Among the different sensory modalities that play a role in sexual selection, acoustic
communication plays an important one. Acoustic communication has been known to be
used for male-male competition (territory maintenance, male aggression during
mating),for advertisement to the opposite sex (mating status, body condition, genetic
quality, nutritional status) and used by females to sample and choose conspecific
preferred males. The use of acoustic communication for sexual display and information exchange has been extensively studied in multiple taxa, including insects, anurans, birds and mammals. Among insects, crickets have proven to be good model systems to study sexual selection based on acoustic communication as most species have an elaborate acoustic communication system, male advertisements, diverse types of
mating incentives for females (such as glandular feeding) and a female dominated
mating system. Generally, in crickets males produce species-specific calls which are
used by females to localize conspecific males. Besides, calls show high levels of
intraspecific variation and are energetically costly to produce. Moreover, as in crickets predominantly the females show phonotaxis towards male calls, calls also can play a role in mate sampling and choice by acting as indicators of preferred male quality.
Despite being studied for many decades there are certain gaps in the studies examining
mate choice in crickets. Some of them are, lack of understanding of the variation of
male calling traits in nature and its role in signal evolution, lack of understanding
regarding the ecological context of mate sampling and the evolution of alternative
mating strategies. Hence, the tree cricket Oecanthus henryi was chosen as a study
system to address these gaps in the understanding of female choice based on acoustic signals.
In the tree cricket Oecanthus henryi, males call and females use calls to localize
conspecific males and hence potentially females can choose males based on acoustic
cues. To understand the evolution of female preference for male acoustic cues it is
important to understand the variation in the calling songs in the field and identify
repeatable call features that are reliable indicators of preferred male traits
(morphological, developmental or genetic). I measured repeatability of male call traits in
the field to understand their variation, reliability and consistency. Carrier frequency was the only call trait that was highly repeatable and hence was reliable and consistent.
Following this I examined whether any of these call traits were indicators of male
morphological traits (such as male size and fluctuating asymmetry) which are known to
be preferred by females. It was found that carrier frequency was negatively correlated
with body size; hence carrier frequency was both reliable and indicated male size. I also
found that females preferred larger males during mating, as revealed by the longer
mating durations and longer spermatophore retention time. Interestingly, though this
study indicated that females could in principle use lower call carrier frequency to localize preferred larger males, simultaneous choice experiments done in the laboratory revealed that the females do not use this cue. These contrasting results may be because females are incapable of discriminating small differences in frequency or because they use non-acoustic cues for mate choice.
However, whichever cues the females use to discriminate between males in the
laboratory conditions, often these preferences are not realized in the field. The main reason behind this is that searching for preferred mates in the field can be costly and this might force females to choose sub-optimal males. Theoretical models predict that male movement and spacing in the field should influence female sampling tactics and in turn, females should drive the evolution of male movement and spacing to sample them optimally. Moreover, simultaneous sampling of males using the best-of-n or comparative Bayes strategy should yield maximum mating benefits to females. Many of the theoretical mate sampling strategies involves recall of the quality and location of individual males, which in turn requires male positions to be stable within a night.
Calling males of O. henryi showed high site fidelity within a night, potentially enabling
female sampling strategies that require recall. To examine the possibility of
simultaneous acoustic sampling of males, I estimated male acoustic active spaces
using information on male spacing, call transmission and female phonotactic threshold.
Males were found to be spaced far apart and active space overlap was rare. I then
examined female sampling scenarios by studying female spacing relative to male
acoustic active spaces. Only 15% of sampled females could hear multiple males,
suggesting that simultaneous mate sampling is rare in the field. Moreover, the relatively large distances between calling males suggest high search costs, which may favor threshold strategies that do not require memory.
Using the insights gathered from these two studies I examined a unique calling
behaviour from leaf holes, baffling, observed in this species. Baffling behaviour has been found in multiple species of the genus Oecanthus where the males call from selfmade holes in leaves rather than calling from leaf edges (their natural calling surface) thus increasing their loudness many fold. I started by examining the natural history of baffling and found that baffling is an extremely rare behaviour in the field. However field observations and laboratory experiments revealed that many males can baffle and hence it is not an obligatory behaviour shown only by a few males. It was hypothesized that one reason for the rarity of baffling could be resource limitation. It was found that baffling males prefer larger leaves possibly due to higher SPL gains achieved by baffling on the larger leaves, which is a limited resource in the field. However this alone was insufficient to explain extreme rarity of bafflers in the field. Hence I examined which males were using this behaviour in the field. Using field observations and laboratory
experiments it was found that less preferred males (smaller and quieter) baffled more
which provided them with higher calling SPL and greater sound-field volume and thus a
higher number of potential mates. Moreover, baffling also increased the mating
duration for the less preferred males thus providing more time to these males for sperm
transfer. The females could not differentiate between an inherently loud caller and a caller whose SPL was increased artificially (as if it was baffling). Hence I concluded that baffling is probably a cheater strategy used by the less preferred males to fool the females into approaching them and mating for longer durations.
To my knowledge, this is the first study that has estimated male call variation in the field to understand its role in female choice in tree crickets. Moreover this is also the first study to examine the ecological context of mate choice in tree crickets. This is also the first study to examine the advantages of baffling behaviour and its potential evolutionary implications.
the different sensory modalities that play a role in sexual selection, acoustic communication plays an important one. Acoustic communication has been known to be used for male-male competition (territory maintenance, male aggression during mating),for advertisement to the opposite sex (mating status, body condition, genetic quality, nutritional status) and used by females to sample and choose conspecific preferred males. The use of acoustic communication for sexual display and information exchange has been extensively studied in multiple taxa, including insects, anurans, birds and mammals. Among insects, crickets have proven to be good model systems to study sexual selection based on acoustic communication as most species have an elaborate acoustic communication system, male advertisements, diverse types of mating incentives for females (such as glandular feeding) and a female dominated mating system. Generally, in crickets males produce species-specific calls which are used by females to localize conspecific males. Besides, calls show high levels of intraspecific variation and are energetically costly to produce. Moreover, as in crickets predominantly the females show phonotaxis towards male calls, calls also can play a role in mate sampling and choice by acting as indicators of preferred male quality.
Despite being studied for many decades there are certain gaps in the studies examining mate choice in crickets. Some of them are, lack of understanding of the variation of male calling traits in nature and its role in signal evolution, lack of understanding regarding the ecological context of mate sampling and the evolution of alternative mating strategies. Hence, the tree cricket Oecanthus henryi was chosen as a study system to address these gaps in the understanding of female choice based on acoustic signals.
In the tree cricket Oecanthus henryi, males call and females use calls to localize conspecific males and hence potentially females can choose males based on acoustic cues. To understand the evolution of female preference for male acoustic cues it is important to understand the variation in the calling songs in the field and identify repeatable call features that are reliable indicators of preferred male traits (morphological, developmental or genetic). I measured repeatability of male call traits in the field to understand their variation, reliability and consistency. Carrier frequency was the only call trait that was highly repeatable and hence was reliable and consistent. Following this I examined whether any of these call traits were indicators of male morphological traits (such as male size and fluctuating asymmetry) which are known to be preferred by females. It was found that carrier frequency was negatively correlated with body size; hence carrier frequency was both reliable and indicated male size. I also found that females preferred larger males during mating, as revealed by the longer mating durations and longer spermatophore retention time. Interestingly, though this study indicated that females could in principle use lower call carrier frequency to localize preferred larger males, simultaneous choice experiments done in the laboratory revealed that the females do not use this cue. These contrasting results may be because females are incapable of discriminating small differences in frequency or because they use non-acoustic cues for mate choice.
However, whichever cues the females use to discriminate between males in the laboratory conditions, often these preferences are not realized in the field. The main reason behind this is that searching for preferred mates in the field can be costly and this might force females to choose sub-optimal males. Theoretical models predict that male movement and spacing in the field should influence female sampling tactics and in turn, females should drive the evolution of male movement and spacing to sample them optimally. Moreover, simultaneous sampling of males using the best-of-n or comparative Bayes strategy should yield maximum mating benefits to females. Many of the theoretical mate sampling strategies involves recall of the quality and location of individual males, which in turn requires male positions to be stable within a night. Calling males of O. henryi showed high site fidelity within a night, potentially enabling female sampling strategies that require recall. To examine the possibility of simultaneous acoustic sampling of males, I estimated male acoustic active spaces using information on male spacing, call transmission and female phonotactic threshold. Males were found to be spaced far apart and active space overlap was rare. I then examined female sampling scenarios by studying female spacing relative to male acoustic active spaces. Only 15% of sampled females could hear multiple males, suggesting that simultaneous mate sampling is rare in the field. Moreover, the relatively large distances between calling males suggest high search costs, which may favor threshold strategies that do not require memory.
Using the insights gathered from these two studies I examined a unique calling behaviour from leaf holes, baffling, observed in this species. Baffling behaviour has been found in multiple species of the genus Oecanthus where the males call from self- made holes in leaves rather than calling from leaf edges (their natural calling surface) thus increasing their loudness many fold. I started by examining the natural history of baffling and found that baffling is an extremely rare behaviour in the field. However field observations and laboratory experiments revealed that many males can baffle and hence it is not an obligatory behaviour shown only by a few males. It was hypothesized that one reason for the rarity of baffling could be resource limitation. It was found that baffling males prefer larger leaves possibly due to higher SPL gains achieved by baffling on the larger leaves, which is a limited resource in the field. However this alone was insufficient to explain extreme rarity of bafflers in the field. Hence I examined which males were using this behaviour in the field. Using field observations and laboratory experiments it was found that less preferred males (smaller and quieter) baffled more which provided them with higher calling SPL and greater sound-field volume and thus a higher number of potential mates. Moreover, baffling also increased the mating duration for the less preferred males thus providing more time to these males for sperm transfer. The females could not differentiate between an inherently loud caller and a caller whose SPL was increased artificially (as if it was baffling). Hence I concluded that baffling is probably a cheater strategy used by the less preferred males to fool the females into approaching them and mating for longer durations.
To my knowledge, this is the first study that has estimated male call variation in the field to understand its role in female choice in tree crickets. Moreover this is also the first study to examine the ecological context of mate choice in tree crickets. This is also the first study to examine the advantages of baffling behaviour and its potential evolutionary implications.
| Identifer | oai:union.ndltd.org:IISc/oai:etd.ncsi.iisc.ernet.in:2005/2641 |
| Date | January 2015 |
| Creators | Deb, Rittik |
| Contributors | Balakrishnan, Rohini |
| Source Sets | India Institute of Science |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Relation | G26712 |
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