25.6 THIS IS HOW WE DO IT: Can males increase sperm investment in response to the presence of another male?

As we’ve seen, in a population of animals, when a female typically mates with more than one male, sperm competition occurs. This can result in the evolution of increased sperm production and testis size—as illustrated dramatically in chimps.

But what impact does sperm competition have on the individual male? Some researchers explored this question, beginning with the hypothesis that “males respond to a risk of sperm competition with increased sperm investment.”

Like many hypotheses, this one may be a bit too broad. For example, it doesn’t identify any particular species, or what qualifies as a “risk of sperm competition.” In addition, for a hypothesis to be useful, it must be falsifiable. But it may be difficult to determine whether males are unable to respond to the risk of sperm competition with increased sperm investment, or whether males are able to respond, but simply do not perceive a risk of sperm competition.

These potential difficulties don’t make it impossible to address the hypothesis. Rather, they are issues to keep in mind when modifying the hypothesis and considering how to test it—that’s just part of scientific thinking. So how can we improve on the hypothesis?

Keep it simple. That is, try to control for as many potential sources of variation in the measure of interest—in this case, sperm investment—as possible. For example, males might differ from one another in their sperm investment even under similar conditions.

Controlling for differences among individuals (for a variety of traits) is a common goal when designing an experiment. An effective way to do this is to use a large number of individuals, and to test each individual in two different situations. In other words, it is probably not ideal to measure one group of males for sperm investment when there is a risk of sperm competition and to measure another group of males for sperm investment when there is no risk of sperm competition. It’s better to use a single group of males and to test each male under both conditions. That way, regardless of whether a male has relatively high or low sperm investment (compared with other males), we can determine whether his sperm investment is increased when there is a risk of sperm competition. And that’s the effect we’re interested in.

Some researchers took this approach as they set out to test this idea. First, they made a couple of additional refinements to the hypothesis, and thus to how they would test it. They decided to limit their study to one species—meadow voles (Microtus pennsylvanicus). They chose voles because, in the wild, litters are usually sired by more than one male, so males regularly experience sperm competition, and because the voles can be maintained under carefully controlled laboratory conditions. They also decided to simulate situations of sperm competition by exposing males to the odors of other male meadow voles. Can you suggest some other ways they might have simulated situations in which sperm competition occurred—and the pros and cons of those alternatives?

Their experimental approach was simple and straightforward.

The set-up The researchers paired a male vole with a sexually receptive female in two different contexts. In one, the voles were placed together in a cage that contained odors of another adult male vole (i.e., urine and feces from the other vole’s cage). This was called the “risk of sperm competition” (RSC) context. In the other—the control context—the male and female were placed in a cage that did not contain odors from another male vole.

Ten males were used. Five were randomly assigned to the control context first, and then, 30 days later, to the RSC context. The other five were assigned to the RSC context first and, 30 days later, to the control context. This set-up minimized the possibility of any potential impact of the order in which the male voles experienced each context

The measurements When the mating was completed, the researchers collected all of the semen from the female vole’s reproductive tract, and determined the male’s sperm investment—measured as the number of sperm present.

The results In all 10 voles, the sperm investment was greater in the RSC context. The average sperm investment in the control context was 98 (±18) million sperm, and in the RSC context was 169 (±18) million sperm—significantly higher. The researchers also noted that in the cage with odors from another male (RSC context), in every case the male vole first explored the cage, investigating the odor.

In a subsequent experiment, the researchers found there was no increase in sperm investment when the male voles were exposed to the odors of males from another species. Why does using the odor of another species serve as a type of control in this study?

The researchers concluded that male mammals may increase their sperm investment in response to an odor from another male of the same species. Could an individual’s ability to alter his sperm investment in response to chemical cues in his environment have adaptive value? How might you explore this question further?