Sexual selection intensity promotes the evolution of delayed maturation by sex in birds

A new study pub­lished in Evol­u­tion Let­ters reveals that sex dif­fer­ences in mat­ur­a­tion times arise in bird spe­cies where one sex is rarer and com­petes more strongly for mates than the oth­er, sup­port­ing the pre­dic­tion that sexu­al selec­tion selects for delayed mat­ur­a­tion. Here, lead author Dr. Ser­gio Ancona explains his findings.

The age at sexu­al mat­ur­a­tion, that is the age when organ­isms are physiolo­gic­ally cap­able of breed­ing, is enorm­ously vari­able across spe­cies and has per­vas­ive impacts on fecund­ity, breed­ing beha­viour and pop­u­la­tion num­bers. Sex dif­fer­ences in age-to-mat­ur­a­tion are both com­mon and strik­ing in nature, and although they have enchanted nature lov­ers for years, the causes of these dif­fer­ences are not fully under­stood. In my research group, we are fas­cin­ated by sex dif­fer­ences in gen­er­al, and we find sex-dif­fer­ences in age-to-mat­ur­a­tion par­tic­u­larly intriguing, as they may have import­ant implic­a­tions for sexu­al con­flict and life his­tory evol­u­tion. Sem­in­al ideas by archi­tects of mod­ern evol­u­tion­ary the­ory, not­ably R. Fish­er (1930) and D. Lack (1968), are par­tic­u­larly inspir­ing for us. They pro­posed that intense sexu­al com­pet­i­tion among males favours the evol­u­tion of delayed male mat­ur­a­tion, since pro­longed mat­ur­a­tion may allow males to fully devel­op their cap­ab­il­it­ies for fight­ing, court­ing and breed­ing suc­cess­fully. This allur­ing idea is often used to explain delayed male mat­ur­a­tion rel­at­ive to females in poly­gyn­ous spe­cies (where one male mates with mul­tiple females and each female only mates with a single male), but it has not been tested across a wide range of taxa. The interest in test­ing this hypo­thes­is motiv­ated our research. We, how­ever, went a little bey­ond the con­jec­tures of Fish­er and Lack and incor­por­ated a new ele­ment in our approach: we explored the impact of the social envir­on­ment, as indic­ated by the adult sex ratio (usu­ally expressed as the pro­por­tion of males in the adult pop­u­la­tion), on the evol­u­tion of sex-dif­fer­ences in age-to maturation.

Using advanced phylo­gen­et­ic com­par­at­ive ana­lyses and a com­pre­hens­ive data­set that includes pub­lished inform­a­tion on age of sexu­al mat­ur­a­tion, body size and sexu­al com­pet­i­tion for males and females, as well as data on adult sex ratio for wild bird pop­u­la­tions of 201 bird spe­cies from 59 fam­il­ies, we show that the sex that is lar­ger, more promis­cu­ous and rarer matures later than the sex that is smal­ler, less promis­cu­ous and more abund­ant. Fur­ther explor­a­tion of likely caus­al con­nec­tions between sex dif­fer­ences in age-to-mat­ur­a­tion, two indic­at­ors of the strength of sexu­al selec­tion (the extent of sexu­al size dimorph­ism estim­ated as sex dif­fer­ences in adult body mass and sex dif­fer­ences in the level of promis­cu­ity) and the adult sex ratio sug­gests that female-skewed adult sex ratios drive the evol­u­tion of male-skewed sexu­al size dimorph­ism and male poly­gamy, which in turn pro­mote the evol­u­tion of delayed mat­ur­a­tion in males rel­at­ive to females. Con­versely, male-skewed adult sex ratios pro­mote the evol­u­tion of female-skewed sexu­al size dimorph­ism and female poly­gamy, and these con­di­tions favour evol­u­tion of delayed mat­ur­a­tion in females rel­at­ive to males. Thus, we con­clude that the adult sex ratio, a proxy of the social envir­on­ment, drives sexu­al com­pet­i­tion, which in turn influ­ences mat­ur­a­tion. These excit­ing res­ults high­light the sig­ni­fic­ance of both sexu­al selec­tion and social envir­on­ment for sex dif­fer­ences in age-to-mat­ur­a­tion, and con­trib­ute more broadly to our under­stand­ing of the evol­u­tion­ary forces that gen­er­ate sexu­al dimorphism.

Dr Ser­gio Ancona is a Research Asso­ci­ate at the Insti­tuto de Eco­lo­gía, Uni­ver­sid­ad Nacion­al Autónoma de México (UNAM). The ori­gin­al art­icle is freely avail­able to read and down­load from Evol­u­tion Letters.