Sample Data

The following data were collected by students in the non-majors Science and Society course at Morehouse College in Fall 2009.  A total of 26 replicates of 15 whole and 15 split mung beans were conducted.  Each replicate contained one female and one male bean beetle and egg laying was permitted for 48 hours. The average per bean mass of 15 whole beans was 0.060g and that of the split beans was 0.028g.  There were significantly more eggs laid on whole beans (260) compared to split beans (26) (binomial test, p<0.000001) with an average per replicate of 10 eggs on whole beans and only 1 egg on split beans (paired t-test, t=4.22 df=25 p=0.0003) (Figure 1).  A total of 16 replicates of 15 large and 15 small (all whole beans) also were conducted. The average per bean mass of 15 large beans was 0.071g and that of the small beans was 0.054g.  Again, there were significantly more eggs laid on large beans (169) compared to small beans (130) (binomial test, p=0.027) with an average per replicate of 10.6 eggs on large beans and only 8.1 eggs on small beans (paired t-test, t=3.48 df=15 p=0.003) (Figure 1).  Note that split beans differ from whole beans in both size and quality, which likely accounts for the much greater discrimination by female bean beetles.

Figure 1.  Female discrimination of bean size in short exposure experiments.  The mean+SE of the number of eggs laid by female bean beetles per replicate are shown for 16 replicates containing 15 large and 15 small mung beans, and 26 replicates of 15 whole and 15 split mung beans.  The differences in eggs laid on large versus small and whole versus split were significant (paired t-tests, p<0.05 for both comparisons).

A long-exposure protocol experiment was conducted by students at the University of Northern British Columbia in Spring 2010.  A total of 76 replicates were prepared that consisted of 10 whole and 10 split mung beans on which a single inseminated female was permitted to lay eggs until she died.  The average per bean mass of the whole beans was 0.059g and that of split beans was 0.036g in this experiment.  As in the short-exposure experiment, there was a significant bias toward whole beans.  The bias was highly significant in this long-exposure experiment with more eggs laid on whole beans (2020) than on split beans (151) (binomial test, p<0.000001).  The average number of eggs per replicate laid on whole beans was 27 while on split beans it was 2 (paired-t test, t=23.8 df=75 p<0.0001) (Figure 2).  A total of 76 replicates of 10 large and 10 small (all whole beans) also were conducted.  The average per bean mass of 10 large beans was 0.089g and that of the small beans was 0.037g.  There was a highly significant bias by females toward larger beans as there were significantly more eggs laid on large beans (1786) compared to small beans (860) (binomial test, p<0.000001) with an average per replicate of 23.5 eggs on large beans and only 11.3 eggs on small beans (paired t-test, t=12.67 df=75 p<0.0001) (Figure 2).

Figure 2. Female discrimination of bean size in long-exposure experiments.  The mean+SE of the number of eggs laid by female bean beetles per replicate are shown for 76 replicates containing 10 large and 10 small mung beans, and 76 replicates of 10 whole and 10 split mung beans.  The differences in eggs laid on large versus small and whole versus split were highly significant (paired t-tests, p<0.000001 for both comparisons).