Evidence of an evolutionary precursor to human language affixation in a non-human primate

Abstract

Human language, and grammatical competence in particular, relies on a set of computational operations that, in its entirety, is not observed in other animals. Such uniqueness leaves open the possibility that components of our linguistic competence are shared with other animals, having evolved for non-linguistic functions. Here, we explore this problem from a comparative perspective, asking whether cotton-top tamarin monkeys (Saguinus oedipus) can spontaneously (no training) acquire an affixation rule that shares important properties with our inflectional morphology (e.g. the rule that adds -ed to create the past tense, as in the transformation of walk into walk-ed). Using playback experiments, we show that tamarins discriminate between bisyllabic items that start with a specific 'prefix' syllable and those that end with the same syllable as a 'suffix'. These results suggest that some of the computational mechanisms subserving affixation in a diversity of languages are shared with other animals, relying on basic perceptual or memory primitives that evolved for non-linguistic functions.

Full text

Animal behaviour
Evidence of an evolutionary
precursor to human
language affixation in a
non-human primate
Ansgar D. Endress*, Donal Cahill, Stefanie Block,
Jeffrey Watumull and Marc D. Hauser
Department of Psychology, Harvard University, Cambridge, MA 02138, USA
*Author for correspondence (ansgar.endress@m4x.org).
Human language, and grammatical competence
in particular, relies on a set of computational
operations that, in its entirety, is not observed
in other animals. Such uniqueness leaves open
the possibility that components of our linguistic
competence are shared with other animals,
having evolved for non-linguistic functions.
Here, we explore this problem from a compara-
tive perspective, asking whether cotton-top
tamarin monkeys (Saguinus oedipus) can spon-
taneously (no training) acquire an affixation
rule that shares important properties with our
inflectional morphology (e.g. the rule that adds
–ed to create the past tense, as in the transform-
ation of walk into walk-ed). Using playback
experiments, we show that tamarins discriminate
between bisyllabic items that start with a specific
‘prefix’ syllable and those that end with the same
syllable as a ‘suffix’. These results suggest that
some of the computational mechanisms subser-
ving affixation in a diversity of languages are
shared with other animals, relying on basic per-
ceptual or memory primitives that evolved for
non-linguistic functions.
Keywords: Animal cognition; evolution of language;
morphology; language acquisition
1. INTRODUCTION
While it is clear that only humans have a language fac-
ulty, it is less clear which components of this system are
unique to humans, and which unique to language. In
fact, although attempts to teach non-human animals
to produce simplified languages have largely failed
(Te r r a c e et al. 1979;Savage-Rumbaugh et al. 1993),
and studies of their natural communication show
only weak evidence of homologous or analogous abil-
ities (Hauser 1996;Liebal et al. 2004;Cheney &
Seyfarth 2005;Arnold & Zuberbu
¨hler 2006;Suzuki
et al. 2006), different animals show perceptual compe-
tences that may well feed into language processing in
humans (Kuhl & Miller 1975;Kluender et al. 1987;
Ramus et al. 2000).
Here, we build on the above tradition exploring
aspects of perceptual competence, asking whether ani-
mals have non-linguistic abilities that are necessary for
some forms of language-specific, grammatical compu-
tations (Hauser et al. 2001;Fitch & Hauser 2004;
Gentner et al. 2006;Murphy et al. 2008). We start
from the observation that, across the world’s
languages, morphological transformations adding
verbal material to the word-edges (i.e. prefixation
and suffixation) are much more frequent than trans-
formations adding verbal material in other positions
(Greenberg 1957). For example, the English past par-
ticiple is formed by adding the ‘ed’ suffix to the end of a
stem (as in talk-ed ), while the German past participle is
formed by adding the ‘ge’ prefix to the beginning of a
stem and either the ‘en’ or the ‘t’ suffix to its end (as
in ge-sag-t, ‘said’). In these and other languages,
word-edges appear well suited for some linguistic
transformations (Nespor & Vogel 1986;McCarthy &
Prince 1993).
Here we ask whether a non-human animal—the
cotton-top tamarin monkey—has the requisite mech-
anisms for learning formally similar prefixation and
suffixation patterns. Our goal, therefore, is not to
show that animals such as tamarins have language,
but rather, that certain components of our expressed
languages rely on domain-general mechanisms of
learning and memory that are likely to be shared
with other animals, including, we suggest, the capacity
to extract patterns of temporal ordering.
In brief, we exposed subjects to a sequence of bisyl-
labic items conforming to a common pattern. For
example, they heard a sequence of ‘stem’ syllables all
preceded by the same prefix syllable. Following this
familiarization, they were exposed to new bisyllabic
items. Half were preceded by the same prefix syllable
as during familiarization, and half were followed by
that syllable, and thus violated the familiarization
pattern. We asked whether tamarins would respond
more to bisyllabic items violating the familiarization
pattern than to items consistent with it.
2. MATERIAL AND METHODS
The detailed methods are described in Hauser et al. (2001); here, we
highlight only critical differences.
(a)Participants
We tested 14 adult tamarins (seven males; mean age 8.2 years)
socially housed in a colony room. For medical reasons, one subject
completed only the suffixation condition, and one only the prefixa-
tion condition.
(b)Materials
We used naturally recorded syllables as stimuli from native speakers
of American English. The affix syllable was always ‘shoy’ uttered by a
male speaker. The familiarization stems (see below) were ‘bi, ka, na,
to, gu, lo, ri and nu’, pronounced by a female speaker, and ‘ba, pu,
di, ki, lu, ro and mo’ pronounced by a male speaker with a lower
voice than that of the speaker of the affix syllable. We used a mixture
of different speakers of different genders to prevent subjects from
using low-level cues (such as pitch differences between vowels) for
their generalizations.
The test stems were the syllables ‘brain, breast, wasp, snake
and swan’, all pronounced by a different female speaker; we used
words because speakers found it easier to read English words than
phonemic transcriptions.
Syllables were recorded individually, normalized to a duration of
400 ms and then RMS amplitude normalized.
(c)Design
We first familiarized subjects to bisyllabic items conforming to either
a prefixation or suffixation pattern, and then tested them on new
items that either violated or were consistent with the familiarization
pattern. Our dependent measure was an orienting response (see
below) towards the speaker playing back a test item. Based on
prior work using the same method, we predicted that tamarins
would orient more to violations of the familiarization pattern than
to items consistent with it.
Biol. Lett. (2009) 5, 749–751
doi:10.1098/rsbl.2009.0445
Published online 8 July 2009
Received 5 June 2009
Accepted 19 June 2009 749 This journal is q2009 The Royal Society