A re-assessment of Rensch's rule in tuco-tucos (Rodentia: Ctenomyidae: Ctenomys) using a phylogenetic approach

Abstract

Keywords:Body mass Model II regression Phylogenetic reduced major axis Scaling Sexual size dimorphism a b s t r a c t Sexual size dimorphism (SSD) is affected by a large number of factors, mating system being one of the most relevant. Almost 70 species of subterranean rodents of the genus Ctenomys are considered highly polygynic, and polygyny jointly with absence of paternal care of the young, favours high SSD. In this respect, Rensch's rule predicts that SSD scales with body size so that when males are larger than females SSD tends to increase with body size. We studied SSD and Rensch's rule in 28 taxa of Ctenomys using a phylogenetic approach employing the method of phylogenetic reduced major axis (pRMA) to perform reduced major axis (RMA) model II regression in the form of log 10(male mass) on log 10(female mass). The RMA regression slope (ˇ) was statistically tested to accept or reject the null hypothesis that ˇpRMA = 1.0. A slope significantly >1.0 would signal concordance with Rensch's rule. Our results showed that despite a high degree of male-biased SSD as expected from polygynic species, Rensch's rule is not verified in this rodent group. The causes of the non-concordance with Rensch's rule as well as its taxonomic level of application are discussed in terms of current models of SSD.

Cats and dogs cross the line: domestic breeds follow Rensch’s rule, their wild relatives do not

“…Our results have shown that two emblematic domesticated species, the dog and the cat follow a sexual dimorphism trend in size that conforms to the empirical pattern known as Rensch's rule which is not present at the interspecific level in their respective families (Martínez et al, 2014;Bidau, Martínez, 2016). We have also demonstrated that in dogs, body size variation and SSD between breeds vary more than in the Canidae as a whole, while cats show a much lower size variation than that of the Felidae but SSD variation between breeds is comparable to that of the whole family.…”

“…SSD tends to have low or no phylogenetic signal (Martínez et al, 2014;Stevens, Platt, 2015;Bidau, Martínez, 2016;Martínez, Bidau, 2016), and as said above there is no convincing mechanism to explain Rensch's rule or its frequent inversion. The possible reason is that so many mechanisms are involved in the production of SSD that a single trend in scaling of SSD with body size does not exist.…”

“…between the sexes either male-biased as in most mammals and birds, or female-biased as in many invertebrates (Fairbairn, 2013). SSD is not a species trait comparable to body size which can be measured in kilograms or milligrams or in units of length: it is an unitless statistical parameter that varies according to the measurement used to estimate size (e. g. body mass and length in canids; Bidau, Martínez, 2016). Measurements of SSD using two different proxies for body size (e. g. mass and length) may or may not be not statistically correlated (e. g. correlation in felids, lack of correlation in canids; Martínez et al, 2014;Martínez, Bidau, 2016).…”

“…Evidence for Rensch's rule at the interspecific level in different animal taxa is contradictory, and the mechanisms involved when it does occur are less understood than those explaining SSD (see Discussion). However, a number of cases including mammalian species are known where the rule operates intraspecifically (e. g. Bidau, Martí, 2008a;Martínez, Bidau, 2016).…”

“…The study of body size and SSD evolution through domestication in dogs and cats is highly relevant for four important reasons: (i) our previous comparative studies have demonstrated that canids and felids do not follow Rensch's rule at the crossspecies level and that the two families greatly differ from each other in their levels of SSD in relation with their different reproductive strategies and social systems. Felids are polygynic and show high levels of male-male competition to access females, thus sexual selection favours large male size and increased SSD while canids are essentially monogamous, sexual male-male contests are rare and sexual selection does not favor exaggerated body size of males, hence lower SSD (Martínez et al, 2014;Bidau, Martínez, 2016); (ii) because in domestic breeds size and SSD are not only a consequence of natural or sexual selection in their wild ancestors but also of self-domestication, artificial and unconscious selection along their domestication pathways, changes in both traits could be expected since body size (thus, SSD) is extremely sensible to an enormous set of ecological and evolutionary factors. Changes in SSD could also be reflected in the presence or absence of Rensch's rule; (iii) it has been shown for dog and cat breeds that body size is negatively associated with lifehistory traits such as longevity (Galis et al, 2007;Greer et al, 2007;OʼNeill et al, 2015) and, in cats SSD is correlated with retroviral infections and age at maturity (Pontier et al, 1998); and (iv) both domestic species include many breeds with large variation in body size and the data are easily accessible.…”