Adult sex ratio influences mate choice in Darwin’s finches

Abstract

The adult sex ratio (ASR) is an important property of populations. Comparative phylogenetic analyses have shown that unequal sex ratios are associated with the frequency of changing mates, extrapair mating (EPM), mating system and parental care, sex-specific survival, and population dynamics. Comparative demographic analyses are needed to validate the inferences, and to identify the causes and consequences of sex ratio inequalities in changing environments. We tested expected consequences of biased sex ratios in two species of Darwin’s finches in the Galápagos, where annual variation in rainfall, food supply, and survival is pronounced. Environmental perturbations cause sex ratios to become strongly male-biased, and when this happens, females have increased opportunities to choose high-quality males. The choice of a mate is influenced by early experience of parental morphology (sexual imprinting), and since morphological traits are highly heritable, mate choice is expressed as a positive correlation between mates. The expected assortative mating was demonstrated when the Geospiza scandens population was strongly male-biased, and not present in the contemporary Geospiza fortis population with an equal sex ratio. Initial effects of parental imprinting were subsequently overridden by other factors when females changed mates, some repeatedly. Females of both species were more frequently polyandrous in male-biased populations, and fledged more offspring by changing mates. The ASR ratio indirectly affected the frequency of EPM (and hybridization), but this did not lead to social mate choice. The study provides a strong demonstration of how mating patterns change when environmental fluctuations lead to altered sex ratios through differential mortality.

Triad hybridization via a conduit species

“…The lineage was initiated by hybridization of an immigrant Geospiza conirostris with a resident G. fortis, resulting in the formation of a reproductively isolated population that displayed allometric transgression, that is, a deviation from the two species in the relationship between beak size and body size (27,78). Transgressive morphology is implicated in reproductive isolation because mate choice is based in part on beak and body size (61,62,72) and the relationship between the two (79).…”

“…Known hybrids were observed to breed from 1983 onward when an El Niño event caused a major change to the environment (56,60). Hybrids were found to backcross according to the same rules that govern intraspecific mate choice: finches choose mates on the basis of imprinting on paternal song and parental morphology, and occasionally hybridize when they learn the song of another species (61,62). Frequencies of alleles at 16 unlinked polymorphic loci are used here to assign 3,165 individuals probabilistically (≥0.9) to the three species (63), and, if the 0.9 criterion is not met, they are assigned to hybrids (admixtures).…”

Evolution of female coloration: What have we learned from birds in general and blue tits in particular

“…This finding was found in the blue tit (García-Navas et al, 2013;Schlicht & Kempenaers, 2013), the dark-eyed junco , Junco hyemalis (Gerlach, McGlothlin, Parker, & Ketterson, 2012), the hihi, Notiomystis cincta (Walker, Ewen, Brekke, & Kilner, 2014)), the mountain bluebird, Sialia currucoides (Balenger, Scott Johnson, Mays Jr, & Masters, 2009), and the white-crowned sparrow, Zonotrichia leucophrys (Poesel, Gibbs, & Nelson, 2011). Interestingly in addition, in two species of Darwin finches (Geospiza fortis & G. scandens) for which mating patterns change when environmental fluctuations alter sex ratios, females of both species were more frequently polyandrous in male-biased populations, and fledged more offspring by changing mates (Grant & Grant, 2019). Lastly, in cooperative breeding species, a positive Bateman gradient was found in females but not in males for the superb starling, Lamprotornis superbus (Apakupakul & Rubenstein, 2015).…”

“…Shorter-term experiments would also be of interest. For example, ecological factors such as density (Aronsen, Berglund, Mobley, Ratikainen, & Rosenqvist, 2013), food availability (Janicke, David, & Chapuis, 2015) or sex ratio (Grant & Grant, 2019) could be modified to test how environmental conditions affect the opportunity for sexual and social selection in females.…”

Making the best of a bad job? Chick mortality and flexible female brood care in Snowy Plovers

“…Re-mating opportunities determine who gains more from deserting the family and who gains more from care (Balshine-Earn & Earn, 1998;Eldegard & Sonerud, 2009;Fujioka, 1989;Keenleyside, 1985;Pilastro, Biddau, Marin, & Mingozzi, 2001;Roulin, 2002;Thomson et al, 2014). Recent studies have shown that reproductive behaviour is often tied to sex ratios of the adult population (Grant & Grant, 2019;Kokko & Jennions, 2008;Liker, Freckleton, & Székely, 2013) with biased sex ratios altering potential for sequential polygamy in each sex. In that case, the rarer sex has a higher opportunity for polygamy and is more likely to desert the brood (Eberhart-Phillips et al, 2018).…”