Factors that Affect Reproductive Success in Male and Female Monarchs
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Dr. Karen Oberhauser Overview of Karen's Research | Meet the Scientist | Karen's Research Questions | Reproduction Home OverviewMale butterflies and moths transfer spermatophores during mating that contain more material than is necessary simply to fertilize eggs. In some cases, these spermatophores contain up to 10% of the male’s mass, which is clearly a significant investment! Much of my study of reproduction in monarch butterflies has focused on this investment: Why do males transfer more than just sperm to females during mating? How do the materials that males transfer affect females? Where do females obtain the resources that they put into eggs? One to these questions has been to think in terms of resource allocation. This is a fairly common approach; many biologists attempt to understand how and why organisms allocate resources the way they do. Biologists usually define the fitness of an individual organism in terms of its reproductive success, or the number of surviving offspring it produces. If we define resources as things that organisms obtain from their environments and need to survive and reproduce, the study of resource allocation is actually the study of investment; how do organisms invest the resources they obtain in ways that make them most likely to survive and produce many successful offspring? Figure 1 illustrates two main areas in which organisms invest resources—reproductive effort (offspring production) and somatic effort (growth and survival). We often assume that there are tradeoffs in these investments. If an organism is investing resources in one thing, like mating, those resources may not be available for something else, like survival. Thus, resources that a male monarch puts into a spermatophore are not available for keeping him alive, or for using in future matings.
Reproductive effort includes both effort invested in offspring themselves (parental effort), and effort invested in obtaining a mate (mating effort). In most animals, females invest much more in each offspring than males. Eggs are generally much larger than sperm, and female investment often includes more than just this large egg. In mammals, for example, females nourish the developing offspring for a long time during its development, both before and after it is born. Males generally invest more in obtaining mates. Figure 2 illustrates this difference in male and female investment in reproduction. Because females tend to invest more in each offspring, the total number of offspring that they can produce is not usually limited by the number of times they mate. Males, on the other hand, often maximize their reproductive success by increasing the number of times that they mate. Thus, there is often competition among males for access to females with which they can mate, and males that mate more often will have more offspring.
There is a great deal of variation in the amount of effort that males invest in a single mating. In some species they invest the minimum required to fertilize eggs, while in others, like monarch butterflies, they invest quite a bit more than this. From the male’s perspective, there is likely to be a tradeoff between investment in a current mating and future matings; resources that a male uses in mating with one female may not be available for future matings. Two circumstances might make it beneficial for males to invest more in a single mating than is necessary to fertilize eggs, and Karen Oberhauser's research suggests that both of these might play a role in monarchs:
Let’s think about females. We might expect that females should mate just enough times to fertilize all of their eggs, since reproductive success is not usually determined by the number of times a female mates. Also, extra matings may be costly because of things like predation or disease transfer. However, even though female monarchs receive enough sperm from one male to fertilize all of the eggs they could lay in their whole life, they usually mate more than once. In addition, they mate during the overwintering period when it might be weeks or even months before they will have eggs ready to fertilize. Oberhauser has studied two possible explanations for these "unnecessary matings," and again, both appear to be important in monarchs.
Karen began her research by focusing on the effects of male monarchs’ investment in mating on male reproductive success. She then looked at this investment from a broader perspective, studying how it affected females and the monarch mating system. This kind of pathway is common to many biologists; narrow questions lead to more and more questions, and eventually a broader understanding of an organism’s biology is achieved.
References: Chapman, R. 1982. The insects: Structure and function. 3rd Edition. Harvard University Press, Cambridge MA. Drummond, B. A. 1984. Multiple mating and sperm competition in the Lepidoptera, pp. 291-371. In R. L. Smith (ed.), Sperm Competition and the Evolution of Animal Mating Systems. Academic Press, Inc. Orlando. Forsberg J.and C. Wiklund. 1989. Mating in the afternoon: time-saving in courtship and mating by female of a polyandrous butterfly, Pieris napi L. Behav. Ecol. Sociobiol. 25:349-356. Haribal, M. and J.A. Renwick. 1995. Oviposition stimulants for the monarch butterfly: Flavonol glycosides from Asclepias curassavica. Phytochemisty 41:139-144. Herman, W. 1975. Endocrine regulation of posteclosion enlargement of the male and female reproductive glands in monarch butterflies. Gen. & Comp. Endocrin. 26:534-540. Lai-Fook, H. 1982. Structural comparison between eupyrene and apyrene spermiogenesis in Calpodes ethlius (Herperiidae, Lepidoptera. Can. J. Zool. 60:1216-1230. Silberglied, R.E., J.G. Shepherd and J.L. Dickinson. 1984. Eunuchs: the role of apyrene sperm in Lepidoptera? Amer. Nat. 123:255-265.
Meet the Scientist
Dr. Karen Oberhauser I have been studying monarch butterflies since I started graduate school in 1984. I first chose to work with monarchs because I was interested in species in which males invest materially in their offspring. Male monarchs, like other butterflies and moths, transfer nutrients to females during mating, and monarchs have the added advantage of being very easy to raise and study in captivity. However, I have since become interested in many other aspects of monarch biology, and, along with an exceptional group of graduate students at the University of Minnesota and colleagues throughout the United States and Mexico, now study everything from caterpillar distribution to how monarchs know when it’s time to migrate. For the last five years, I’ve become more and more involved in sharing my work with people outside of the "ivory tower" of colleges and universities. I work with teachers and pre-college students in Minnesota and throughout the United States using monarchs to teach about biology, conservation, and the process of science. I’ve also become more and more concerned with the impacts that humans have on monarchs and other organisms, and with the precarious balance between human needs and the needs of other species with which we share the planet. I think that learning as much as we can about our fellow inhabitants, and sharing the amazing things that we find out, will tip the balance in a direction that will be better for all of us! I have an undergraduate degree from Harvard University; taught high school biology, chemistry and earth science for three years before starting graduate school at the University of Minnesota; and am now an adjunct professor of Ecology, Evolution and Behavior at the University of Minnesota. I have two daughters, one husband, two cats, and add thousands of monarch caterpillars to my household each summer. In my spare time I read novels that have little to do with science, run, and watch my daughters play hockey.
Reproductive
Success in Male and Female Monarchs
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