It is argued that scaling relationships are best viewed as reaction norms, and that the evolution of exaggerated morphological traits results from genetic changes in the slope and/or shape of these scaling relationships.
The potential for male competition to drive rapid divergence in weapon morphology remains one of the most exciting and understudied topics in sexual selection research today.
Both overall body size and relative horn length significantly affected the outcome of fights over tunnel ownership, suggesting that alternative reproductive tactics may favor divergence in male horn morphology, with long horns favored in males large enough to guard tunnels, and hornlessness favored in smaller males that adopt the “sneaking” behavioral alternative.
Combined, the two alternative reproductive tactics used by male O. taurus appear to favour opposite horn phenotypes, which may explain the paucity of intermediate morphologies in natural populations of O. Taurus.
It is shown that costs of weapon production also may drive patterns of weapon evolution, a critical but overlooked role in sexual selection and has implications for understanding the evolution of animal morphology.
Results from experiments designed to assess the relative importance of genetic and environmental factors as determinants of male horn morphology support recent theories on sexual selection which predict that male ornaments will evolve to be reliable indicators of male quality.
This work uses partial sequences from four nuclear and three mitochondrial genes to develop a phylogenetic hypothesis for a worldwide sample of 48 species from the dung beetle genus Onthophagus and characterize the evolutionary radiation of beetle horns.
In the case of beetle horns, the tradeoff in size is manifest as a significant negative genetic correlation among the involved body parts and constitutes a developmental source of genetic constraint on the evolution of body form.
Recent advances in the fields of growth regulation and endocrinology are reviewed and used to construct a developmental model of static allometry expression in insects that serves as the foundation for a research program that will result in a deeper understanding of the relationship between growth and form.
A general mechanistic model for the evolution of exaggerated traits is presented, proposing that sensitivity to the insulin response pathway can explain variation among individuals and illustrating how enhanced sensitivity to insulin/IGF signaling in a growing ornament or weapon would cause heightened condition sensitivity and increased variability in expression among individuals.