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Studying mammals: The social climbers
Studying mammals: The social climbers

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5.2 Social and ecological factors

Researchers think that the increase in creative thinking capacity was driven by group living because, in primates, increasing neocortex size is linked to increasing group size. We know that, even if there is not a strict hierarchy, monkeys are aware of the rank and kinship of each individual in their group, both in relation to themselves and in relation to each other. So as group size increases, each individual needs to keep track of a rapidly increasing number of relationships, which may be facilitated by the association areas of the neocortex. This relationship between group size and neocortex size is further supported because other aspects of social living are all related to neocortex size - factors such as the size of grooming cliques, the amount of tactical deception shown by individuals of the species, the amount of time juveniles spend playing and the length of the juvenile period, which could be regarded as the period for learning and socialisation.

But social factors are not the only aspects of primate life that are linked to neocortex size. Other researchers have shown that neocortex size is also linked to ecological factors such as the percentage of fruit in the diet and the size of the home range. As ripe fruiting trees are usually unevenly distributed throughout the home range, individuals need to move quickly from one food resource to another. Researchers suggest that individuals who were more able to hold a mind map of the locality in their head may have been more successful at finding food and this could be linked with an increase in neocortex size.

You have met several ways of presenting numerical data in this course. Figure 4 showed the number of alarm calls of vervet monkeys grouped into categories, 1-5 calls, 6-10 calls, etc., and then each category was represented by a different-coloured line. Bar charts were introduced in Figure 5, grouping data about the range of the red colobus monkeys. Figure 9a and b are also bar charts, with the length of each bar representing the absolute or the relative size of the brains of various mammals.

The graphs in Figure 11, used in Activity 12, are rather different. They are called scatter plots. In this example, the vertical axis (also called the y-axis) of each of them has a logarithmic scale. (You'll notice that Figure 11a has a logarithmic scale on the horizontal axis, i.e. the x-axis, too). In all three graphs, the x-axis represents the ratio of the neocortex volume to the volume of the rest of the brain. In general, scatter plots can be used to see if there is a relationship (also called a correlation) between the two variables plotted on the axes. Note that there are no units on the x-axis because ratios are relative measures.

Figure 11
Figure 11: Dunbar, R. I. M. (1989) Neocortex size as a constraint on group size in primates, Journal of Human Evolution, vol. 20, pp. 478 and 484, copyright š 1992, with permission from Elsevier ©
Figure 11: Dunbar, R. I. M. (1989) Neocortex size as a constraint on group size in primates, Journal of Human Evolution, vol. 20, pp. 478 and 484, copyright š 1992, with permission from Elsevier
Figure 11 Relationships between neocortex size (ratio of neocortex volume to volume of rest of brain) and (a) group size, (b) home range, and (c) day range for anthropoids and prosimians

Activity 12

Study Figure 11 carefully, including the figure caption and the key provided. Can you decide at this point which of the three factors - mean group size, home range area and day range - is more likely to have an influence on the evolution of the neocortex? Write down your answer and then work through the explanation of Figure 11 that follows.

Figures 11a, b and c are scatter plots. Look at plot (a). Each symbol represents the mean value of group size plotted against the value of neocortex size for a particular anthropoid species. To see if a relationship between two variables exists, you need to look at the pattern of the scatter. Firstly you need to establish a 'line-of-best-fit' through the scatter of points so that the points are equally distributed on either side of the line.

Draw a line-of-best-fit through the points in Figures 11a, b and c. Now look at the way in which the points are distributed around each best-fit line. If the points lie close to the line, there is a stronger relationship between the two plotted variables than if the points lie further away from the line. When considering relationships shown on scatter plots, the steepness of the line also needs to be taken into account. A reasonably steep line (say, drawn at an angle of about 45 degrees), with minimal scatter, would mean that the values plotted on the vertical (y) axis change considerably for a small change in the values plotted on the horizontal (x) axis, showing a strong relationship. An almost horizontal line (nearly parallel to the x-axis) shows there is little change in y as values in x increase, meaning the relationship between the two variables is weak. Similarly, an almost vertical line (nearly parallel to the y-axis) shows that there is little change in x as y increases.

Question 20

Question: Look at Figure 11a, b, and c. Taking into consideration the scatter of points about your line-of-best-fit and the steepness of the line, can you order the plots by the strength of the relationships between the two variables plotted, starting with weakest?


You probably found it easy to put (c) into first place as the weakest relationship. Because the scatter is wide, it is difficult to decide where exactly to put the line through the points and the slope of the resulting line is quite shallow. You can fit a line more easily through (a) and (b), but deciding which relationship is the stronger is quite difficult.

To make comparisons between relationships easier, statisticians calculate a value called r (the symbol for a term called the correlation coefficient), which takes into account the position of the line, its slope and the spread of the points about the line and enables them to state whether the relationship is significant, i.e. 'Is it strong enough to be meaningful?'. The r values for (a), (b) and (c) are 0.87, 0.77 and 0.55; in the particular circumstances in which this test is being applied here, a value of 0.55 or above is taken as significant. Surprisingly the relationship between day range and neocortex size (c) is significant despite the large amount of scatter. (Strictly speaking, the relationship is between the logarithm of day range and neocortex size, but it's easier if we use the more convenient shorthand.) The relationship between group size and neocortex size (a) is slightly stronger than the relationship between total home range and neocortex size (b) although both relationships are significant. So the correct order from the weakest to the strongest is (c), (b), (a).

It's appropriate at this point to understand that this type of correlation can only measure the strength of a relationship between two variables. No correlation, no matter how strong, can prove that a change in one variable is causing the change in another. In this example, it's not possible to be certain that increasing social and ecological factors were the driving force behind increasing neocortex size. It's possible (though unlikely) that a third unidentified factor caused both variables to change hand-in-hand.

Figure 11a includes data for many primates, both solitary and group-living. When these data are looked at more closely, it's found that the neocortex size of solitary anthropoids is not greatly different from the neocortex size of prosimians that are solitary, whereas the neocortex size of group-living anthropoids is considerably larger than for solitary anthropoids and prosimians, thus providing further support for the theory.

But differences of opinion amongst researchers remain. As both group size and total home range size (and the other factors mentioned previously) correlate well with neocortex size, researchers are split; a few support the importance of ecological factors, but the majority believe that social factors are more important. Some very recent research, however, suggests that both sets of factors may have played an important role in the evolution of the primate brain. By collating from the literature all the examples of innovation, tool use and social learning in primates, researchers have shown that all three measures are correlated with neocortex size. Many of these examples of innovation, tool use and social learning are related to obtaining food, suggesting that both ecological and social factors have played a part in the evolution of the large primate brain that has led to the abilities of the most successful primate - humans.