Factors that Affect Reproductive Success in Male and Female Monarchs
What Factors Affect the Size and Composition of Monarch Spermatophores?(See also Oberhauser 1988 and 1992) Abstract | Background | Methods | Results | Discussion | References | Karen's Research Questions During mating, male monarch butterflies transfer sperm and other substances packaged in a gelatinous body called a spermatophore. This spermatophore represents a significant material investment by males. I studied how male size, age and mating history affect the size and composition of monarch spermatophores. Males transferred about 5-10% of their body mass during mating. Spermatophores increased in size with male age, but just after a mating, males produced very small spermatophores. Spermatophores are composed mainly of water and protein, with protein comprising about 21% and 65% of their wet and dry mass, respectively. Male monarchs, like all Lepidoptera, produce spermatophores during mating. These spermatophores consist of a sperm sac embedded into a gelatinous body formed from male accessory gland sections. Research by Boggs and Gilbert (1979) demonstrated that lepidopteran spermatophores contain nutrients used by females in egg production, and my early research focused on quantifying the investment that male monarchs made in spermatophores. Spermatophore content is also important. Previous research (Marshall 1982) has shown that lepidopteran spermatophores are made up of water, protein, and small amounts of other molecules such as fats and sterols (cholesterol is an example of a kind of sterol, although it has not been found in monarch spermatophores). The component that is most likely to affect both male and female fitness is protein, since adult monarchs obtain very little protein in their diet. The only source of protein for males is the food they eat as larvae, but females can use the protein that comes in spermatophores. This may increase the number of eggs they can lay. MethodsUnmated females were put into outdoor screen cages (2m x 2m x 2m) and allowed to mate with males of different ages, sizes, and mating histories. Soon after pairs separated (just after daybreak), I removed some mated females from the mating cage and dissected them in insect saline, a solution similar in composition to the intracellular fluids in insects. I removed spermatophores from the bursa copulatrix, blotted them on tissue paper, and weighed them to the nearest 0.01 mg on an analytical balance. I then froze the spermatophores for later analysis of water and nitrogen content. I obtained water content by placing spermatophores in a 65°C drying oven overnight, removing them to a desiccator, and allowing them to come to room temperature. Dried spermatophores were then weighed to the nearest 0.01 mg, and their dry mass compared to their wet mass to measure water content. I used a technique called a micro-Kjeldahl analysis to determine nitrogen content. ResultsSpermatophore Mass: Age was the most important predictor of spermatophore mass from males that had never mated before (figure 1). Older males transferred larger spermatophores, and spermatophore size continued to increase as males get older. The time that had elapsed since their last mating was most important for males that had mated previously (figure 1). Males that had just mated transferred very small spermatophores, and size increased as males waited longer between matings. Other factors that had significant effects on spermatophore size were mass at eclosion for unmated males, and both mass at eclosion and the number of previous matings for males that had already mated. Table 1 shows regression equations that illustrate the effects of all of these factors; larger males tended to transfer larger spermatophores, and males that had mated more times in the past transferred smaller spermatophores.
Figure 1. Spermatophore masses plotted against their most important predictors. For unmated males, the most important predictor of spermatophore mass was the age of the male. For previously-mated males, the most important predictor was the time that had elapsed since the last mating of the male.
Table 1: Predictors of Spermatophore Mass
Spermatophore Water and Nitrogen Content: I measured the proportion of water and nitrogen in spermatophores transferred by all five male groups. By looking up the approximate proportion of nitrogen in the common amino acids in spermatophores, I could estimate how much protein is in spermatophores. Figure 2 and Table 2 show the average proportions of water, nitrogen and protein. Spermatophores are composed mainly of water (like most components of living organisms), and most of their dry mass is protein. Spermatophores from males that had mated one day previously had a higher proportion of water and a lower proportion of nitrogen. Table 2: Spermatophore Components
*When biologists measure the components of substances, they often consider both wet weight and dry weight. This is because most living tissue is composed mostly of water. So we weigh a substance with the water in it, then dry it in a drying oven to remove all of the water and reweigh it.
Figure 2. Proportions of water, protein and other components in spermatophores. These proportions vary slightly for males that mated one day previously (see text). DiscussionTwo conclusions can be drawn from the data on spermatophore mass. First, male monarchs appear to transfer all or almost all of the accessory gland material that is available when they mate. Spermatophore mass from virgin males increased with male age, suggesting that as more accessory gland material was produced, it was transferred to females and not saved for future matings. In addition, the small size of spermatophores from males that had just mated suggests that there was very little accessory gland material available one day after mating. Second, male monarchs make a nontrivial material investment in mating, producing spermatophores than can represent up to 10% of their body mass. It took four days after one mating until they were able to produce spermatophores equal in mass to those produced by unmated males. A similar pattern has been reported in other Lepidoptera (Rutowski 1984, Svärd 1985, Rutowski and Gilchrist 1986, Svärd and Wiklund 1986). Spermatophores transferred one day after a previous mating contain more water and less nitrogen than other spermatophores. This suggests that water and non-protein components are more readily available than protein for use in making spermatophores right after a male has depleted his accessory glands by mating. These results clarify possible benefits both sexes gain from large spermatophores, and much of my subsequent research has been focused on quantifying these benefits. I have shown that females that receive larger spermatophores wait longer to remate (see Mating Frequencies in Male and Female Monarchs). Thus males that transfer larger spermatophores will gain more offspring from a mating because they will fertilize the female’s eggs over a longer time period. Large spermatophores will provide the female with more nutrients, especially protein, that can be used to produce more eggs (see What factors affect the number of eggs that female monarchs lay?). return to Karen's Research Questions References
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