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Representational signalling in birds
Christopher S Evans
Linda Evans
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Biol. Lett. (2007) 3, 811
doi:10.1098/rsbl.2006.0561
Published online 14 November 2006
Representational signalling in birds
Christopher S. Evans* and Linda Evans
Centre for the Integrative Study of Animal Behaviour,
Macquarie University, Sydney, New South Wales 2109, Australia
*Author for correspondence ().
Some animals give specific calls when they
discover food or detect a particular type of predator.
Companions respond with food-searching
behaviour or by adopting appropriate escape
responses. These signals thus seem to denote
objects in the environment, but this specific
mechanism has only been demonstrated for
monkey alarm calls. We manipulated whether
fowl (Gallus gallus) had recently found a small
quantity of preferred food and then tested for a
specific interaction between this event and their
subsequent response to playback of food calls. In
one treatment, food calls thus potentially
provided information about the immediate
environment, while in the other the putative message was
redundant with individual experience. Food calls
evoked substrate searching, but only if the hens
had not recently discovered food. An identical
manipulation had no effect on responses to an
acoustically matched control call. These results
show that chicken food calls are representational
signals: they stimulate retrieval of information
about a class of external events. This is the first
such demonstration for any non-primate species.
Representational signalling is hence more
taxonomically widespread than has previously been
thought, suggesting that it may be the product of
common social factors, rather than an attribute of
a particular phylogenetic lineage.
1. INTRODUCTION
Some animal calls have the unusual property of seeming
to denote environmental events. Such referential signals
are produced in response to specific stimuli (e.g.
approach of a particular predator and discovery of
food) and are sufficient to evoke from companions the
full suite of appropriate responses (e.g. adaptive escape
behaviour and food search). Given the cognitive
sophistication implied by such systems, it was logical for initial
research to concentrate on non-human primates,
beginning with vervet monkeys (Cercopithecus aethiops;
Seyfarth et al. 1980). A flurry of recent papers has
revealed that referential signalling may be relatively
widespread. For example, it is also present in other
cercopithecines (Zuberbu hler 2000a,b, 2001), tufted
capuchins (Cebus apella nigritus; Di Bitetti 2003),
lemurs (Macedonia 1990), at least one non-primate
mammal (suricates (Suricata suricatta); Manser 2001;
Electronic supplementary material is available at http://dx.doi.org/
10.1098/rsbl.2006.0561 or via http://www.journals.royalsoc.ac.uk.
Manser et al. 2001) and several species of birds,
including fowl (Evans et al. 1993; Evans & Marler
1994; Evans & Evans 1999), ravens (Bugnyar et al.
2001), yellow warblers (Dendroica petechia; Gill & Sealy
2004) and black-capped chickadees (Poecile atricapilla;
Templeton et al. 2005).
Referential signalling is controversial because it
potentially extends the parallels between animal
communication and language (Hauser 1996; Evans 1997;
Fitch 2005), long considered the principal exception to
an otherwise clear pattern of evolutionary continuity
(Darwin 1871). Some linguists accept that there is
evidence for a primitive type of reference in animal
communication (Bickerton 1990; Pinker 1994); others
stress the apparent lack of volitional control and
conclude that such analogies are not compelling
(Lieberman 1994). Similarly, some biologists have
objected that referential signals may reveal only the
subsequent behaviour of the sender (Smith 1991) or
that it is not useful to think of animal signals as
containing information at all (Owings & Morton 1998).
In sum, conventional studies of call production and
playback experiments can establish only that animals
behave as if their signals describe external events.
The central issue in this debate is both
straightforward and empirically accessible: it concerns the
cognitive processes that must be invoked to explain the
observed pattern of receiver behaviour. Words derive
their meaning from mental representations that
correspond to stimulus categories. If referential signals and
language are truly analogous, then they should similarly
evoke representations of the eliciting event (i.e.
stimulate retrieval of stored information that then determines
receiver response). This property would correspond to
Gallistels nominal representation (1990), which is the
lowest level of cognitive complexity: it would establish
that calls stand for something in the environment. The
design of most previous st (...truncated)