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Research
Acoustic Mating Behaviour Research
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Variation in Acoustic Mate Signalling:
Crickets vary extensively in the amount of effort they put into acoustically signalling to attract a mate. Some males signal for up to fourteen hours each night, but others signal for only a few hours, a few minutes, or not at all (Bertram, Johnson, Clark et al. 2004; Bertram and Warren 2005). Males also vary in how loud they signal, how long they signal without taking a break, and in how often they signal (Bertram, Johnson, Clark et al. 2004; Bertram and Warren 2005). My students and I explore the environmental and genetic underpinnigs of this variation. |
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Environmental Influences:
We have quantified many environmental components influencing signalling effort. Signalling effort is not influenced by photoperiod (Bertram and Bellani 2002), the presence of female crickets or parasitoid flies (Bertram, Orozco and Bellani 2004), age (Bertram 2000), size (Bertram 2000; Bertram, Schade and Elser 2006), residual mass (Bertram 2000; Bertram, Schade and Elser 2006), or development time (Bertram, Schade and Elser 2006). Signalling effort is, however, correlated with flight capability. Fliers signal with twice the overall calling effort than non-fliers (Bertram 2006). Signalling effort may also be strongly influenced by diet quality. We are presently exploring how dietary nitrogen and phosphorus may underlie variation in fitness conferring behaviours (Bertram, Schade and Elser 2006). Our research revealed a strong relationship between body C:N:P and long-distance mate attraction signals produced by male Texas field crickets (Gryllus texensis). We found signalling effort was strongly and positively correlated with %P and strongly negatively correlated with N:P. Further, callers had more total body P than non-calling satellite males. Diets limited in essential elements can reduce growth rates, reproduction, and survival in arthropod herbivores, suggesting that a stoichiometric imbalance between the needs of the organism and the relative scarcity of nutrients in nature may underlie variation in condition-dependent behaviours.
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Acoustic Signalling and Quantitative Genetic Components:
My recent research explores the additive genetic and genotype by environment interactions influencing acoustic mate signalling behaviour. In a preliminary study, we estimated the heritability for total nightly signalling time in the Texas field cricket (Gryllus texensis). The Texas field cricket's total nightly signalling time is a widely cited example of a sexually selected character that is highly heritable. This heritability estimate of hr=0.50 and hr=0.53 for the high and low lines, respectively, arose from a selection experiment. The study was conducted on crickets from San Antonio, Texas over twenty-five years ago. Because most of the behavioural and evolutionary research on G. texensis has been conducted on crickets in Austin, Texas, not in San Antonio, we decided to estimate the heritability on the Austin Texas population. We reared cricket fathers, sons, and half-siblings and quantified their total nightly calling time. Signalling time appears to exhibit minimal heritability and little of the total variance appears to be additive genetic. We have also collected some preliminary data on the heritability of other signalling traits, and are in the process of analyzing this data.
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Acoustically Orienting Parasitoids:
Many animals use acoustic signals to attract mates but their signals simultaneously expose them to predators or parasites. Male Texas field crickets, for example, signal acoustically to attract potential mates but their calls also attract a parasitoid (Ormia ochracea; Tachinidae). These parasitoid females acoustically orient to signalling males and lay their larvae on and around the cricket. The larvae burrow inside to feed and grow. The parasitoid larvae emerges 7-10 days later, killing the host. Our research has revealed that males alter their calling behaviour after they are parasitized. Male acoustic mate signalling effort becomes greatly reduced in the late stage of parasitism (Orozco and Bertram 2004).
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Behavioural Syndromes: Behavioural syndromes are suites of correlated behaviours expressed either within a given behavioural context (e.g. mating) or between different contexts (e.g. foraging and mating). Many studies have revealed strong behavioural correlations across contexts, even when some behaviours are seemingly maladaptive. These findings have important ramifications: they imply that behavioural plasticity may be limited; they compel us to examine behaviours across contexts; and (most importantly) they suggest that individual behaviours may not be shaped optimally but instead may be shaped in combination with other traits. Recently, there has been an upsurge of behavioural syndrome research across a range of taxa. This compelling research has, however, focused primarily on birds and mammals. The taxonomic breadth of syndromes remains largely unknown, especially in terms of insects. With Alex Wilson (PhD student in Jean-Guy Godin's laboratory) we are investigating whether the field cricket Acheta domesticus exhibits behavioural syndromes. Specifically, we are examining whether individuals display behavioural correlations across mating, activity, and anti-predatory contexts. This will be one of the first studies to examine whether terrestrial insects display behavioural syndromes. |
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Trade-offs Among Acoustic Mate Signalling Components:
Given the conflicting selection pressures influencing acoustic signals, we investigated whether male crickets made trade-offs among their multi-component acoustic signals to reduce their risk of becoming prey, or because of energetic, biomechanical, or physiological limitations. We revealed that high effort males trade-off bout duration with hourly bout number and trilling amplitude with hourly bout number. Low effort males did not exhibit these tradeoffs. Our results suggest high effort signallers may experience different selective regimes than low effort signallers (Bertram and Warren 2005). |
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Relationship Between Signalling, Allometry, and Physiology: Male crickets exhibit extensive variation in their acoustic mate attraction signals. Some males signal for hours a night, others rarely signal. Some males produce very attractive calls, others don't. Jeff Dawson, Charles Darveau and I are collaborating on a project investigating the biomechanic, allometric, and physiological underpinnings that affect behavioural variation associated with mate signalling displays. |
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Pre-Copulatory Versus Post-Copulatory Mating Behaviour: Males signal acoustically to attract mates and females select mates based on several aspects of male acoustic signals. Males that produce the sexiest calls are thought to benefit the either directly or indirectly. Trevor Pitcher, Lauren Fitzsimmons, Samantha Klaus and I are testing whether males that produce the sexiest signals also provide the best sperm. We are quantifying correlations between signalling quality and quantity, and sperm number, viability, and motility in crickets. |
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Electronic Acoustic Recording System:
Females often prefer to mate with males that produce loud, long, and/or leading calls. However, because of time and personnel costs, only a handful of studies have quantified among-male variation in these temporal signalling parameters; even fewer studies have estimated the heritability of these traits. To alleviate this problem, we designed and built an inexpensive electronic acoustic recorder that can quantify the temporal components of up to 128 acoustically signalling insects simultaneously (Bertram and Johnson 1998; Bertram, Johnson, Clark et al. 2004). Our acoustic recorder determines when each male signals, how much time he spends signalling, how loud he signals, the duration of his signalling bouts, and when he signals in relation to his neighbours. Our recorder design is now being used by other researchers as highlighted by a recent Nature publication (432:1024-1027). |
Indicies for Quantifying Diversity
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Quantifying Division of Labour and Diversity:
We have been using information theory to quantify division of labour. We divide Shannon’s mutual entropy by marginal entropy, rendering it robust over changes in number of individuals or tasks (Gorelick et al 2004). Our statistic has applications in diverse fields unrelated to socio-biology. Tasks (columns) and individuals (rows) can be redefined to a broader array of interactions, in which individuals specialize among tasks and/or tasks specialize among individuals. We have been using our statistic to quantify diversity, especially of multiple interactions. Our statistic can now be utilized by a host of fields including biology, sociology, biogeography, and landscape ecology (Gorelick and Bertram, 2007). |
Study Organisms
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Crickets and Parasitoid Flies: For the last several years I have been using field crickets as my model organism. Males signal to attract mates and exhibit heritable variation in many of their signalling components. Acoustic signalling is highly variable among males: signals vary in amplitude, duration, pulse rate, number of pulses per chirp, inter-chirp intervals, percentage of missed pulses, and dominant frequency. Female crickets discriminate between potential mates based on their nightly signalling time, suggesting the potential for sexual selection to influence these mating signals. There is also a strong potential for natural selection to influence their mating signals, as signalling males are often attacked and killed by acoustically orienting parasitoid flies. Female flies lay their larvae on and around the signalling cricket. The larvae burrow inside the cricket. A week later the larval fly burrows out of the host, killing the cricket. Acoustic signalling is, therefore, beneficial as it enhances mating success, and yet costly as it increases the chances of dying young. |
Collaborators
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