Figure 2.

Resource allocation tradeoffs in developing O. taurus revealed by treatment with the JH analog methoprene. Solid bars, animals receiving methoprene in acetone; open bars, acetone-treated controls. (a) Topical application of JH during the second and third days of the final larval instar induced males to reduce allocation to horns. Males just above the critical size for horn production developed significantly shorter horns relative to their body size than control males (Mann–Whitney U test on relative horn length: U_7,7 = 9, Z = −1.981, P = 0.0476). Bars indicate the residual horn length (mean ± SEM), calculated as the difference between actual horn length and that expected for a male of the same body size. Expected values were generated from the best-fit curve relating horn size to body size in unmanipulated males (25). Females never develop horns and were not affected by the JH treatment (Mann–Whitney U test: U_13,13 = 61, Z = −1.205, P = 0.228). (b) JH-induced diminution of male horns was accompanied by a significant increase in the size of male compound eyes (Mann–Whitney U test: U_7,7 = 1, Z = −3.003, P = .0027). Bars indicate the mean ± SEM of residual eye size, calculated from the mean relationship between eye surface area and body size of unmanipulated males. Female eye size was unaffected by JH treatment (Mann–Whitney U test: U_13,13 = 75, Z = −0.487, P = 0.626), indicating that increased eye size in males resulted from reduced allocation to horns rather than as a direct response to JH.