Trade-off between horns and other functional traits in two Onthophagus species (Scarabaeidae, Coleoptera)

Abstract

Beetle horns are extraordinarily diversified secondary sexual structures used for mate choice and male-male combat. Due to an interaction of nutritional, hormonal and genetic factors, their polyphenic development is metabolically expensive and occurs in the virtually closed system of the pre-pupal stage, after the developing larva has stopped feeding. Previous studies showed the occurrence of resource competition resulting in a trade-off between horns and other morphological structures. These studies also revealed functional associations between autoecology and horns, as a function of their physical location (i.e. head versus pronotum), and suggested that constraints imposed by trade-offs on adult morphology may have profound evolutionary consequences, such as ecological and reproductive isolation. In this study, we compared trade-off patterns between horns and other functional traits (eyes, antennae, legs, head, epipharynges and genitalia) in two congeneric species bearing horns located in the same anatomical area, but with different morphologies. Specifically, we considered Onthophagus taurus, characterised by a pair of long, lunated cephalic horns, and Onthophagus fracticornis, expressing a single cephalic horn. We demonstrated that, even when horns are located in the same physical position on the insect's body, differences in horn morphology can bring about differences in how functional traits respond to horn investment. These differences are interpretable in the light of the hierarchy of functions carried out by these structures and their component parts in each species.

The First Morphometric Study of the Horn Morphological Pattern in a Geotrupidae: The Case of the Dor BeetleCeratophyus rossiiJekel, 1865

“…Experimental studies on Onthophagus species have also suggested that, if there is an overlap between growth periods, even distant structures, such as other horns and copulatory organs, can engage in a developmental tradeoff (Moczek and Nijhout, 2004;Parzer et al, 2008;Pizzo et al, 2012). In Ceratophyus rossii no evidence supporting the occurrence of a tradeoff between head and pronotal horn development was found: there was instead a weakly significant, positive correlation between head and pronotal horn length residuals.…”

“…in Onthophagus (Emlen, 2001;Emlen et al, 2005;Macagno et al, 2009;Pizzo et al, 2012); Xylotrupes (Rowland, 2003) and Calchosoma (Kawano, 1995)): when these patterns are accompanied by non-linear allometry and by a clear bimodality in horn length frequency distributions, they can reveal a threshold-dependent mechanism of horn expression, with a developmental switch point separating major and minor morphs within males. However, in Ceratophyus rossi males, even if larger males seem to be not simply expanded versions of the smallest ones, no such bimodality in horn length or in pronotum width frequency distributions was found.…”

“…We calculated horn length residuals (the difference between observed and expected values) for each horn type, and we then tested for possible correlations between them by using a standard linear regression model (Pizzo et al, 2012). Resource allocation trade-offs during development should be evidenced by a negative correlation between horn length residuals (Emlen, 2001).…”

“…On the dorsal view, we measured pronotum width (PW) which was used as a proxy for the overall body size (Emlen, 1994;Moczek, 1998;Parzer and Moczek, 2008;Pizzo et al, 2012).…”

“…This can result in morphological trade-offs in which the development of the exaggerated trait (the horn) can result in other structures (e.g. other horns, genitalia, other functional traits, such as eyes or wings) growing proportionally smaller (Emlen, 2001;Kawano, 1995Kawano, , 1997Klingenberg and Nijhout, 1998;Moczek, 2009;Moczek and Nijhout, 2004;Nijhout and Emlen, 1998;Nijhout and Wheeler, 1996;Okada and Miyatake, 2009;Painting and Holwell, 2013;Parzer and Moczek, 2008;Pizzo et al, 2012;Simmons and Emlen, 2006).…”