Overview of Our Research on the Variable Field Cricket

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The evolution of animal communication systems is a coevolutionary process; the types of male signals that evolve will depend on female responses to male signals, and the evolution of female responses to male signals will depend on the costs and benefits of mating with males that produce different signal types. Evolutionary changes in one sex will thus affect how selection acts on the other sex.

The general goals of our research on the variable field cricket, Gryllus lineaticeps, are to understand how environmental, social and genetic factors individually affect the evolution of male traits and females preferences, how the interaction between these factors affects how male and female traits evolve, and how evolutionary changes in the behavior of one sex affects the nature of selection on the other sex. Our research on these issues has so far been focused on four broad, interrelated areas: (1) the effect of natural and sexual selection on the evolution of male singing behavior; (2) the costs and benefits of female mating preferences; (3) the evolution of signals of post-mating direct benefits; and (4) female searching behavior during mate choice.

You can see our laboratory facilities <HERE>.

The variable field cricket is one of the most common field crickets of coastal and central California. Males produce a calling song to attract females at a distance, and females select mates, in part, based on variation in male calling song. The calling song of this species consists of a series of short, rapidly repeated chirps, and males vary in the rate at which they produce chirps and the duration of their chirps.

The Evolution of Male Singing Behavior

Effect of Song Variation on Mate Attraction
In two-speaker choice tests, females prefer calling songs with higher chirp rates to songs with lower chirp rates, and they prefer calling songs with longer chirp durations to songs with shorter chirp durations (Wagner 1996; Wagner & Reiser 2000; Wagner et al. 2001a). Sexual selection through female choice thus appears to favor the evolution of higher chirp rates and longer chirp durations. Females also have preferences based on variation in male courtship song (Wagner & Reiser 2000).  

Effect of Song Variation on the Energetic Costs of Singing
Higher calling song chirp rates are energetically more expensive for males to produce (Hoback & Wagner 1997), and males provided a lower quality diet, who thus have less energy to invest in signaling, produce substantially lower chirp rates (Wagner & Hoback 1999). The production of high chirp rates thus appears to be constrained by male nutritional condition. We were not able to detect an energetic cost of producing longer chirp durations, and diet quality has no effect on chirp duration.

Effect of Song Variation on Predator Attraction
The phonotactic parasitoid fly, Ormia ochracea, is an important predator of this species. These flies locate their field cricket hosts by homing in on male song. Male crickets that produce higher chirp rates and longer chirp durations have a higher probability of attracting flies, and thus a greater risk of predation (Wagner 1996; Wagner & Smith in prep; Wagner & Basolo in prep). You can <VIEW> a short video clip showing flies attracted to a speaker broadcasting male song (quicktime format).

Quantitative Genetics of Song Variation
Both chirp rate and chirp duration show heritable variation (Olvido et el. in prep). Both traits are thus capable of evolving in response to sexual and natural selection. The two traits are not genetically correlated. They are thus capable of evolving independently of each other.  

Evolutionary Consequences of Predation
To examine whether fly predation has affected the evolution of male signing behavior, we are currently comparing male singing behavior among populations that are and are not attacked by the flies. If fly predation has an evolutionary effect on male singing behavior, males from populations with flies should produce lower chirp rates and/or shorter chirp durations than males from populations without flies.

The Evolution of Female Mating Preferences

The Benefits of Mating
Female G. lineaticeps that mate more frequently live longer and produce more eggs, even when all that they receive from males is sperm and seminal fluid (Wagner et al. 2001b). Females thus appear to receive seminal fluid products from males that increase their fitness.  

The Benefits of Mating Preferences
Females not only appear to receive beneficial seminal fluid products from males, but they also appear to receive more beneficial seminal fluid products from males that produce preferred song types. Females that mate with males with higher chirp rates have higher lifetime fecundity and fertility, while females that mate with males that produce longer chirp durations live longer (Wagner & Harper 2003). Females thus directly benefit from their mate choices. Furthermore, because of a negative phenotypic correlation between male chirp rate and chirp duration, females may have to trade off one benefit against the other.  

The Relative Importance of Different Mating Benefits
When chirp rate and chirp duration both vary, only chirp rate has a significant effect on female mating responses (Wagner & Basolo 2007). This result suggests that chirp rate is more important to females than chirp duration, and that the reproductive benefits correlated with male chirp rate are thus more important than the life span benefits correlated with male chirp duration.  

Potential Costs of Mating Preferences
There are two important types of costs that might affect the evolution of female mating preferences: association costs and search costs. Females are at risk of being parasitized by the parasitoid flies when they are around males, and because the flies prefer the same types of males that female prefer, females should be at greatest risk when they are in association with preferred males. Females are also at risk of being eaten by predators, such as black widows and tarantulas, when they move around on the ground sampling potential mates. If females incur search costs, and if females with stronger preferences must search more to find an acceptable male, females with stronger preferences may incur greater search costs. Cassie Martin is currently examining female association costs.

Evolutionary Consequences of Predation
To examine whether fly predation has affected the evolution of female mating preferences, we are currently comparing female mating preferences among populations that are and are not attacked by the flies. If fly predation has an evolutionary effect on female preferences, females from populations with flies should have weaker chirp rate and/or chirp duration preferences than females from populations without flies.

The Evolution of Signals of Post-Mating Direct Benefits

For female choice based on signals of direct benefits to evolve, there need to be mechanisms that constrain the production of deceptive signals by low quality males and mechanisms that constrain high quality males from cheating on the benefits promised by their signals. Cheating by high quality males is particularly a problem when females do not receive direct benefits from males until after mating has occurred.

Constraints on Deceptive Signaling by Low Quality Males
Deceptive signaling by low quality males can be constrained if producing attractive signals and providing mating benefits are both energetically expensive. If so, only males in good condition may be able to afford to produce both attracttive signals and high quality benefits. Previous results with G. lineaticeps have shown that higher chirp rates are energetically more expensive for males to produce, and that diet quality affects male chirp rate (see above). Furthermore, indirect evidence suggests that spermatophores are costly for males to produce and that males providing greater reproductive benefits to females incur higher costs of spermatophore production (Wagner 2005). Only males in good nutritional condition may therefore be able to support the costs of producing both high chirp rates and spermatophores that provide reproductive benefits to females.

Constraints on Cheating by High Quality Males
One factor that may favor the transfer of high quality benefits by males that are capable of doing so is the expression of conditional mating tactics in females based on benefit quality (see Wagner & Harper 2003). If females who receive lower quality benefits behave in ways that penalize the males who provided the lower quality benefits, males that provide higher quality benefits will benefit more from a given investment in signaling (there will be differential benefits of signaling). Our results so far suggest that females are less likely to re-mate with males that provide lower quality benefits (Wagner et al. in preparation). This should result in reduced success in sperm competition for males that provide lower quality benefits. We are currently testing two additional hypotheses about female behaviors that would penalize males that provide lower quality benefits: (1) females terminate sperm transfer earlier when they receive lower quality benefits from a male; and (2) females mate sooner with another male when they receive lower quality benefits from their current mate.

Female Sampling Behavior During Mate Choice

Very little is known about how female animals sample males during mate choice. An understanding of female sampling behavior is important, however, because female sampling strategies will affect female mate choices. There are a number of ways in which females in acoustic species might sample males. First, they may not compare males at all, instead showing fixed responses to different song types. Second, they may simultaneously compare the songs of males that they can hear, choosing whichever of these simultaneously sampled males produces the most preferred song type. Third, they may move between males and sequentially compare male songs, either choosing whichever of these sequentially sampled males produces the most preferred song type (a best of N approach) or adjusting their threshold as they sample, accepting the first male that produces a song type that is acceptable based on this sample-adjusted threshold. Finally, they might use a combination of simultaneous and sequential sampling.

We recently began work examining female sampling behavior in G. lineaticeps. Females of this species change their responses to male songs based on the song types they have previously heard; exposure to a more attractive song reduces female responses to less attractive song (Wagner et al. 2001a). Females thus do not have fixed responses to different song types. In addition, a series of experiments, using a variety of experimental designs, suggest that a female's response to the calling song of a nearby male is unaffected by the attractiveness of the calling song of a more distant male (Wagner & Basolo in preparation). Females thus do not appear to simultaneously compare male songs.