Dogs' capacity to 'catch' human yawns has
recently attracted the attention of researchers
in the field of animal cognition. Following
recent studies suggesting that contagion yawning
in humans, and some other primates, is
empathy-related, some authors have considered
the possibility that the same mechanism may
underlie contagious yawning in dogs. To date,
however, no positive evidence has been found,
and more parsimonious hypotheses have been put
forward. The present study explored the
'contagion-only' hypothesis by testing whether
the mere sound of a human yawn can be sufficient
to elicit yawning in dogs, in a way that is
unaffected by social-emotional factors.
Unexpectedly, results showed an interesting
interplay between contagion and social effects.
Not only were dogs found to catch human yawns,
but they were also found to yawn more at
familiar than unfamiliar yawns. Although not
allowing for conclusive inferences about the
mechanisms underlying contagious yawning in
dogs, this study provides first data that
renders plausible empathy-based, emotionally
connected, contagious yawning in these
animals.
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Introduction
In recent years, a renewed interest in the
study of contagious yawning has been growing
among researchers from various Welds of
investigation (see Campbell and de Waal 2010),
mostly because of a theoretical link with
empathy that has been supported empirically.
Platek et al. (2003) showed that humans who
performed better at self-recognition and
theory-of-mind, two abilities that contribute to
complex empathy, performed more contagious
yawning. More recently, Palagi et al. (2009)
found that yawn contagion in gelada baboons is
best predicted by emotional closeness to the
yawner&emdash;which is consistent with the
observation that empathic tendencies are
strongest or most likely to arise, as a function
of familiarity (Preston and de Waal 2002; de
Waal 2008)&emdash;and Campbell and de Waal
(2011) demonstrated an ingroup-outgroup bias in
contagious yawning in chimpanzees.
Apart from primate species [(humans
(Provine 1986, 1989; Anderson and Meno 2003;
Senju et al. 2007), chimpanzees (Anderson et al.
2004), stump-tail macaques (Paukner and Anderson
2006), and gelada baboons (Palagi et al.
2009)], contagious yawning has, to date,
only been demonstrated in dogs. In 2008,
Joly-Mascheroni et al. published a study in
which twenty-one out of twenty-nine dogs yawned
after seeing a human experimenter acting full
yawning movements with vocalizations while none
yawned after seeing that same experimenter
displaying non-yawning mouth opening actions
without vocalizations. Even though, in more
recent studies, Harr et al. (2009), and also
O'Hara and Reeve (2010), found very limited
evidence for contagious yawning in dogs, it has
been recognized that at least some individuals
do yawn contagiously at humans. Interestingly, a
Wrst attempt to test the empathy-based,
emotionally connected, contagious yawning in
dogs found no evidence in support of the
prediction that dogs should be more likely to
yawn contagiously at familiar than unfamiliar
humans (see O'Hara and Reeve 2010). Instead, it
was suggested that if dogs 'catch' human yawns,
then contagion might rely on a less cognitively
stringent mechanism than empathy, such as some
behavioral Wxed action pattern that is
hard-wired and simply needs a releasing stimulus
(Yoon and Tennie 2010). The present study aimed
at further exploring this so-called
'contagion-only' hypothesis by testing whether
an isolated presentation of the sound of a human
yawn may be suYcient to induce yawning in
domestic dogs, in an 'encapsulated way that is
unmodulated by social contextual factors'
[as proposed by Yoon and Tennie (2010)].
Interestingly, results were obtained that add
more conXicting data to the debate on the
possible mechanism underlying the phenomenon of
dogs yawning contagiously at humans.
Discussion
Besides showing that the mere sound of a
human yawn can elicit yawning in dogs, obtained
results also highlight the eVect of a social
variable (familiarity/unfamiliarity) on the
level of contagion, with dogs yawning more at
familiar yawns than at unfamiliar ones.
Interestingly, obtained data on attention seems
to rule out the possibility that attention
diVerences per se may have accounted for the
observed pattern of yawning responses. It could
have been that the voice of the owners attracted
more attention orienting and/or engagement,
which prompted dogs to process familiar yawns
more deeply or eVectively. Results, however,
show that dogs seemed to pay similar attention
to both the familiar and the unfamiliar yawn
stimuli [for a similar observation in
chimpanzees, see Campbell and de Waal
(2011)].
What does the observed social bias suggest
about the mechanism underlying the phenomenon of
dogs yawning contagiously at humans? Social
modulation of contagious yawning, as observed in
primate species [gelada baboons (Palagi et
al. 2009) and chimpanzees (Campbell and de Waal
(2011)], has been interpreted as supporting
the suggesting that catching another's yawn may
be an empathic response. The idea that dogs, as
a domestic species living in close contact with
humans, may be capable of some level of empathy
toward people is not new. According to
Joly-Mascheroni et al. (2008), it is conceivable
that these animals have developed the capacity
to represent humans' actions and to modulate
their own behavioral and autonomic responses
accordingly, and that this underlies contagious
yawning [see also Topál et al. (2009)
and Silva and Sousa (2011) for an extensive
theoretical discussion on dogs' potential to
empathize with humans].
If empathy is the mechanism underlying
contagious yawning also in dogs, then, how can
we explain the conXicting results presented by
O'Hara and Reeve (2010), which showed no
evidence for empathy-based, emotionally
connected yawning contagion in dogs? According
to Campbell et al. (2009), the Wrst challenge in
discussing results across studies on contagious
yawning relates to the methodological details
that diVer between studies and to the fact that
these details have not been studied for their
eVect on contagious yawning. To test for social
modulation of contagious yawning, O'Hara and
Reeve (2010) exposed dogs to familiar and
unfamiliar yawns, presented by a live model or
in an LCD screen, and compared the number of
elicited yawns between conditions that diVered
only in familiarity/ unfamiliarity. It is
important to note, as it contrasts with the
procedure follow in the present study, that,
during experimental trials, dogs tested by these
authors were encouraged to focus on these visual
stimuli, meaning that there was some human-dog
interaction that may have diverted the animals'
attention.
But what if there is no unitary underlying
releasing mechanism across all species capable
of yawning contagiously and the phenomenon of
dogs yawning contagiously at humans is indeed
not empathy-related [as suggested by the
results pointed out in O'Hara and Reeve's study
(2010)]? It is known that dogs can actively
generate an internal representation of the
owner's face when they hear the owner's voice
(Adachi et al. 2007). Therefore, if one
considers that it may be the perceptual image of
yawn that triggers yawning (see Sarnecki 2008),
then the explanation for the social bias
observed here could be diVerences in the
capacity to form mental representations from
familiar and unfamiliar auditory input. That is,
the perceptual image generated from hearing
unfamiliar yawns would then be more obscure and
less deWned than that generated from hearing
familiar stimuli, thus eliciting less
yawning.
Clearly, there is a need for future research
to focus on the key variables in the
presentation of yawns, as this will facilitate
comparisons across studies (see Campbell and de
Waal 2010) and will allow Wrm conclusions to be
drawn as to whether or not contagious yawning in
dogs is empathyrelated. If it emerges that it
is, then giving the growing range of roles being
played by dogs in human society (as service and
therapy dogs, for example), it could turn out a
useful complementary tool for selecting the most
appropriate dogs (in terms of empathic
processing) for speciWc tasks. Finally, it would
also be important that future studies explore
the potential eVects of a vast range of
individual traits (from basic life history to
temperament and degree of attachment to humans)
on dogs' susceptibility to contagion. Following
Sarnecki's view (2004), a theory of yawning
contagion should explain not merely why human
yawns are contagious to dogs, but also why, for
some individuals, they are not.