SummaryThe animal world is replete with vibrant colours: these are often used as display signals and selection has solved a fundamental problem in information transfer by enhancing the detectability of these signals against the backgrounds on which they are perceived by the particular sensory systems of their receivers

Ryan, Michael J.

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Sensory Ecology: See Me, Hear Me 

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Laboratory of Cell Biology, Building 50,
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Lung and Blood Institute,
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Maryland 20892, USA.
*E-mail: hammerj@nhlbi.nih.gov
DOI: 10.1016/j.cub.2007.10.010
Dispatch
R1019Sensory Ecology: See Me, Hear Me
The animal world is replete with vibrant colours: these are often used as
display signals and selection has solved a fundamental problem in
information transfer by enhancing the detectability of these signals
against the backgrounds on which they are perceived by the particular
sensory systems of their receivers.Michael J. Ryan
In his landmark publication that
launched Information Theory,
Claude Shannon [1] stated that
‘‘the fundamental problem of
communication is that of
reproducing at one point either
exactly or approximately
a message selected at another
point’’. Although in its simplest
version communication is a dyadic
interaction between a signaler and
a receiver, there is always the
intervening transmission channel,
and this channel is often noisy
and it causes information to be lost.
The fundamental problem in signal
evolution is to enhance efficacy in
a noisy world.
Noisy channels can vary with
habitat, location, and time. Studies
of various adaptations of animals
to increase the efficacy of signal
transmission have long been
somewhat of a cottage industry in
acoustic communication [2–4].
Sounds do not always transmit the
same in all habitats. High frequency
sounds usually attenuate (lose
energy) more with distance than do
low frequencies, and rapidly
pulsed sounds usually degrade
(lose quality) more than tones. The
situation is often more severe inforest than in field. Many animals
have evolved calls or songs that
enhance signal transmission in
their particular environment.
Furthmore, the background
sounds fromwind,waves and other
species can mask signals. Another
adaptive strategy is to producesignals that are less likely to
be masked by other sounds in
the environment. Recently, for
example, it was shown that great
tits in urban environments use
higher frequencies to avoid the
industrial din of cities [5].
Our understanding of parallel
issues in visual communication
have lagged behind. There were
earlier and insightful studies [6–9],
but the visual ecology of
communication got a jump start
from two sources, one conceptual
and one technical. In Endler’s [10]
presentation of sensory drive, he
described the myriad constraintsA
B
F
G
C H
ID
E J
Current Biology
Figure 1. Display coloration
in male and female African
dwarf chameleons.
On the left are shown exam-
ples of male display colora-
tion, with each individual in
the display posture (laterally
compressed, casque raised,
gular pouch expanded). Red
arrows show where reflec-
tance spectra were taken.
(A) B. transvaalense, Wood-
bush; (B) B. sp. Ngome;
(C)B.damaranum; (D)B.caff-
rum; (E) B. setaroi. On the
right are shown examples
of female display coloration,
with each individual showing
display coloration. Red ar-
rowsshowwhere reflectance
spectra were taken. (F) B.
caffrum; (G) B. ventrale; (H)
B. transvaalense, Graskop;
(I) B. taeniabronchum; (J) B.
transvaalense. (Photos re-
produced with permission
from [12].)
Current Biology Vol 17 No 23
R1020Visual
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Color
pattern
reflectance
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35
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ef
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 (%
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Color contrast
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Brightness contrast
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Current Biology
Figure 2. Coevolution of
habitat, vision and color in
surfperch.
Hypsurus caryi (rainbow
surfperch) and Embiotoca
lateralis (striped surfperch)
occupy distinct optical hab-
itats in the nearshore kelp
forest environment: one
that favors detection by
color contrast (H. caryi)
and one that favors detec-
tion by brightness contrast
(E. lateralis). Each species
has visual pigment and
color pattern reflectance
properties (from a blue
patch) that have diverged
towards the habitat-spe-
cific bias (H. caryi solid
lines; E. lateralis dashed
lines). (Figure courtesy of
M.E. Cummings.)and selection forces under which
visual communication evolves.
His paper coincided with a wave
of interest in the mechanisms
underlying sexual selection by
female choice, and made it clear
that signal evolution results from
a compromise of various selection
forces and the contingencies
imposed by the environment and
the sensory biology of the receiver.
A more pragmatic motivator for
visual ecology studies was
technical. It is far easier to quantify
and synthesize acoustic signals
than it is to measure optical
properties of signals and their
environments, let alone synthesize
them. Critical technical advances
in studies of sound occurred with
the advent of the sonograph and
portable tape recorders in the
middle of the last century [11].
These advances have recently
been matched in the visual realm
with field portable
spectrophotometers that have
become affordable for many field
researchers in the last two decades
or so. Although video playbacks
and robotics still do not match the
simplicity and robustness of
acoustic playbacks of synthetic
sounds, they are beginning to close
this gap as well.A recent paper by Stuart-Fox
et al. [12] is an example of what has
been birthed by the conceptual and
technical advances in visual
ecology. We all know that the world
is a biologically diverse place even
as we are stunned by how quickly
that diversity is being lost. Given
our own sensory biases, that
diversity astounds us most when
perceived through our eyes. The
myriad of colours of butterflies in
a field, fishes on a coral reef, or
birds in a tropical forest can
impress even the most obtuse
observer. Another group that
exhibits astounding visual diversity
is the African dwarf chameleons
(Figure 1). These animals are
stunning in many ways; their
adornment crests are spectacular
and the unsynchronized
movements of their eyes
somewhat creepy. Foremost,
however, is the striking variation in
the colors and patterns of the 21
taxa in this group. These animals
vary in their microhabitats, and
thus the background against which
they display. Stuart-Fox et al. [12]
measured the spectrum of the light
available to be reflected by signals,
the reflectance of the signals
themselves, and the background
against which the signals areperceived. They combined these
data with a model that accounts
for various properties of ‘the
chameleon’ visual system and
estimated the detectability of the
signal as its contrast against the
background and against other
body parts. Both male and female
coloration contrasts predictably
with habitat to enhance signal
detectability, and some of the time
habitat shifts are accompanied by
shifts in display coloration. There is
also an interesting sexual
dimorphism. Males achieve signal
efficacy through the evolution of
a diversity of colours, while females
do so in black and white (Figure 1).
The chameleon study [12] argues
strongly that habitat influences the
evolution of the signal end of the
communication dyad, but what
about the receiver? The critical
aspect of the visual model used by
the authors is photopigment
sensitivity. According to the
authors, there is little variation in
this visual parameter among the
small handful of chameleons
investigated (only one being
a dwarf chameleon). They thus
assume no evolution at the receiver
end of the dyad. In other cases,
however, the evolution of visual
signals is a more complicated
dance between both signals and
receivers. This is exemplified
wonderfully by a recent series of
studies of surfperch by Cummings
[13–15]. These fishes occupy one of
themost diverse optical habitats on
earth, the kelp forests off the coast
of California (Figure 2). As with the
dwarf chameleons, the optical
habitat varies among species,
as does the color spectrum of
the fishes’ signals, but so does
the visual pigment sensitivity. The
visual sensitivity among species
varies predictably to enhance
target detection (for example,
foraging items against
background). But this can be
achieved in two ways, either by
enhancing chromatic (‘color’)
contrast or luminance (‘brightness’)
contrast. In terms of visual pigment
tuning, the solutions are mutually
exclusive, anddifferent species rely
on one or the other strategy and
have visual systems adapted for
prey detection in the visual world
they inhabit. The next step in this
evolutionary dance is the evolution
Dispatch
R1021of the male’s display colors. Their
signals have evolved to match the
females’ visual sensitivity, but
they too can choose among
strategies. If that species relies
on chromatic contrast to find
food, then males evolve signals
that exploit the contrast channel,
but if the species uses
brightness contrast then male
signals exploit that channel.
Cummings presents strong
phylogenetic data to support
this coevolutionary process, with
Occam’s razor supporting a
specific chain of events: changes
in habitat leading to changes in
vision and subsequent changes in
color patterns.
These studies are just two of
a burgeoning field of animal
behavior. They integrate
analyses of sensory systems
and communication behavior
and interpret their results in
a phylogenetic context. Such
studies are becomingSignal Transductio
and Integrated Cir
Cognition
Signal transduction systems that me
expression to specific sensory inputs
Recent studies have focused on mec
between different information-proces
Melinda D. Baker2
and Jeffry B. Stock1,2
What are the decision-making
mechanisms by which
a bacterium controls the activities
of its genes and proteins to adapt
to changing environmental
conditions? How do cells regulate
the expression and activities of
thousands of different genes and
proteins to efficiently control
motility, metabolism, growth and
differentiation? The structures and
functions of most regulatory
components are well understood.
Transcription factors and RNA
polymerases interact with DNA to
regulate gene expression. In some
cases, small molecule secondparadigmatic of integrative
biology. And they are giving
us some important insights into
how the splendour of colours
all around us have come into being.
References
1. Shannon, C.E. (1948). A mathematical
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cuits in Bacterial
diate adaptive changes in gene
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hanisms that allow crosstalk
sing modalities.
messengers bind to
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Department of Integrative Biology,
University of Texas, Austin,
Texas 78712, USA.
E-mail: Mryan@mail.utexas.edu
DOI: 10.1016/j.cub.2007.09.004But how do disparate regulatory
modules interact to function as
a cell? How are bacterial
information processing networks
organized?
The classic paradigm for
a regulatory module is the
system that controls lactose
utilization in Escherichia coli [1].
The genes for lactose uptake and
metabolism are regulated by
a transcription factor that binds
lactose so that E. coli in the
gut tend to turn on lactose
metabolism when they sense the
presence of lactose (for example,
after the mammalian host drinks
milk). The structures of the
genes and proteins that
constitute this molecular switch
have been resolved at atomic
resolution and their functional
interactions have been
exhaustively investigated. E. coli
exhibit a preference for glucose
over lactose. When they sense
the availability of glucose, the
lactose genes are repressed.
This mechanism involves
additional transcription factors
that are controlled by
a regulatory module that is


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Sensory Ecology: See Me, Hear Me

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