Horn allometry and shape pattern of the tuscan stenoendemic dor beetle Ceratophyus rossii (Coleoptera, Geotrupidae)

Several thousand species of beetles develop horns or horn-like structures; the sizes of these horns relative to the sizes of the beetles bearing them is often disproportionated, and the diversity of their forms surprisingly. Horns appear to have arisen repeatedly within the scarab beetles but the family most predominated by species with exaggerated horns is that of Scarabaeidae (dung beetle); whitin Geotrupidae (dor beetle), horned species, even though represented, are less frequent. If the horns of Scarabaeidae are largely studied and the species of the genus Onthophagus used as model systems for evo-devo studies, reserchers deserved no special attention for geotrupids’ horns, whose allometric and shape pattern is largely unknown. Ceratophyus rossii Jekell, 1865, is a geotrupid beetle species stenoendemic to the Tuscany, considered at risk of extinction (and for that reason protected by regional laws) for its extremely reduced and fragmented distribution area, and for the progressive suitable habitat lost. It is a rare member of the winter beetle fauna of the protected Mediterranean maquis of San Rossore and the Site of Community Importance (SCI) of the Oasis of Burano. The Cerathophyus horns occur at the same body regions as the horns of more ‘‘typical’’ horned scarabs: the anterior surface of the pronotum, which is a body region that was already universally enlarged in scarabs and thought to be an adaptation to a burrowing lifestyle, and the dorsal surface of the head. We carachterised for the first time the cephalic and pronotal horn allometries of this geotrupid, distinguishing between linear and non linear relationships, we defined horns shape variation along these allometries and we looked for resource allocation and developmental trade-off between the two simultaneous horns. We finally compared this pattern with that of the other better studied horned beetles.