IB Evaluation Practice 2
We have previously discussed how a positive correlation has been identified between the change in diet and brain size during hominid evolution. A number of studies have been conducted to analyze this correlation, some of which we will examine and discuss in class. The following prompt and corresponding data are related and examine this question.
For this activity, you do not need to include a discussion of the error/limitations as you are only focusing on the analysis of the data presented. Use all three graphs to make a single conclusion addressing the relationship between human brain size and diet.
The high energy costs of large human brains is evident in Figure 1. which shows the scaling relationship between brain weight (grams) and resting metabolic rate (RMR) (kcal/day) for humans, 35 other primate species, and 22 non-primate mammalian species. The solid line denotes the best fit regression for non-human primate species, and the dashed line denotes the best fit regression for the non-primate mammals. The data point for humans is denoted with a star. As a group, primates have brains that are approximately three times the size of other mammals (relative to body size). Human brain sizes, in turn, are some 2.5–3 times those of other primates.
Figure 2 shows the association between dietary quality and body weight in living primates, including modern human foragers. The diet quality (DQ) index reflects the relative proportions (percentage by volume) of (a) structural plant parts (s; e.g., leaves, stems, bark), (b) reproductive plant parts (r; e.g., fruits, flowers), and (c) animal foods (a; including invertebrates): The index ranges from a minimum of 100 (a diet of all leaves and/or structural plant parts) to 350 (a diet of all animal material).
Figure 3 shows relative brain size versus relative dietary quality for the 33 different primate species for metabolic, brain size, and dietary data. Relative brain size for each species is measured as the standardized residual (z-score) from the primate brain versus body mass regression, and relative DQ is measured as the residual from the DQ versus body mass regression.
Using these descriptions and the three graphs, state a conclusion, with justification, based on a reasonable interpretation of the data.
Figure 1. Log–log plot of brain weight (BW, g) versus RMR (kcal/day) for humans, 35 other primate species, and 22 non-primate mammalian species. The primate regression line is systematically and significantly elevated above the nonprimate mammal regression. For a given RMR (resting metabolic rate), primates have brain sizes that are three times those of other mammals, and humans have brains that are three times those of other primates.
Figure 2. Plot of DQ versus log body mass for 33 primate species. DQ is inversely related to body mass (r = −0.59 [total sample]; −0.68 [nonhuman primates only]; P < 0.001), indicating that smaller primates consume relatively higher quality diets. Humans have systematically higher quality diets than predicted for their size. (Adapted from Leonard, W.R. et al., Comp. Biochem. Physiol., Part A, 136, 5, 2003.)
Figure 3. Plot of relative brain size versus relative DQ for 31 primate species (including humans). Primates with higher quality diets for their size have relatively larger brain size (r = 0.63; P < 0.001). Humans represent the positive extremes for both measures, having large brain:body size and a substantially higher quality diet than expected for their size. (Adapted from Leonard, W.R. et al., Comp. Biochem. Physiol., Part A, 136, 5, 2003.)
This criterion assesses the extent to which the student’s report provides evidence of evaluation of the investigation and the results with regard to the research question and the accepted scientific context.
Nearly Meets (2)