Anisogamy is unrelated to the intensity of sexual selection

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.

Methods
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: 

name
description
notes

species

species latin name

sperm_head_length
sperm head length  (μm)

sperm_head_length_reference
reference of sperm head length  (μm)

sperm_total_length
sperm length  (μm)

sperm_total_length_reference
reference of sperm total length  (μm)

testis_mass
combined testis mass (g)

testis_mass_reference
reference of combined testis mass (g)

other_testis_size
other testis size data

other_testis_size_reference
reference of

egg_mass
fresh egg mass (g)
fresh egg mass or newborn mass (g)

egg_mass_reference
reference of fresh egg mass (g)

other_egg_size
other egg size data

other_egg_size_reference
reference of other egg size data

clutch_size
clutch size

clutch_size_reference
reference of clutch size

male_body_mass
male body mass (g)

male_body_mass_reference
reference of male body mass (g)

female_body_mass
female body mass (g)

female_body_mass_references
reference of female body mass (g)

female_body_length
female body length (mm)

female_body_length_reference
reference of female body length (mm)

male_body_length
male body length (mm)

male_body_length_reference
reference of male body length (mm)

other_body_size
other body size data

other_body_size_reference
reference of other body size data

sexual_size_dimorphism
log(male size) - log(female size)

sexual_size_dimorphism_how_was_measured
what kind of body size

dI_lnCVR
effect size of opportunity of selection

dI_lnCVR_VAR
measurement error variance for dIs_lnCVR

dIs_lnCVR
effect size of opportunity of sexual selection

dIs_lnCVR_VAR
measurement error variance for dIs_lnCVR

dbeta_g
effect size of Bateman gradient

dbeta_g_VAR
measurement error variance for dbeta_g

selection_indices_reference_Janicke_et_al
reference of selection (from Janicke)

sperm_mass
sperm mass (g)

sperm_mass_reference
reference of sperm mass (g)

sperm_volume
sperm head volume (μm3)

sperm_volume_reference
reference of sperm head volume (μm3)

parental_care
parental care bias: if care: 0 - just female, 1- shared but rather female care, 2- equal care, 3-shared but rather male care, 4- just male care, - no parental care

parental_care_reference
reference of parental care