Jacqueline MacPherson

Guest Blogger – Annabel Scott , BSc Environmental Stewardship –  Glasgow University

In a recent study by Wilson et al. (2013), the facial structure of capuchin monkeys was examined to see whether differences in this link to different personality traits.

Sixty-four capuchins were examined from three institutes: our Living Links monkeys here in Edinburgh Zoo, the Language Research Centre at Georgia University and the Laboratory of Comparative Ethology at the National Institute of Health.  Front facing photographs were used to find the measurements for each capuchin.  fWHR (facial width to height ratio) was determined by the ratio of bizygomatic width to upper face height as shown below.   Lower face/face height (eyelids-chin/height of whole face) and face width/lower face height (bizygomatic width/height of whole face) were also calculated.

 

 

 

Personality ratings were collected for each individual by a measure used in non-human primates.  These were scored on a 7-point scale, which ranged from no signs of the trait (1) to extreme display (7).  Five traits were looked at; assertiveness, openness, attentiveness, neuroticism and sociability.  Below are photos that illustrate the differences in capuchin face morphology.

 

 

Examinations of lower face/face showed a significant effect of age as the ratio increases, however no sign of sexual dimorphism.  Neuroticism was found to be non-dimorphic in capuchins, but in humans both neuroticism and lower face/face height are sexually dimorphic.  fWHR is not sexually dimorphic in humans.  Capuchins with higher ratios of lower face/face height (longer lower face) were found to be more neurotic and less attentive.  Therefore facial morphology of capuchins determines three personality traits: assertiveness, attentiveness and neuroticism.

 The following graph (Fig. 1.) shows how face width/lower face height has a significant age × sex interaction, with males showing a higher face width/lower face height ratio than females.  These sex differences increase across the life span.  Humans also exhibit sexual dimorphism in this facial metric, however women show higher ratios than men and this also increases with age.

fWHR and face width/lower face height both showed a link to assertiveness.  One possible reason that these facial metrics relate to personality is due to the connection with status and leadership traits.  Status in humans can be based on force or friendship, lower face/face height however, may be driven by vigilance and attention span, therefore linked to a social form of status.

Openness and sociability appear to affect sociality and cognition in capuchins.  Sexual dimorphism may be linked to differences in morphology, so future work with these species may help to understand what determines species-specific personality traits and why they are associated with facial structure.

Further studies could look at sex-specific age growth in capuchin facial metrics and could also examine the effects of location and body weight.  Examining the lower face/face height further could tell us the origins of status effects on well-being and emotional traits, which could be linked to status in humans.

Reference

Wilson, V., Lefevre, C.E., Morton, F.B., Brosnan, S.F., Paukner, A. and Bates, T.C. (2013). Personality and facial morphology: Links to assertiveness and neuroticism in capuchins (Sapajus [Cebus] apella), Personality and Individual Differences. (IN PRESS)

 

 

 

 

 

 

 

 

 

The ability to learn patterns, ultimately the rules of language has rarely been studied in new world primates. In this study the researchers use species specific sounds/tones to determine if the squirrel monkeys were able to decipher when the correct order and incorrect order of these sounds were played. This is the first study that has used species specific frequencies in order to test their abilities. Other studies relied on humans producing syllables, which may not have been the most appropriate way to measure the monkeys’ capabilities.

Two ordered sound patterns were tested with the same rule being applied of A Bⁿ A.

Test 1 – Low Tone= A, and High Tone = B (eg. Low, High, High, High, Low)

Test 2 – High Tone = A and Low Tone = B (eg. High, Low, Low, High)

The results suggest that the monkeys were able to consistently decipher when the rules were being broken in both the test settings.

These finding suggests that the ancestor of both squirrel monkeys and humans which existed approximately 36 million years ago would also have been able to understand the rules involved in pattern learning, thus most living apes and monkeys today should also be able to do this. This skill may have evolved as a cognitive ability rather than a direct pre-cursor to language.

Ravignani, A., Sonnweber, RS., Stobbe, N. and Fitch, TW (2013). Action at a distance: dependency sensitivity in a New World primate. Biology Letters. 9. 20130852

Ruth Sonnweber, one of the scientists from this study is now continuing similar work here with our squirrel monkeys in Living Links, however instead of sound stimuli she has been using touch screen to determine their ability to learn patterns.

 

 

Chimpanzee Alarm Call Production Meets Key Criteria for Intentionality

Determining the intentionality of primate communication is vital to understanding the evolution of human language. Although intentional signalling has been studied for some great ape gestures, comparable evidence is currently lacking for their vocal signals.

In this study the following criteria were used to analyse if the Sonso group of chimpanzees in the Budongo Forest in Uganda were communicating with intention.

 

 

 

 

 

 

The researchers presented the chimpanzees with a python model with up to 4 observers recording the calls and behaviours of each focal individual.

 

 

 

 

 

 

 

Figure – The snake image represents the location of the python
model, concealed by leaves. Observers and their main roles are defined. The
chimpanzee image depicts the focal chimpanzee, who could be accompanied by
other group members depending on the experimental condition.

The results of this study indicate that some chimpanzee calls meet the criteria for intentionality, for example most chimpanzees who had seen the snake model gave calls and demonstrated gaze alternation and audience checking with a fellow chimpanzee. ‘Waa Barks’ (WB) and ‘Alarm Huus’(AH) were produced in the presence of socially important individuals who had not seen the snake. Furthermore , WB’s and AH’s were increased in the presence of friends vs. non-friends.

The researchers believe that their findings disagree with the gestural theories of language origins and instead support a multimodal origin for human language.

Video illustrating gaze alternation and looking at a group member before producing waa barks. Video is filmed from position 2. Focal adult female Nambi reacts to the arrival of her adult son Musa by turning and looking at Musa before producing her first waa barks of the trial. Nambi then looks immediately back at the snake, showing gaze alternation between the recipient and the snake whilst calling. During Nambi’s waa bark production, Musa stands bipedally.

 

Schel, AM., Townsend,SW.,Machanda, Z.,Zuberbuhler, K., and Slocombe, KE. (2013). Chimpanzee Alarm Call Production Meets Key Criteria for Intentionality. PLoS ONE 8 (10): DOI: 10.1371/journal.pone.0076674