DOI: 10.5061/dryad.x3ffbg7fn
title: Sexual size dimorphism and sexual selection in artiodactyls

description: Sexual size dimorphism is biased towards males in most mammalian species.
 The most common explanation is precopulatory intra-male sexual selection.
 Large males win fights and mate more frequently. In artiodactyls, previous
 tests of this hypothesis consisted on inter-specific correlations of
 sexual dimorphism with group size as a surrogate for the intensity of
 sexual selection (Is). However, group size is not a proper measure of
 sexual selection for several reasons, as is largely recognised in other
 mammalian taxa. I conducted an inter-specific test on the role of sexual
 selection in the evolution of sexual dimorphism using variance in genetic
 paternity as a proxy for the Is. I reviewed the literature and found 17
 studies that allowed estimating Is=V/(W2), where V and W are the variance
 and mean number of offspring per male, respectively. A phylogenetic
 generalised least squares analysis indicated that dimorphism (Wm/Wf)
 showed a significant positive regression with the intensity of sexual
 selection but not group size (multiple r2= 0.40; F3,17 = 12.78, p =
 0.002). This result suggest that sexual selection may have played a role
 in the evolution of sexual size dimorphism in Artiodactyla. An alternative
 hypothesis based on natural selection is discussed.
description: I conducted a literature search during July-September 2019 using Web of
 Knowledge to locate genetic paternity studies in artiodactyls species. I
 identified a total of 34 publications for 17 species. From these studies,
 I selected those that either provided an estimate of Is or the necessary
 data to estimate this measure. The final data set included Is estimates
 from 17 studies that represent 17 species (Table 1). When multiple
 estimates of variance were available from one study (e.g., estimates
 derived from different methods or with different levels of confidence), I
 used the most conservative estimate or the method defined as preferable by
 the authors. Both peccary species were assigned a value of Is = 0 because
 the studies did not provide sufficient information to estimate Is.
 Nevertheless, the studies found that: (i) reproduction is randomly or
 evenly distributed among males in Pecari tajacu (Cooper et al. 2010) and
 (ii) there are no significant differences in the number of young sired per
 year among Tayassu pecari males (Leite et al. 2018). I defined size
 dimorphism as the ratio of mean male adult body mass (kg) to mean female
 adult body mass (kg). Most body mass values were obtained from
 Perez-Barbería and Gordon (2000), who performed an extensive bibliographic
 search to derive a dataset of body mass (kg) that comprises as much
 information as possible about population and subspecies variability for
 the artiodactyl species used in the analyses. They did not provide
 information for the following species, so I searched for alternative
 sources: Lama pacos (Bonavia 2008), P. tajacu (Cooper et al. 2011
 mentioned the lack of dimorphism), Sus scrofa (Tack 2018) and T. pecari
 (Leite et al. 2018 mentioned the lack of dimorphism). Most data regarding
 group size were obtained from the PanTHERIA 1.0 database (Jones et al.
 2009). Data not available in this database were obtained from
 www.ultimateungulate.com for: Capreolus capreolus (Marchal et al. 1998),
 Rangifer tarandus (Alendal et al. 1979), Muntiacus crinifrons, Ovis
 Canadensis, Capra ibex and T. pecari.