title: Cope's Rule and the Universal Scaling Law of Ornament Complexity.
abstract: Luxuriant, bushy antlers, bizarre crests, and huge, twisting horns and tusks are conventionally understood as products of sexual selection. This view stems from both direct observation and from the empirical finding that the size of these structures grows faster than body size (i.e., ornament size shows positive allometry). We contend that the familiar evolutionary increase in the complexity of ornaments over time in many animal clades is decoupled from ornament size evolution. Increased body size comes with extended growth. Since growth scales to the quarter power of body size, we predicted that ornament complexity should scale according to the quarter power law as well, irrespective of the role of sexual selection in the evolution and function of the ornament. To test this hypothesis, we selected three clades (ammonites, deer, and ceratopsian dinosaurs) whose species bore ornaments that differ in terms of the importance of sexual selection to their evolution. We found that the exponent of the regression of ornament complexity to body size is the same for the three groups and is statistically indistinguishable from 0.25. We suggest that the evolution of ornament complexity is a by-product of Cope's rule. We argue that although sexual selection may control size in most ornaments, it does not influence their shape.
authors.affiliation: Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse (DISTAR), Università degli Studi di Napoli Federico II, Largo San Marcellino 10, 80138 Naples, Italy.
authors.forename: Pasquale
authors.initials: P
authors.lastname: Raia
authors.forename: Federico
authors.initials: F
authors.lastname: Passaro
authors.forename: Francesco
authors.initials: F
authors.lastname: Carotenuto
authors.forename: Leonardo
authors.initials: L
authors.lastname: Maiorino
authors.forename: Paolo
authors.initials: P
authors.lastname: Piras
authors.forename: Luciano
authors.initials: L
authors.lastname: Teresi
authors.forename: Shai
authors.initials: S
authors.lastname: Meiri
authors.forename: Yuval
authors.initials: Y
authors.lastname: Itescu
authors.forename: Maria
authors.initials: M
authors.lastname: Novosolov
authors.forename: Mattia Antonio
authors.initials: MA
authors.lastname: Baiano
authors.forename: Ricard
authors.initials: R
authors.lastname: Martínez
authors.forename: Mikael
authors.initials: M
authors.lastname: Fortelius
mesh_terms: D000818:Animals; D005075:Biological Evolution; D049628:Body Size; D049830:Cephalopoda / Q000033:anatomy & histology / Q000254:growth & development; D003670:Deer / Q000033:anatomy & histology / Q000254:growth & development; D025061:Dinosaurs / Q000033:anatomy & histology / Q000254:growth & development; D005580:Fossils; D017709:Fractals; D010802:Phylogeny; D012641:Selection, Genetic; D012727:Sex Characteristics