Critters often utilize conspicuous features or behaviors to seek out and evaluate mates. Think of the male Indian peacock's brightly-colored blue-green tail, which appears to change color when viewed from different angles.
Another example is the recent evidence that song quality is used by some female birds to assess potential mates. Scientists at the University of Antwerp in Belgium reported in 2004 that the song rate of male flycatchers was altered when their immune system was artificially repressed, which in turn hindered their mating success.
The evolutionary rationale for such conspicuous traits and behaviors among males is that they help suitable males and females find each other (aid reproduction). This contributes to the survival of the species.
Indirect costs of conspicuous behavior.
However, conspicuous traits and behaviors may sometimes come with a cost. Standing out in such an obvious manner may attract predators or parasites.
I knew this intuitively from an early age. I survived elementary school "dodge ball" in gym class by hiding in the far corner of the gym (the other students used dodge ball as an excuse to deliberately hit me in the head and get away with it; fun times).
Extensive research has focused on the benefits and costs of conspicuous traits and behavior. There is comparatively little research on the indirect costs shouldered by females.
This is an important question to address. Evolutionary pressure may favor females who choose less-conspicuous traits and behaviors in mate selection (such females may be eaten and parasitized less frequently).
Cassandra Martin and William Wagner Jr (University of Nebraska-Lincoln, United States) have begun to fill in this gap in our knowledge. Their research on Gryllus lineaticeps (crickets) and Ormia ochracea (fly parasites) has broad evolutionary implications that have yet to be studied in detail.
Importance of studying female crickets.
This cricket is an appropriate insect to study; previous research has established that male chirping attracts both females and parasites. This observation has been made of numerous cricket species by many scientists over the past few decades.
However, it is unknown whether females are placed at greater risk of parasitization by seeking out chirping males. This is the question the scientists wished to address.
The larger significance here is that it is the females who lay the eggs. If the female cricket is unhealthy, few eggs will be laid, and reproduction will be far less successful.
Note that the cost of parasitism in this case is quite severe, even in the short term. A cricket that has been parasitized inevitably dies, and early stages of parasitism hinder female reproduction, both of which would be expected to have evolutionary consequences.
How were the crickets studied?
The scientists studied lab-reared crickets descended from field-collected females collected in the Santa Monica Mountains National Recreation Area (southern California, United States). They were reared under standard laboratory conditions.
These lab-reared crickets were then caged, and transported to the original collection site. The cages were placed above speakers playing either a high (4.2 per second) or low (1.8 per second) chirp rate.
After the field experiments, the crickets were returned to the laboratory, and checked for parasites after no more than 15 days (longer than the time required for parasite development within the cricket).
Increased parasitism observed among female crickets.
The scientists found that female crickets were almost twice as likely to be parasitized when they were placed near the high-rate chirping sound than near the low-rate chirping sound. Twnety-nine out of 40 (72.5%) crickets were parasitized in the former, and 15 out of 38 (39.5%) were parasitized in the latter.
Note that the absolute parasitism risk is exaggerated in these scientists' experiments, because they don't quite mimic undisturbed field conditions that would enable a male and female cricket to quickly retreat underground (away from parasites) after finding one another. However, the relative parasitism risk is most likely retained.
Interestingly, male crickets were not statistically more likely to be parasitized in the presence of high-rate chirping sounds. The scientists propose several explanations for this curious result, including the possibility that since the female crickets are larger and more active, they attracted flies away from the males (both the female and male [muted] crickets were in the same cage).
Remember that parasitized crickets die. This isn't analogous to a person getting head lice, a minor irritant; being parasitized is a serious, life-terminating cost of seeking out the ideal cricket mate.
Evolutionary implications.
Chirping helps male crickets attract females. However, in these experiments, there is an indirect (yet severe) cost shouldered by the females: increased chance of parasitism, leading to hindered reproduction and early death.
If the male crickets are parasitized immediately before mating, that's not necessarily a big deal, in terms of successful reproduction. However, if the female crickets are parasitized immediately before mating, their egg production will be hindered, hindering reproduction of the species.
By seeking out the ideal male cricket, something that one may expect would only help survival of the species, female crickets are also placed at higher risk of parasitism and death. More research is needed with these and other species, to investigate the possible relevance of this cost/benefit observation towards long-term natural selection.
for more information:
Martin, C. M., & Wagner Jr, W. E. (2010). Female Field Crickets Incur Increased Parasitism Risk When Near Preferred Song PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009592