.

Adult vervet monkeys are more discriminatory when eliciting alarm calls. Infants and juveniles calls however, are less discriminating as they attribute most terrestrial mammals with leopard calls, flying birds with eagle calls and stick-like figures with snake calls (although, compared to infants, juveniles are more discriminant when making alarm calls). In spite of that, adult vervet monkeys seem to elicit eagle alarm calls to different species of raptors and non-raptors (see illustration below). We can infer that adult vervet monkeys attribute eagle alarm calls to birds with the same silhouette as martial eagles. As vervet monkeys get older, they seem to have a better association between predator species and types of alarm calls. Vervet monkeys generally pay more attention to adult alarm calls than those of juveniles or infants.

Alarm calls made by infant, juvenile and adult vervet monkeys in response to sightings of birds of prey (raptors) and non-raptors. The number of calls cited for each age group refers to the total number of calls that were analysed (Gould & Gould, 1999). Click on illustration for larger view of the image.

The study of vervet monkey alarm calls by Seyfarth et al. (1980) laid an important ground work to better understand the complexity of animal communications. By showing that vervet monkeys make different alarm calls to different predatory species, we can posit that vervet monkeys have the ability to categorize different species. The ability to discriminate between terrestrial mammal, flying birds and snake-like objects starts during infancy in vervet monkeys. As they get older, they are better at associating predators with specific alarm calls.

An infant vervet monkey with its mother. Photo by Lip Kee from Flickr.

The ability to over generalize during infancy is evident in both vervet monkeys and humans. For example, upon learning the word "dog", human infants would refer to quadruped mammals they see as "dog". As the infant grows, so does the ability to associate the semantic meaning of words they learned. However, the acquisition of alarm calls in vervet monkeys is different than the acquisition of speech (language) in humans. Alarm calls in vervet monkeys are instinctual and not learned. Humans, however, have to learn their language. Failing to do so during the "critical period" generally will result in the inability to learn language in later years. Feral child are examples of human infants that lack linguistic input during their critical period of language acquisition.

Most of us interpret animal alarm calls as an uncontrollable auditory response to fear or pain, akin to humans yelping if we had our finger caught in a door. While this is not entirely false, some animal calls actually convey information from the caller to the listener. Seyfarth et al. (1980) posit that vervet monkey alarm calls are actually basic semantic signals or symbolic signals because each alarm calls seem to mean something to these vervet monkeys. While we don't know if these alarm calls actually mean "leopard" or "run up to the tree", we do know that it conveys specific information to their conspecific about approaching predators.

I will be blogging Part II of this post later this week, where I will explain in details the experiments done by Seyfarth and Cheney on vervet monkey alarm calls.

References:
Cawthon Lang KA. 2006 January 3. Primate Factsheets: Vervet (Chlorocebus) Taxonomy, Morphology, & Ecology. http://pin.primate.wisc.edu/factsheets/entry/vervet. Accessed 2011 March 9.

Fichtel, C. Perry, S. Gros-Louis, J. 2005. Alarm calls of white-faced capuchin monkeys: an acoustic analysis. Animal Behaviour 70(1): 165-176. doi: 10.1016/j.anbehav.2004.09.020.

Gould, JL. Gould, CG. 1999. The Animal Mind. Scientific American Library.

Ouattara, K. Lemasson, A. Zuberbühler, K. 2009. Campbell's Monkeys Use Affixation to Alter Call Meaning. PLoS ONE 4(11). doi:10.1371/journal.pone.0007808.

Seyfarth, RM. Cheney, DL. Marler, P. 1980. Monkey responses to Three Different Alarm Calls: Evidence of Predator Classification and Semantic Communication. Science 210(4471): 801-803.

Zuberbühler, K. Jenny, D. Bshary, R. 1999. The Predator Deterrence Function of Primate Alarm Calls. Ethology 105: 477–490. doi: 10.1046/j.1439-0310.1999.00396.x.

Zuberbuhler, K. 2000. Referential labelling in Diana monkeys. Animal Behaviour 59(5): 917-927. doi: 10.1006/anbe.1999.1317.

Sexual And Natural Selection: Why Humans Are Still Evolving

Comparative photo between a masculine-looking (left) and feminine-looking (right) male. Photo from Dienekes' Anthropology Blog.

On Dienekes' Anthropology Blog, he shares with us a study on the correlation between female mate choice (sexual selection) and national health index (DeBruine et al., 2010). Female tend to prefer more masculine-looking males in countries where the national health index is low. Consequently, females tend to prefer more feminine-looking males in countries where the national health index is high. Head over to Dienekes's blog to read about his blog post, "Preference for masculine/feminine-looking men and national health".

With natural selection at work, females are predicted to be much shorter and stouter in the future. Photograph by Hans Neleman/ Getty on Macleans.ca

Also read about "Evolution favours shorter and heavier women—like it or not", an article that foresees the evolution of females to be that of much shorter and stouter. Stephen Stearn, professor of evolutionary biology at Yale University thinks that humans continue to evolve even when we're in a post-industrial society. While there are no large-scale genetic changes, Stearn believes that natural selection is still at work.

“One [could express] the result as: women are going to get shorter and fatter,” he explains. But he prefers a different bent: “There is natural selection against women being slender.” Stearns’s work shows that plumper, shorter women tend to bear more children—who carry on those same traits. His analysis drew on data from the Framingham Heart Study: a survey, begun in 1948, that collected medical information from 5,209 subjects, and monitored them and their offspring for 60 years. 

The weight part of the equation, says Stearns, is straightforward: “A woman has to have about 20 per cent body fat to ovulate and conceive.” But he admits that he “can’t give a good explanation of why they are getting shorter.” A separate study by Open University’s Daniel Nettle found that shorter women are more likely to be in long-term, offspring-producing relationships—perhaps, he hypo thesized, because men evolved to disfavour tall women, who tend to reach puberty later. 

A Cure For Malaria

Do you know that the difference between human and chimpanzee genome is only 2% ? That genetic difference involves mostly genes that are related to the immune system, infectious disease and also vulnerability to parasites (Sapolsky, 2006). Chimpanzees are resistant to malaria while we humans are scrambling to find a cure for this epidemic.

Researchers are now trying to understand how chimpanzees are resistant to malaria and use that knowledge to find a cure.

Erica Tassone, a biology graduate student, is currently working on malaria research with Stone and Vellerri for her dissertation project, which she started in fall 2007.

“I look at genes associated with malaria resistance in chimpanzees and see how they’re the same or different from those genes in humans,” Tassone said.

Although Tassone said she is also interested in doing research on how genes act in a cell, her main concentration now is the disease research.

“[I hope] to get a better understanding of how humans and chimpanzees have evolved and developed resistance to diseases,” Tassone said.

Click here to read the full news article.

Citation:
Sapolsky R. 2006. The 2% Difference. Discovermagazine.com http://discovermagazine.com/2006/apr/chimp-genome/article_view?b_start:int=1&-C=

Our Toes Were Made For Running

Ever wonder why our toes are stubby instead of long and slender like our fingers? Long toes require more energy and generate more shock as compared to short toes, according to biomechanical analysis. Shorter toes are an adaptation favored by natural selection. Australopithecines, our first bipedal ancestor, had longer toes than our genus Homo but smaller than those of great apes.

"When you're walking, before you push off to start the next step, your other foot has already hit the ground. You've transferred some of your body weight. Your toes have to do much more work in running, to push you." (Rolian et. al, 2009)

Click here to read more.

Citation:
Rolian C. Lieberman DE. Hamill J. Scott JW. Werbel W. 2009. Walking, Running And The Evolution of Short Toes in Humans. Journal of Experimental Biology 212 (5): 713-721.