Mating Sex Ratio Research Articles

Evolution of male strategies with sex-ratio-dependent pay-offs: connecting pair bonds with grandmothering.

Men's provisioning of mates and offspring has been central to ideas about human evolution because paternal provisioning is absent in our closest evolutionary cousins, the great apes, and is widely assumed to result in pair bonding, which distinguishes us from them. Yet mathematical modelling has shown that paternal care does not readily spread in populations where competition for multiple mates is the common male strategy. Here we add to models that point to the mating sex ratio as an explanation for pairing as pay-offs to mate guarding rise with a male-biased sex ratio. This is of interest for human evolution because our grandmothering life history shifts the mating sex ratio from female- to male-biased. Using a difference equation model, we explore the relative pay-offs for three competing male strategies (dependant care, multiple mating, mate guarding) in response to changing adult sex ratios. When fertile females are abundant, multiple mating prevails. As they become scarce, mate guarding triumphs. The threshold for this shift depends on guarding efficiency. Combined with mating sex ratios of hunter-gatherer and chimpanzee populations, these results strengthen the hypothesis that the evolution of our grandmothering life history propelled the shift to pair bonding in the human lineage.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

Modelling the Evolution of Traits in a Two-Sex Population, with an Application to Grandmothering.

We present a mathematical simplification for the evolutionary dynamics of a heritable trait within a two-sex population. This trait is assumed to control the timing of sex-specific life-history events, such as the age of sexual maturity and end of female fertility, and each sex has a distinct fitness trade-off associated with the trait. We provide a formula for the fitness landscape of the population and show a natural extension of the result to an arbitrary number of heritable traits. Our method can be viewed as a dynamical systems generalisation of the Price equation to include two sexes, age structure and multiple traits. We use this formula to examine the effect of grandmothering, whereby post-fertile females subsidise their daughter's fertility by provisioning grandchildren. Grandmothering can drive a shift towards increasingly male-biased mating sex ratios due to a post-fertile life stage in females, while male fertility continues to older ages. Our fitness landscapes show a net increase in fitness for both males and females at longer lifespans, and as a result, we find that grandmothering alone provides an evolutionary trajectory to higher longevities.

Evolution of longevity, age at last birth and sexual conflict with grandmothering

We use a two-sex partial differential equation (PDE) model based on the Grandmother hypothesis. We build on an earlier model by Kim et al. (2014) by allowing for evolution in both longevity and age at last birth, and also assuming that post-fertile females support only their daughters’ fertility. Similarly to Kim et al. (2014), we find that only two locally stable equilibria exist: one corresponding to great ape-like longevities and the other corresponding to hunter–gatherer longevities. Our results show that grandmothering enables the transition between these two equilibria, without extending the end of fertility. Moreover, sensitivity analyses of the model show that male competition, arising from a skew in the mating sex ratio towards males, plays a significant role in determining whether the transition from great ape-like longevities to higher longevities is possible and the equilibrium value of the average adult lifespan. Whereas grandmothering effects have a significant impact on the equilibrium value of the average age at last birth and enable the transition to higher longevities, they have an insignificant impact on the equilibrium value of the average adult lifespan.

Grandmothering life histories and human pair bonding

The evolution of distinctively human life history and social organization is generally attributed to paternal provisioning based on pair bonds. Here we develop an alternative argument that connects the evolution of human pair bonds to the male-biased mating sex ratios that accompanied the evolution of human life history. We simulate an agent-based model of the grandmother hypothesis, compare simulated sex ratios to data on great apes and human hunter-gatherers, and note associations between a preponderance of males and mate guarding across taxa. Then we explore a recent model that highlights the importance of mating sex ratios for differences between birds and mammals and conclude that lessons for human evolution cannot ignore mammalian reproductive constraints. In contradiction to our claim that male-biased sex ratios are characteristically human, female-biased ratios are reported in some populations. We consider the likelihood that fertile men are undercounted and conclude that the mate-guarding hypothesis for human pair bonds gains strength from explicit links with our grandmothering life history.

Effects of mating sex ratios in Indian peafowl (Pavo cristatus) on production performance at Wildlife Research Institute, Faisalabad (Pakistan).

Summary Effects of mating sex ratios on production performance in Indian peafowl were investigated at Punjab Wildlife Research Institute, Gatwala Faisalabad. Research was conducted from May through August 2007 to explore the most productive mating sex ratio. The sex ratios have a significant effect on egg production. Maximum mean eggs (1.63) were produced by the sex ratio 1:2 while maximum egg fertility (79.63%) and hatchability (61.59%) was noted in the sex ratio 1:3. Sex ratios had a significant effect on egg weight. Sex ratios have shown no effect (except number of mountings) on courtship behavior of peafowl. Statistically less feed was consumed by sex ratio 1:3 and better feed conversion was shown by sex ratio 1:1.

Reproductive and behavioral aspects of red-winged tinamous (Rhynchotus rufescens) in groups with different sex ratios

The aim of this research study was to evaluate the reproductive performance of tinamous submitted to five different male:female ratios. The study was carried out with 72 birds in a randomized experimental design with 4 replications. Tinamous were housed in cages, using the ratios of one (1:1), two (2:1), three (3:1) and four (4:1) females per male, and also one male was housed with three females individually (3R:1), in a rotational system. Reproductive records of the breeding season from September 2004 to March 2005 were used. The reproductive traits studied were: number of eggs laid, fertility, and percentage of eggs damaged and cracked by pecking. Nonparametric analyses of these traits were performed using Kruskal-Wallis test. Two replications of treatments 1:1 and 4:1, and one of treatment 2:1 were video-taped for three days, 12 hours/day. The videotapes were sampled according to the scan method to fit an ethogram. Birds were also watched for one hour per day to study dominance and agonistic behavior. None of the reproductive traits was affected by mating sex ratio (p<0.05). Female dominance could be related to displacement behavior (r=1.00), and male sitting in immobility plus sitting in activity behaviors were related to lower number of damaged eggs (r=-0.90). Social dominance was indirectly determined by displacement behavior in the study situation. A large number of damaged eggs occurred in all treatments, thereby not allowing a clear conclusion on the best male:female ratio.

Influence of mating sex ratio in commercial pheasant flocks on bird health and the production, fertility, and hatchability of eggs

1. The effects of two mating ratios (8 : 1 and 12 : 1 females : males) on bird health, egg production and quality, fertility (employing macroscopic and microscopic techniques) and hatchability were examined for pheasants ( Phasianus colchinus ) on a commercial game farm, to provide a baseline data set for production of pheasants under specific conditions. 2. Mating ratio did not significantly affect mortality or pathology of breeding birds. 3. Egg production was significantly higher for the 8 : 1 mating ratio over the first 5 weeks of the trial but this difference disappeared during the second half of the trial. 4. Incidence of rejected eggs increased as the season progressed, mainly due to poor shell quality, but it was not significantly affected by mating ratio. 5. Fertility of eggs produced from birds under an 8 : 1 mating ratio was consistently and significantly 4% higher than fertility of eggs from the 12 : 1 mating ratio. The incidence of eggs showing no sign of mating (no sperm) was twice as high for the 12 : 1 pens (13.7% of the sample). There was good agreement between macroscopic and microscopic examination of fertility in eggs. Higher fertility in the 8 : 1 pens appeared to be related to a higher proportion of the females being recruited and mated. 6. Hatchability was significantly improved by adopting an 8 : 1 mating ratio, presumably due to the higher fertility of the eggs. Candling of the eggs improved hatchability but only for the 12 : 1 pens probably because more clear eggs were removed from the tray.