Males and females often display different behaviours and, in the context of reproduction, these behaviours are labelled sex roles. The Darwin–Bateman paradigm argues that the root of these differences is anisogamy (i.e., differences in size and/or function of gametes between the sexes) that leads to biased sexual selection, and sex differences in parental care and body size. This evolutionary cascade, however, is contentious since some of the underpinning assumptions have been questioned. Here we investigate the relationships between anisogamy, sexual size dimorphism, sex difference in parental care and intensity of sexual selection using phylogenetic comparative analyses of 64 species from a wide range of animal taxa. The results question the first step of the Darwin–Bateman paradigm, as the extent of anisogamy does not appear to predict the intensity of sexual selection. The only significant predictor of sexual selection is the relative inputs of males and females into the care of offspring. We propose that ecological factors, life-history and demography have more substantial impacts on contemporary sex roles than the differences of gametic investments between the sexes.

We followed the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) [1] statement to search systematically the literature using the exact same method given by Janicke et al. (2016) [2], to augment Janicke et al’s dataset. We searched the ISI Web of Knowledge (Web of Science Core Collection, from 1945 to 2017) in 2017 3rd of March the following “topic” search terms and format: (“Bateman*” OR “opportunit*for selection” OR“opportunit*for sexual selection”OR“selection gradient*”), where asterisk (*) represents any group of characters, including no character. We obtained 754 candidate publications for further investigation. We compared these studies to Janicke’s database, and we checked all the items absent in their database. We were able to extend the dataset of Janicke et al. only with one new species (Lamprotornis superbus where data was extract from the supplementary material of Apakupakul and Rubenstein(2015)), otherwise, we used their data. Hermaphrodite species (Physa acuta, Biomphalaria glabrata) were excluded. Overall, our dataset contains data from 64 species. 

1. Moher D et al. 2009 Preferred reporting items for systematic reviews and meta-analyses: The PRISMA statement (Chinese edition). J. Chinese Integr. Med. 7, 889–896. (doi:10.3736/jcim20090918)

2.  Janicke T, Ha derer IK, Lajeunesse MJ, Anthes N. 2016 Darwinian sex roles confirmed across the animal kingdom. Sci. Adv. 2, e1500983–e1500983. (doi:10.1126/sciadv.1500983)

This dataset contains the following information: 

To represent the phylogenetic relationships between species, we used the most recent comprehensive phylogeny (timetree.org [3] that included all but seven species in our dataset. 

Species added to the phylogeny trees.
Added species    Source
Colpula lativentris    [4]
Gerris gilettei    [4]
Hippocampus subelongatus    [5]
Ischnura gemina    [6]
Labidomera clivicollis
Megabruchidius dorsalis    [7]
[7]
Strongylocentrotus purpuratus    [8]
 

4.    Li M, Tian Y, Zhao Y, Bu W. 2012 Higher level phylogeny and the first divergence time estimation of heteroptera (insecta: Hemiptera) based on multiple genes. PLoS One 7. (doi:10.1371/journal.pone.0032152)
5.    Teske PR, Beheregaray LB. 2009 Evolution of seahorses’ upright posture was linked to Oligocene expansion of seagrass habitats. Biol. Lett. 5, 521–3. (doi:10.1098/rsbl.2009.0152)
6.    Swaegers J, Janssens SB, Ferreira S, Watts PC, Mergeay J, McPeek MA, Stoks R. 2014 Ecological and evolutionary drivers of range size in Coenagrion damselflies. J. Evol. Biol. 27, 2386–2395. (doi:10.1111/jeb.12481)
7.    Kergoat GJ, Le Ru BP, Genson G, Cruaud C, Couloux A, Delobel A. 2011 Phylogenetics, species boundaries and timing of resource tracking in a highly specialized group of seed beetles (Coleoptera: Chrysomelidae: Bruchinae). Mol. Phylogenet. Evol. 59, 746–760. (doi:10.1016/j.ympev.2011.03.014)
8.    Lee YH. 2003 Molecular phylogenies and divergence times of sea urchin species of Strongylocentrotidae, Echinoida. Mol. Biol. Evol. 20, 1211–1221. (doi:10.1093/molbev/msg125)