Factors that Affect Reproductive Success in Male and Female Monarchs
Dr. Karen Oberhauser
Department of Ecology, Evolution and Behavior
University of Minnesota
St. Paul MN
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Overview
Male 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:
- Males may not have a good chance of obtaining any other matings, so they aren’t
really losing anything by investing more than they have to in one mating, or
- Extra investment might offset any tradeoffs if it has a good chance of increasing
the number or quality of offspring from the current mating.
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.
- Males may force females to mate even when this is not in the female’s best
interests, or
- Females may gain additional nutrients by mating more than once.
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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