Signal divergence and design in sympatric morphs of an electric fish species flock
Mechanisms by which 'species recognition signals' diverge in animal lineages
without extrinsic barriers to gene flow remain poorly understood. Empirical
cases lag behind theory in this arena of evolutionary biology. Promising places
to explore sympatric signal diversification are animal groups of recent origin, in
which species-typical signals are distinct and easy to quantify. An excellent
example is a species flock of mormyrid fishes from Gabon (genus Brienomyrus).
All mormyrids produce discrete electric organ discharges (EODs) to detect objects
in their environment and to communicate. These fishes have been difficult to study
in the wild due to their nocturnal habits and inaccessibility. Among mormyrids, the
Brienomyrus species flock exhibits particularly striking EOD waveform
diversification. In Chapter 1, I describe electric signaling and group predation
by mormyrids using the first video recordings of any freely-behaving species
(Mormyrops anguilloides) in its natural habitat (Lake Malawi). In Chapter 2,
I introduce the reader to Gabon's Brienomyrus species flock, and I apply
quantitative methods to characterize EOD waveform variation among
morphologically-distinct species. In Chapter 3, I use new genetic markers to show
that members of the species flock with unique appearances and distinctive EODs are
reproductively isolated. Given this context, I describe an atypical system of
Brienomyrus morphs belonging to the flock's magnostipes complex.
I show that EODs of these morphs have diverged in the absence of extrinsic barriers
to gene exchange. In Chapter 4, I assay EOD discrimination behavior using electrical
playbacks, and I make physiological recordings of Knollenorgan electroreceptors.
These experiments demonstrate that EODs serve as functional displays for the
magnostipes complex, in which distinct signals are linked to body size
differences between morphs. In Chapter 5, I construct theoretical simulations of
potential relevance to the magnostipes complex. These simulations describe
conditions in which sympatric speciation is expected to occur by strong mating
preferences, alone. Some mode of natural or sexual selection apparently underlies
the occurrence of multiple EOD dimorphisms within the magnostipes complex.
EODs may signal alternative behavioral or life history strategies within lineages,
or selective influences driving sympatric speciation may be recruiting EODs for
behavioral isolation between incipient species. Regardless of the mechanism, my
results set the stage for future work on this potentially informative model of
sympatric divergence in animal recognition signals.