Cope’s rule and the universal scaling law of ornament complexity

Pasquale Raia*, Federico Passaro, Francesco Carotenuto, Leonardo Maiorino, Paolo Piras, Luciano Teresi, Shai Meiri, Yuval Itescu, Maria Novosolov, Mattia Antonio Baiano, Ricard Martínez, Mikael Fortelius

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


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 from0.25.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.

Original languageEnglish
Pages (from-to)165-175
Number of pages11
JournalAmerican Naturalist
Issue number2
StatePublished - 28 Jul 2015


  • Allometry
  • Biological scaling
  • Heterochrony
  • Ornamental structures
  • Sexual selection


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