Sexual dimorphism in toothed whales (Odontoceti) follows Rensch’s rule

Rensch’s rule entails that male-biased sexual size dimorphism scales allometrically with body size and is assumed to generally apply to polygynous taxa. However, so far only few mammalian groups have been shown to conform to it. Toothed whales (Odontoceti) not only span a substantial range of body sizes, but are commonly presumed to be predominantly polygynous, thus representing a promising candidate group to test for Rensch’s rule. Here we compiled a dataset of sex-specific body lengths in 57 species of toothed whales and demonstrate that sexual size dimorphism in this group does indeed follow Rensch’s rule. When focusing on selected subgroups of toothed whales, conformity to the rule was prominent among lineages of the speciose superfamily Delphinoidea, while it was not found in the beaked whales of the family Ziphiidae. These results support the assumption that polygynous and polygynandrous mating systems and marked precopulatory intrasexual competition between males are common among toothed whales. However, female-biased sexual size dimorphism as well as monomorphism occur at notable frequencies as well, suggesting that reproductive strategies are nevertheless variable among these marine mammals.

Intrasexually selected weapons

We propose a practical concept that distinguishes the particular kind of weaponry that has evolved to be used in combat between individuals of the same species and sex, which we term intrasexually selected weapons (ISWs). We present a treatise of ISWs in nature, aiming to understand their distinction and evolution from other secondary sex traits, including from 'sexually selected weapons', and from sexually dimorphic and monomorphic weaponry. We focus on the subset of secondary sex traits that are the result of same-sex combat, defined here as ISWs, provide not previously reported evolutionary patterns, and offer hypotheses to answer questions such as: why have only some species evolved weapons to fight for the opposite sex or breeding resources? We examined traits that seem to have evolved as ISWs in the entire animal phylogeny, restricting the classification of ISW to traits that are only present or enlarged in adults of one of the sexes, and are used as weapons during intrasexual fights. Because of the absence of behavioural data and, in many cases, lack of sexually discriminated series from juveniles to adults, we exclude the fossil record from this review. We merge morphological, ontogenetic, and behavioural information, and for the first time thoroughly review the tree of life to identify separate evolution of ISWs. We found that ISWs are only found in bilateral animals, appearing independently in nematodes, various groups of arthropods, and vertebrates. Our review sets a reference point to explore other taxa that we identify with potential ISWs for which behavioural or morphological studies are warranted. We establish that most ISWs come in pairs, are located in or near the head, are endo- or exoskeletal modifications, are overdeveloped structures compared with those found in females, are modified feeding structures and/or locomotor appendages, are most common in terrestrial taxa, are frequently used to guard females, territories, or both, and are also used in signalling displays to deter rivals and/or attract females. We also found that most taxa lack ISWs, that females of only a few species possess better-developed weapons than males, that the cases of independent evolution of ISWs are not evenly distributed across the phylogeny, and that animals possessing the most developed ISWs have non-hunting habits (e.g. herbivores) or are faunivores that prey on very small prey relative to their body size (e.g. insectivores). Bringing together perspectives from studies on a variety of taxa, we conceptualize that there are five ways in which a sexually dimorphic trait, apart from the primary sex traits, can be fixed: sexual selection, fecundity selection, parental role division, differential niche occupation between the sexes, and interference competition. We discuss these trends and the factors involved in the evolution of intrasexually selected weaponry in nature.