Selection for sexually dimorphic traits and signal diversity in fiddler crabs
Date
2017
Authors
Perez, Daniela
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Abstract
The evolution of diverse sexual signals and traits in animals is
a promising yet
challenging field in behavioural ecology. Sexual features are
under the influence of a
multitude, and often cryptic, mechanisms. In addition, the
composition and strength of
selection varies for each study species. Fiddler crabs are ideal
subjects for investigating
the evolution of diverse sexual traits. Males possess one
sexually dimorphic enlarged
claw. The structure is used as a weapon in conflicts over
territory and signaled in the
form of wave displays as courtship to females and warning to
intruders. The displays are
diverse on the interspecific level, characterized by
species-specific movement patterns.
In this thesis, I explore the forces behind the evolution of
sexually dimorphic claws and
the great diversification of fiddler crab signals.
In the first two chapters of my thesis I search for a clearer
understanding of sexual
selection in shaping the species-specific diversity of wave
displays in fiddler crabs. First, I
investigate if wave displays are cues for species identity in
sympatric populations. I use
robotic crabs in my experiments and verify that females are able
to choose the
conspecific over a heterospecific wave movement. In chapter II, I
direct my focus to
female natural choices and identify their preferences on wave
displays and claw size.
This final approach allows me to reveal other selective
mechanisms, such as constraints
from natural selection in male signaling effort.
In the second part of my thesis, I expand my approach to the
level of social context
and its effects on fiddler crab sexual traits. In chapter III, I
look into the laterality of the
sexually dimorphic claw and examine the different fight
endurances when opponents
are same- or different-handed. I point out how distinct
handedness ratios in fiddler crab
populations set contrasting selective pressures in male-male
coalitions. In chapter IV, I
explore differential social situations in the production of wave
signals, and take a step
further to explore the social contexts that stimulate the
emergence of a rare signal
phenomenon, the synchronous waving.
Finally, in chapter V, I look into signal diversity from a wider
perspective by
applying comparative approach to understand the evolution of the
wave displays. I
gather structural and temporal information of the wave displays
of 28 species to predict
the likelihood and evolutionary paths of presenting waves in
synchrony. Lastly, I
indicate how the phenomenon may stem from the particularities of
a species mating
systems and ecological adaptations.
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Keywords
diverse sexual signals, female choice, social context, behavioural ecology, fiddler crabs, wave displays, robotic crabs, waving synchrony
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