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Second replies: Wells
Instead of responding to my criticism of the way Thomas and Johnson manipulate the data from Michael Majerus's 1998 book, Thomas now claims that I misrepresent several other scientific reports about the resting-places of peppered moths.
For example, Thomas cites Howlett and Majerus (1987), and claims that I disregard portions of it showing that 25% of peppered moths found resting in the wild were on tree trunks. Thomas is apparently basing his claim on Table 4 of that paper, which is simply an earlier version of Table 6.1 in Majerus's 1998 book (which I cited in my previous response, above). Table 4 lists resting positions of 25 peppered moths "found in the wild between 1964 and 1985." Of these, 4 were on exposed tree trunks, and 2 were hidden on unexposed portions of trunks. Six out of 25 is 25%, but four out of 25 is only 16%. Thomas is once again inflating his figures by including unexposed moths.
Furthermore, Howlett and Majerus acknowledged that their results might be skewed because "if most moths rest high up in the canopy or on foliate twigs, these would have been missed…. When the bias in being able to see exposed individuals is taken into account, it seems certain that most B. betularia [the scientific name for peppered moths] rest where they are hidden. We conclude, from the data and in consideration of the way collecting was carried out,… that exposed areas of tree trunk are not an important resting site for any form of B. betularia."
Thomas goes on to criticize my mention of Mikkola (1984), claiming that I fail to point out "that Mikkola found that 40 PERCENT of the moths in his experiment did indeed choose trunks." Table 1 in that paper lists a total of 191 caged peppered moths whose resting behavior Mikkola observed. Of those, 51 (27%) rested on trunks, 77 (40%) rested on branches, and 63 (33%) rested on the roof, walls and floor of the cage. Thomas's 40% figure can be obtained only by ignoring the 63 moths resting elsewhere in the cage - in other words, by once again manipulating the data.
Furthermore, Mikkola noted that his experimental procedure produced many more moths on trunks than would have been likely in the wild. He observed: "Many Peppered Moths simply walked up the trunks from the floor…. Most attention was therefore paid to the moths resting on the horizontal branches, the majority of which certainly had alighted from flight and supposedly showed the proper resting behaviour." Mikkola concluded: "It seems clear that the normal resting place of the Peppered Moth is beneath small, more or less horizontal branches (but not on narrow twigs), probably high up in the canopies, and the species probably only exceptionally rests on tree trunks."
Thomas then faults me for my use of Liebert & Brakefield (1987), claiming that I fail "to include the whole story, such as the part where Liebert & Brakefield disagree with Mikkola's conclusions, and where they note that moths WILL be found on lower branches and trunks."
It is true that Liebert & Brakefield disagreed with Mikkola in minor respects. For example, they wrote: "Overall, our observations of pairings and female moths suggest a more varied choice of resting position than proposed by Mikkola. Some will rest on main branches or trunks." But Liebert & Brakefield concluded: "Our observations [of female moths] support Mikkola's earlier conclusion from cage experiments with male moths that the species rests predominantly on branches."
Finally, Thomas claims - without mentioning specifics - that I misrepresent a 1985 paper by Clarke, Mani & Wynne. Clarke et al. (1985) wrote that "all we have observed is where the moths do NOT spend the day. In 25 years we have found only two" - one on a tree trunk, and another on a wall near a mercury vapor trap. To appreciate the significance of this - and the numbers cited in the other papers - it is helpful to note that Steward (1977) listed 52 studies conducted between 1952 and 1974, involving a total of 8,426 peppered moths. Clearly, the one moth reported by Clarke et al. (1985), and the six moths reported by Majerus (1998) as resting on exposed tree trunks, represent only a vanishingly small percentage of all peppered moths studied.
If the reader doubts my interpretation of the data, and has the time and inclination, I recommend consulting the original papers, which should be available in most good university libraries. The clear conclusion that emerges from all of them is that peppered moths do not normally rest on tree trunks in the wild.
R. C. Steward, Ecological Entomology 2 (1977): 231-243.
K. Mikkola, Biological Journal of the Linnean Society 21 (1984): 409-421.
C. A. Clarke, G. S. Mani & G. Wynne, Biological Journal of the Linnean Society 26 (1985): 189-199.
R. J. Howlett & M. E. N. Majerus, Biological Journal of the Linnean Society 30 (1987): 31-44.
T. G. Liebert & P. M. Brakefield, Biological Journal of the Linnean Society 13 (1987): 129-150.
M. E. N. Majerus, Melanism: Evolution in Action (Oxford: Oxford University Press, 1998).