Adaptive Plasticity in Wild Field Crickets’ Acoustic Signaling

Adaptive Plasticity in Wild Field Crickets’ Acoustic Signaling

A new article in PLOS ONE written by

Susan Bertram, Sarah Harrison, Ian Thomson, and Lauren Fitzsimmons

Click here for a copy of our PLOS ONE article

Click here for the Bertram Lab website

http://science.nature.nps.gov/im/units/ucbn/monitor/sagegrouse/sagegrouse.cfm

Figure 1: Changes to the density of competitors is an example of environmental variability commonly faced by organisms in the wild.

Environmental change is a common obstacle that animals have to overcome. Temporal or spatial changes in the weather, food availability, predator density, mate availability, or competitor density are just some examples of the environmental variability commonly faced by animals in the wild (Figure 1).The optimal phenotype (an organism’s morphology, physiology, behavior etc.) for a given environment often changes as environmental conditions change. For example, a caterpillar feeding on oak flowers will develop into a mimic of an oak catkin, while a sibling caterpillar feeding on leaves will develop into a mimic of a twig (Figure 2). These caterpillars may avoid predation by camouflaging themselves to match their environments.

Photo source: Whitman and Agrawal

Figure 2: Nemoria arizonaria caterpillars: (a) summer broods feed on oak leaves and
Develop to resemble oak twigs, (b) while spring broods feed on and resemble oak catkins (Whitman and Agrawal 2009).

Figure 3: Hydrangea macrophylla flowers grown in acidic soil produces blue flowers, while flowers grown in neutral or alkaline soil produce pink flowers.

Figure 3: Hydrangea macrophylla flowers grown in acidic soil produces blue flowers, while flowers grown in neutral or alkaline soil produce pink flowers.

If individuals are unable to adapt to changes in their surroundings, they may experience reduced survival or reproductive success compared to individuals who are able to adapt.Phenotypic plasticity is the ability to change behaviour, appearance, or other characteristics in response to environmental change. For instance, Hydrangea macrophylla produce either blue or pink flowers depending on soil alkalinity (Figure 3). Phenotypic plasticity may be adaptive as long as environmental variation is predictable and the benefits of plasticity outweigh the costs. For instance, phenotypic plasticity in sexually selected traits may be particularly adaptive if there are frequent predictable changes to the socio-sexual environment (e.g. fluctuation in mate density).

In our study, recently published in PLOS ONE, we examined the phenotypic plasticity of male field crickets’ sexually selected acoustic signals for two species: the fall field cricket, Gryllus pennsylvanicus and the spring field cricket, G. veletis (Figure 4).

Figure 3: The fall field cricket (Left- G. pennsylvanicus male) and the spring field cricket (Right- G. veletis male) are both found throughout North America.

Figure 4: The fall field cricket (Left- G. pennsylvanicus male) and the spring field cricket (Right- G. veletis male) are both found throughout North America.

Male crickets produce acoustic signals by raising their forewings and rubbing them together, each closing stroke producing a pulse of sound with 3-4 pulses grouped into a chirp (Figure 5). These acoustic calls are used to attract females, and females choose mates on the basis of the quantity and fine-scale structure of these signals. Female movement and sexual receptivity in the wild often follows predictable fluctuations throughout the day (diel rhythms), which may partially be explained by hourly fluctuations in the biotic (predator and parasite density) and abiotic (light and temperature levels) environment. Therefore, males that synchronize their daily signalling rhythms to match female mating activity are likely to have higher reproductive success. Here we examined how male acoustic signalling in the laboratory changes over the course of the day and compared this to the known reports of diel rhythms in female mating activity in the wild.
We captured wild males and brought them to our laboratory at Carleton University where we recorded their acoustic signalling for 2-4 days. We found that both spring and fall field crickets exhibited phenotypically plastic signalling behavior, with most males signalling more often and more attractively during the time of day when female mating activity is the highest in the wild (fall field cricket: night time and early morning; spring field cricket: early morning and afternoon). Most male crickets (from both species) chirped more often during the time of the day that female mating activity is highest in the wild (Figure 5). However, a few males of each species signalled in a seemingly maladaptive manner during times when female mating activity is lowest in the wild. Diel rhythms in signalling also differed across species, which may be beneficial in preventing lethal hybridization when spring and fall field crickets overlap temporally and spatially in mid-summer.

Figure 4: Waveforms of long-distance mate attraction signals of one G. pennsylvanicus and one G. veletis male.  Figures show typical long-distance mate attraction signal for each species and how signaling typically changes during time periods indicative of low (A & B) and high (C & D) mating activity in the wild. Signal fine-scale properties are indicated as follows: ChD = chirp duration; IChD= interchirp duration; PPCh = pulses per chirp; PD = pulse duration; IPD = interpulse duration; and PP = pulse period, which combines PD and IPD.

Figure 4: Waveforms of long-distance mate attraction signals of one G. pennsylvanicus and one G. veletis male. Figures show typical long-distance mate attraction signal for each species and how signaling typically changes during time periods indicative of low (A & B) and high (C & D) mating activity in the wild. Signal fine-scale properties are indicated as follows: ChD = chirp duration; IChD= interchirp duration; PPCh = pulses per chirp; PD = pulse duration; IPD = interpulse duration; and PP = pulse period, which combines PD and IPD.

Overall, our findings suggest that male field crickets exhibit phenotypically plastic mate attraction signals and that diel rhythms in these signals are synchronized so that they are in phase with diel rhythms in female mating activity, suggesting that signalling plasticity may be adaptive. We have yet to determine how the costs and benefits of the phenotypic plasticity we observed in our species of field cricket ultimate affect fitness.

References:

Whitman, D. W. and A. A. Agrawal. What is Phenotypic Plasticity and why is it Important? Pages 1-63 in: D. W. Whitman and T. N. Ananthakrishna (editors), Phenotypic plasticity of insects: Mechanisms and consequences. Science Publishers, Inc, Enfield, NH

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Deceptive Males: Not All Sexual Signals Are Honest

Calling, Courtship, and Condition in the Fall Field Cricket, Gryllus pennsylvanicus

A new article in PLOS ONE written by

Sarah Harrison, Ian Thomson, Caitlin Grant, and Susan Bertram

Click here for a copy of our PLOS ONE article

Click here for a press release by Carleton University

Click here for the Bertram Lab website

Acoustic and physiological sexual signals

Figure 1. Acoustic and physiological sexual signals

Males often produce signals to attract females to mate. These sexual signals come in many forms: acoustic (e.g. calls of songbirds and crickets), visual (e.g. deer antlers, bright colourful bird plumage, colourful tropical fish scales; Figure 1), and chemical (e.g. pheromones). These signals are thought to provide females with information about male quality and potential mating benefits to help them choose between different males.

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Figure 2. Bright plumage and plump body size in a crested partridge.

For instance, if bright colourful bird plumage is strongly dependent on nutrition, then feather colour might signal that a male is healthy and good at finding high quality food (Figure 2). Mating with males that have colourful plumage might benefit females directly if males share their food, or indirectly if they pass on good genes for health and foraging ability to their offspring. Since poor quality males should be incapable of cheating and producing sexual signals that rival those of high quality males, sexual signals are thought to honestly indicate mating benefits. But are all sexual signals honest, or do males sometimes cheat when trying to attract a mate?

midlife-crisis-1

Figure 3. Are you compensating for something?
Source: http://www.blogger.com/img/blank.gif

Males might be more likely to produce dishonest signals (cheat) if they have a bleak reproductive future. For instance, starving, diseased, old, or dying males might invest all of their remaining energy into attracting females at the risk of dying sooner (Figure 3). This alternative idea, known as the terminal investment hypothesis, suggests that poor quality males might sometimes be able to produce signals that rival the signals of high quality males. These poor quality males would not, however, be able to provide females with the mating benefits that they were dishonestly advertizing.

We questioned whether all sexual signals are honest using fall field crickets (Gryllus pennsylvanicus). These crickets are commonly found throughout much of the U.S. and southern Canada. Males produce two types of mating calls by rubbing their hind wings together: one they use to attract females from a distance, the other they use to court attracted females. Both mate attraction and courtship calls are made up of a series of sound pulses that they group into chirps (Figure 4).

Cricket chirps

Figure 4. Sonograms (top) and waveforms (bottom) of a G. pennsylvanicus long-distance mate attraction call (A & B) and a courtship call (C & D).

We break down these chirps into smaller pieces (call rate, chirp rate, pulse rate, tick rate, number of pulses per chirp etc) to study them. Males can invest more or less energy into calling by changing the temporal properties of these pieces. Higher condition males should have more energy to devote to signalling than low condition males (‘Condition’ is an individual’s ability to find food and turn it into useable energy). Females should benefit from using calls to find mates because high condition males should pass on good genes for condition to their offspring.

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Figure 5. Size variation in G. pennsylvanicus

We recorded each male’s long-distance calls and courtship calls (click on the calls for examples), broke these calls down to their smaller temporal components, and then examined relationships between these temporal components and two different estimates of male condition: 1) body size (i.e. height; Figure 5) and 2) plumpness (i.e. fat).

We found evidence that males both honestly and dishonestly signal their condition. Male mate attraction calls honestly indicated body size, except that small males called with faster chirp rates than large males. Courtship calls dishonestly signalled male plumpness, as lean males produced calls with pulse and chirp rates similar to plump males, with males of intermediate plumpness having the lowest pulse and chirp rates. It costs more energy to call at faster pulse and chirp rates, so faster rates should be harder for low condition males. Our findings reveal that small and lean males cheat when calling to females: they call at equivalent or higher rates than large or plump males, suggesting not all sexual signals are honest.

Blog post “Deceptive Males: Not All Sexual Signals Are Honest” was written by Sarah Harrison, a PhD candidate in the Bertram Lab.

Sarah J. Harrison

Sarah J. Harrison

ResearchBlogging.orgHarrison SJ, Thomson IR, Grant CM, & Bertram SM (2013). Calling, Courtship, and Condition in the Fall Field Cricket, Gryllus pennsylvanicus. PloS one, 8 (3) PMID: 23527313