Attractiveness of primate infants

Issue 16, December 1999

Young mammals need to be fed, protected, groomed and transported around and A these behaviours are costly to the parents, especially the mother. In some species other adults provide care, though the amount and form of care that is given varies. In primates in particular, young animals are often the source of considerable attention for other members of the same group, especially females. One recent study* into the attractiveness of primate infants was carried out on bonnet macaques (Macaca radiata), see Figure 1. The observations were made over a period of four years at a research centre at a university in California.

Figure 1 A bonnet macaque.

The group under study varied in size (due to births and deaths) but consisted of male and female adults, subadults, juveniles and infants. Forty infants were observed in this study, 21 females and 19 males. During her observations, all interactions directed towards the focal mother's infant by other group members were recorded. The researcher recognised eight types of handling: greet, muzzle, touch, groom, inspect, pull, carry and hold. She calculated the "hourly rates of interaction towards infants by dividing the number of interactions directed towards the infant by the number of hours the mother was observed".

*Silk, J. B. 1999. Why are infants so attractive to others? The form and function of infant handling in bonnet macaques. Animal Behaviour, 57, 1021 - 1032.

[We are grateful to Academic Press for permission to reproduce Figures 1, 2 and 4 from the article by Silk J. B. in Animal Behaviour, 57, 1999.]

1. This study was carried out on a captive group of macaques at a university research centre in California. Outline one advantage and one disadvantage of studying captive populations of animals, such as macaques. ( 2 )
2. In her report, the 'researcher states that she focused "upon behaviour directed towards infants when they were on or near (within 3 m of) their mothers during the first six months of life". Why is it important for researchers to be precise in defining such factors as distance from the mother and the age of the infants when writing a report of their study( 2 )
3. This study used focal sampling. Explain what a focal sample is. (1)
4. The observer identified eight forms of infant handling and then recorded the number of acts per hour that each of the behaviours was observed. The data are illustrated in Figure 2. Briefly comment on the pattern of infant handling behaviours that is evident on the graph. ( 1)

Figure 2. Mean rates (acts/h) of different forms of infant handling. [Muzzling occurred when an individual put its face close to the infant's face or body.]


5. The researcher also recorded the number of handling acts per hour received by the infants from other macaques in the group and these findings are shown in Figure 3. What interpretation can be placed on these data? ( 2 )

Figure 3. Mean rates of handling received by infants from male and female adult, subadult and juvenile macaques.


6. Suggest one reason why subadult female macaques might handle the infants more than adult or juvenile female macaques. ( 2 )
7. The researcher also recorded how the number of acts of handling was associated with the age of the infants, see Figure 4. What does the pattern evident in Figure 4 suggest? ( 1 )

Figure 4. Mean rates of handling received by infants of different ages.


8. What type of graph is illustrated in Figure 4? (1)
9. The researcher analysed the findings to see if female infant macaques were handled significantly more than male infants. For this analysis she used the Mann-Whitney test. Why was this an appropriate test to use? ( 1 )
10. The Mann Whitney test gave a U value of 171.5, p = 0.603. What interpretation can be placed on this value? ( 1 )