Sexual selection and sexual conflict

According to much evolutionary thinking, males of promiscuous species are able to produce quantities of sperm that are virtually unlimited and should be selected to mate indiscriminately with all available females. However, sperm are generally delivered in batches (ejaculates or spermatophores) that may include many millions of gametes. Males are limited with respect to the number of ejaculates they can deliver and the time required to restore depleted reserves. Because ejaculates are a limited resource, prudence would be expected in the allocation of ejaculates to females. Among the forces acting to limit the number of females with which a promiscuous male mates are high pregnancy initiation requirements, sperm competition, female choice and control, and the costs and risks of searching. Optimal strategies for ejaculate allocation will vary with the operational sex ratio. Males can and do discriminate among females as potential mating partners. The problems of limited ejaculatory capacity and male choice...

https://www.jstor.org/stable/pdfplus/2461181.pdf

Ejaculate Cost and Male Choice

1 – Sperm Competition and the Evolution of Animal Mating Strategies

The male of Calopteryx maculata (Beauvois) (Odonata) uses its penis not only to transfer sperm to the female but also to remove sperm deposited in the female's sperm storage organs from previous matings. Apparently, no such sperm removal function has previously been attributed to the intromittent organ of any animal.

https://science.sciencemag.org/content/sci/203/4383/916.full.pdf

Dual Function of the Damselfly Penis: Sperm Removal and Transfer

Sperm competition was identified in 1970 as a pervasive selective force in post-copulatory sexual selection that occurs when the ejaculates of different males compete to fertilise a given set of ova. Since then, sperm competition has been much studied both empirically and theoretically. Because sperm competition often favours large ejaculates, an important challenge has been to understand the evolution of strategies through which males invest in sperm production and economise sperm allocation to maximise reproductive success under competitive conditions. Sperm competition mechanisms vary greatly, depending on many factors including the level of sperm competition, space constraints in the sperm competition arena, male mating roles, and female influences on sperm utilisation. Consequently, theoretical models of ejaculate economics are complex and varied, often with apparently conflicting predictions. The goal of this review is to synthesise the theoretical basis of ejaculate economics under sperm competition, aiming to provide empiricists with categorised model assumptions and predictions. We show that apparent contradictions between older and newer models can often be reconciled and there is considerable consensus in the predictions generated by different models. We also discuss qualitative empirical support for some of these predictions, and detail quantitative matches between predictions and observations that exist in the yellow dung fly. We argue that ejaculate economic theory represents a powerful heuristic to explain the diversity in ejaculate traits at multiple levels: across species, across males and within individual males. Future progress requires greater understanding of sperm competition mechanisms, quantification of trade-offs between ejaculate allocation and numbers of matings gained, further knowledge of mechanisms of female sperm selection and their associated costs, further investigation of non-sperm ejaculate effects, and theoretical integration of pre- and post-copulatory episodes of sexual selection.

Sperm competition and ejaculate economics.

1
In double mating experiments with Drosophila melanogaster in which one male had been irradiated, it was confirmed that sperm displacement is extensive, i.e. the second male to mate displaces most of the previously-stored sperm.

2
The predominance of the second ejaculate over the first increases with the interval between the two matings, from about P2= 0.83 (second mating on the first day after the first mating) to about P2= 0.99 (interval between mating = 14 days) where P2 is the proportion of offspring fathered by the second male.

3
A more accurate method for calculating P2 values is developed for experiments in which sperm are ‘labelled’ by irradiation treatment (equation 1).

4
Observations of the reducing egg production of the female throughout life were also obtained. A model is examined which incorporates both the sperm competition and egg production data to predict the reproductive value to a male of a mating with a given type of female, varying in age and mating status. The relative value (in terms of probable numbers of progeny gained) of a mating with a virgin or 4 day post-mating female is about twice that of a 14 day post-mating female, mainly because of the fecundity difference.

5
Some evolutionary aspects of sperm competition and multiple mating in insects are reviewed and discussed.

G. A. Parker

Papers

Sperm (ejaculate) competition in Drosophila melanogaster, and the reproductive value of females to males in relation to female age and mating status