Research Project

How Often Do Males and Females Mate, and What Affects the Timing of Mating?

(see also Oberhauser 1988, 1989, 1992, 1995)

Note: The 1994 experiments reported here were carried out by Christine Jessup, who was an undergraduate at Wellesley College at the time, and is now a grad student at UC Santa Barbara.

Abstract

I conducted several experiments to determine the age at which monarchs first mate, lifetime mating frequencies, and the intervals between mating for each sex. Most males mated for the first time at five to seven days of age, while first matings for females were spread out over a longer time period, with most mating for the first time at ages one to four days. It is likely that first matings for males occur when males are physiologically ready to mate, but the timing of the first mating for females is caused by a combination of female physiology and the ability of males to force females to mate. The distribution of mating frequencies for males is skewed, with many males mating few times. Most females mate an intermediate number of times. This suggests that there is strong sexual selection (selection on traits that increase the ability to attain mates) on male monarchs.

Females that receive large first spermatophores wait longer to remate than females that receive small first spermatophores. This will benefit the males transferring large spermatophores, because eggs laid after a female remates are likely to be fertilized by the second male’s sperm. Despite this, males do not appear to delay mating until they are able to transfer a large spermatophore. It is likely that male monarchs have been selected to mate whenever they have the opportunity. This may result from low summer densities of monarchs, which make the cost of passing up an opportunity to mate very high, even if the mating may not result in a maximum number of offspring.

Introduction

The timing and frequency of mating in any species has many ecological and evolutionary implications, and I have studied these implications throughout the 14 years I’ve been working with monarchs. In this paper, I summarize this work. Aspects of monarch mating behavior, and their relevance to monarch ecology and evolution, that I’ve studied include:

The age at which females first mate in relation to egg production (see Does mating cause monarch eggs to mature?). If females first mate before most of their eggs are developed, there is greater potential for them to incorporate nutrients received in spermatophores (see Spermatophores) into eggs.
The age at which both sexes mate and butterfly movement. When individuals begin mating and laying eggs as soon as they reach the adult stage, there is less chance for different populations to mix together. This makes it more likely that there will be genetic differences between different populations. However, if individuals do not mate right away, and disperse from the location in which they were larvae, there is less chance for individual populations to become genetically distinct.
The effects of the size of spermatophores received by females on intervals between matings. A potential benefit to males of transferring large spermatophores is delaying the time at which females remate. This is advantageous to males, since once the female remates, his sperm will not fertilize as many of her eggs (see Whose sperm fertilize the female’s eggs if she mates more than once?) Intermating intervals by females will also determine whether nutrients that a male transfers in spermatophores are used to help make eggs that his sperm will fertilize, or if they will be used to make eggs that some other male’s sperm will fertilize.
The effects of male mating history on mating likelihood. The size of the spermatophore a male transfers is affected by both his age and the time that has elapsed since a previous mating (see What factors affect the size and composition of monarch spermatophores?). If it is beneficial to males to produce large spermatophores, it might be better for them to wait after one mating before they attempt to mate again.
The distribution of mating frequencies by different males in a population. In many animal species, some males mate many times, while others mate only a few times or even not at all. This usually means that some males will fertilize many more offspring than others. The traits that make males more successful in obtaining mates are caused by a category of natural selection called sexual selection; the higher the variation in male success, the stronger sexual selection is.

Here I report several experiments in which I addressed these issues using captive monarch butterflies, and discuss the implications of my findings.

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