to the sudden loss of horns from a lineage. Subsequent breakdowns in these suppressive mechanisms could just as easily lead to sudden regains of horns. There is no question that the history of beetle horns is a story of repeated gains, losses, and re-gains of these weapons. We suggest that comparative studies of horn evolution and developmental studies of horn growth both attest to the relative ease with which growth of these structures can be turned on or off.
Finally, it does not appear to be difficult to change horn morphology. All three of the mechanisms now thought to be involved with horn development are likely candidates for genetic changes in horn form. We now suspect that subtle genetic changes in just a few elements within these mechanisms might be sufficient to generate all four of the principal trajectories of horn evolution: changes in horn location, shape, allometry and dimorphism.
One hundred and thirty five years ago, Darwin noted that sexual selection appeared to have acted “especially effectively” in scarab beetles (Darwin, 1871, p. 371), and 55 years ago, Gilbert Arrow, then curator of the British Museum, noted that these beetles appeared to have a “special tendency toward the acquisition of horns” (Arrow, 1951, p. 94). Today, we are finally able to elucidate the mechanisms underlying these observations. We now understand a lot about how beetles make a complex structure like a horn, and we are beginning to visualize how these horns might change in form. In essence, we are starting to elucidate what that “special tendency” of the scarabs was, and these insights from development are transforming how we think about the patterns of horn evolution.
We thank Kerry Bright and two anonymous reviewers for providing helpful comments on the manuscript and Mary Liz Jameson, Brett Ratcliffe, and Andrew Smith for help with scarab history and diversity. This work was funded by National Science Foundation CAREER Award no. IBN-0092873 (to D.J.E.).