I love aquatic insects! They do some amazing things and are incredibly interesting animals. That said, I feel like most people know very little about aquatic insects and the role they play in our world. Heck – some people don’t know that aquatic insects even exist! So, for my first From the Literature post, I thought I’d discuss a recent paper dealing with aquatic insects.
In biology, it is thought that males benefit from mating with as many females as possible. Because males usually do not care for their offspring and invest little in producing sperm, it is best for them to mate with as many females as they can, thereby contributing their genes to as many offspring as possible. There are, of course, exceptions to this general rule (the giant water bugs I study are an excellent example!), but it holds true for many species. Because females usually make a greater investment in their children – if nothing else, eggs are much more nutritionally expensive to produce than sperm – they often cannot mate as often as males. As they contribute their genes to fewer offspring, it is to their benefit to choose the best mates, the ones that will likely produce strong and robust children that have a high chance of surviving to adulthood. In essence, there is a battle of the sexes going on: males want to mate all the time with every female they can find while females want to mate with only the best males. It pays for a male to be aggressive and secure as many mates as possible while it pays for a female to be choosy. In essence, there is a trade-off between what males want and what females want: when one sex succeeds, the other suffers.
This idea sets the stage for a recent paper by Dr. Omar Eldakar (currently a postdoctoral fellow at the University of Arizona) and his colleagues published in the November 6, 2009 issue of Science. Eldakar and his team studied a type of aquatic insect called water striders (order: Hemiptera, family: Gerridae). Water striders (also known as Jesus bugs, water spiders, and pond skaters) are long, skinny insects that live and hunt on the surface of water. Water striders are typically found in groups of several individuals called aggregations in calm areas of streams and ponds.
The species Eldakar studied is Aquarius remigis and it is well-known for its battle of the sexes. Many male A. remigis individuals are highly aggressive when pursuing females, lunging at and jumping on their potential mates. While the females often resist mating with these hyper-aggressive males, the behavior has been known to improve mating success. Aggressive males are usually more successful at securing mates than non-aggressive males. This leads to a question: if aggressive males mate with more females than non-aggressive males, why aren’t ALL males aggressive? Eldakar and his colleagues wondered if the fact that most studies of sexual conflict in water striders do not allow individuals to migrate between aggregations might explain why hyper-aggressive males are reportedly so successful. If females are forced to remain in an area with hyper-aggressive males, the males might have a higher mating success than if the females could be choosier about who they mated with.
Eldakar and his colleagues set up an ingenious experiment to test this idea. First, they placed water striders in an artificial pond and observed male aggression, movement of females, and mating attempts/successes. Water striders were able to form their own aggregations and move freely between them. The researchers then divided the pond into several sections, placing males of various aggression levels with females in each section. The same observations were made, though this time individuals were not able to move between groups. Finally, the group compared their observations of the open treatments to the closed treatment to see how movement contributed to the mating success of aggressive and non-aggressive males. They discovered some interesting things.
As had been reported in other water strider studies, hyper-aggressive males had more successful mating attempts than the non-aggressive males in the closed system. If females were not able to move to another aggregation (i.e. their choices in mates were restricted), they mated more frequently with the aggressive males than the non-aggressive males. However, when the striders were able to move between groups, Eldakar observed that females moved to other aggregations when harassed by aggressive males. Aggressive males repelled the females they wanted to mate with! The females would, however, readily mate with the less aggressive males in their new areas. This meant that less aggressive males were able to secure many more mates in open systems than in closed systems. In other words, the females avoided the aggressive jerks and the nice guys were getting the girls!
I think this is a great paper. The experiment was very simple, but it ended up revealing a lot of information. Many scientists call this sort of experiment “elegant” and I think this research certainly qualifies as an elegant experiment. Eldakar was able to refute the findings of several previous papers with an experiment that should be very easy for others to duplicate, one of the conditions for good science. He also clearly reminded biologists of one of the perils of doing behavioral research in the lab: unnatural conditions sometimes lead to unnatural behaviors. As a behavioral researcher myself, I think this isn’t reinforced enough. By modifying what had been done in the past, by allowing aggregations of water striders to form naturally rather than forcing them into pre-determined groups, Eldakar learned that aggressive males are not always the most successful in nature. This corrected what we already knew and gave us some great information that will be useful for many future studies. Thanks, Dr. Eldakar!
Eldakar OT, Dlugos MJ, Pepper JW, & Wilson DS (2009). Population structure mediates sexual conflict in water striders. Science (New York, N.Y.), 326 (5954) PMID: 19892974
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