| dc.description.abstract | The spectacular diversity in the pair-formation strategies among animal groups has attracted the attention of many over the years. The roots of this diversity lie in the diversity of challenges that males and females face in finding a potential mate. Successful localisation of mates requires effective information flow between the sexes regarding location, availability and, in some cases, quality. To this end, animals have evolved different kinds of signals, signalling and search strategies to counter the various challenges and facilitate encounter.
A major factor affecting the information flow between the sexes is the spatio-temporal distribution of the sexes, as signals and sensory physiologies are constrained spatio-temporally. The spatio-temporal distribution of males and females is in turn a function of two major factors: resource distribution and parental investment of the sexes. Depending on whether the resource distribution is patchy or uniform, individuals can be clumped in space or evenly dispersed, influencing their pair-formation strategies. Parental investment can further alter the relative distribution of males and females by affecting the time that males and females can be sexually receptive. Higher parental investment of a particular sex can lead to these individuals being out of the mating pool longer, causing a bias in the ratio of sexually receptive males to females (Operational Sex Ratio) at any given point in time, consequently impacting pair-formation strategies.
Orthopterans are well known models for studies on acoustic communication owing to the diversity in signals and pair-formation strategies. Conventionally, males function as the static signalling sex and use long-range acoustic signals to direct the phonotactic response of silent females. There are deviations observed, with females responding to calling males via acoustic signals, with males (or both males and females) performing localisation.
Onomarchus uninotatus, a canopy inhabiting paleotropical false-leaf katydid, presents a unique departure, wherein the two sexes employ signals that function at different spatial scales. Males broadcast long- range acoustic signals and the females respond via vibratory signals that are then used by males to localise females. Laboratory experiments have established vibratory signals to be an immediate response to male calls even at the threshold of female hearing. Being a canopy species, relying on short-range vibratory signals for localisation across trees and at larger distances appears paradoxical.
In my thesis, I therefore investigated the localisation strategies of Onomarchus uninotatus across two spatial scales, i.e. within and between trees, and aimed to understand if the roles played by the sexes are reflected in their relative parental investment.
For the across-tree scale, I studied the spatial structure of calling males and their preferred calling sites (Artocarpus spp.) in their natural habitat. Using the information on male spacing, call transmission and hearing thresholds, I computed the perceptual spaces of male signals to understand the acoustic environment of calling males and females. It was found that both calling males and females could hear calls of males from neighbouring trees with a probability of 0.76 and 0.59 respectively. Although males were found to be dispersed, significant overlap was seen in their acoustic ranges. I then investigated female flight responses to male acoustic signals in laboratory experiments, wherein male acoustic signals were played back from loudspeakers that were not connected to the substrate on which the females were placed. Females typically tremulated first, followed sometimes by initiation of flight, suggesting that females may perform flight phonotaxis to locate calling males on a different tree.
Using the information gathered from these studies, I then used a simulation framework to elucidate optimal mate encounter strategies at the across-tree spatial scale. The across-tree spatial distribution of the sexes was varied in the simulations and the encounter efficiencies quantified for different movement patterns of both males and females, using the data on spatio- acoustic patterning of callers in this system. Stationary calling males with localising females was found to be the optimal strategy across all spatial distributions.
To understand pair-formation strategies at the within-tree spatial scale, field experiments were conducted in a semi-natural setup to observe interactions between a calling male and a responsive female at two different distances on a branch. Interestingly, females always tremulated first, irrespective of their distance from the caller. At larger distances, some females were found to perform phonotaxis, but only after a bout of tremulation.
Finally, I examined the relative parental investment of the sexes using re-signalling intervals as a proxy. Given that both males and females invest in localisation, parental investments of the sexes were predicted to be comparable. Preliminary results from re-signalling intervals indicate that the sexes have comparable parental investment in this species. | en_US |
