Chimpanzees
show a developmental increase in susceptibility
to contagious yawning: a test of the effect of
ontogeny and emotional closeness on yawn
contagion
Madsen EA, Persson T, Sayehli S, Lenninger
S, Sonesson G.
Department of Cognitive
Science, Lund University, Lund, Sweden ;
Department of Semiotics, Centre for Languages
and Literature, Lund University, Lund, Sweden.
-Madsen EA,
Persson T. Contagious yawning in domestic
dog puppies (Canis lupus familiaris): the effect
of ontogeny and emotional closeness on low-level
imitation in dogs. Anim Cogn.
2013;16(2):233-240
Abstract
Contagious yawning has been reported for
humans, dogs and several non-human primate
species, and associated with empathy in humans
and other primates. Still, the function,
development and underlying mechanisms of
contagious yawning remain unclear. Humans and
dogs show a developmental increase in
susceptibility to yawn contagion, with children
showing an increase around the age of four, when
also empathy-related behaviours and accurate
identification of others' emotions begin to
clearly evince.
Explicit tests of yawn contagion in
non-human apes have only involved adult
individuals and examined the existence of
conspecific yawn contagion. Here we report the
first study of heterospecific contagious yawning
in primates, and the ontogeny of susceptibility
thereto in chimpanzees, Pan troglodytes verus.
We examined whether emotional closeness, defined
as attachment history with the yawning model,
affected the strength of contagion, and compared
the contagiousness of yawning to nose-wiping.
Thirty-three orphaned chimpanzees observed an
unfamiliar and familiar human (their surrogate
human mother) yawn, gape and nose-wipe.
Yawning, but not nose-wiping, was contagious
for juvenile chimpanzees, while infants were
immune to contagion. Like humans and dogs,
chimpanzees are subject to a developmental trend
in susceptibility to contagious yawning, and
respond to heterospecific yawn stimuli.
Emotional closeness with the model did not
affect contagion. The familiarity-biased social
modulatory effect on yawn contagion previously
found among some adult primates, seem to only
emerge later in development, or be limited to
interactions with conspecifics. The influence of
the 'chameleon effect', targeted vs. generalised
empathy, perspective-taking and visual attention
on contagious yawning is discussed.
Introduction
Contagious yawning (henceforth, CY) is
well-established in humans
[1]&endash;[5]. Viewing videos
of others yawning elicits CY in approximately
half of adults, and the thought [1] and
sound [5] of yawning is sufficient to
elicit contagion. Yawn contagion has also been
reported in chimpanzees, Pan troglodytes spp.
[6]&endash;[8], bonobos, Pan
paniscus [9], gelada baboons,
Theropithecus gelada [10], domestic
dogs, Canis lupus familaris [11],
[12] and budgerigars Melopsittacus
undulatus [14]. While CY has been
reported for stumptailed macaques, Macaca
arctoides, elevated levels of concomitant
self-directed scratching, suggested that the
yawns derived from tension, rather than
contagion [15]. The only solitary
species tested for CY, the tortoise, Geochelone
carbonaria [16] has shown no evidence of
contagion. While the ultimate function of
yawning remains disputed (for a review, see
[17]), it has been suggested to carry
thermorgulatory [18] and non-verbal
communicative functions, and its contagiousness
to serve the adaptive function of synchronizing
group behaviour [19], with respect to
arousal [20] and attention [21].
On a proximate level, the facial expressions
that individuals adopt, tend to influence their
emotional experiences (e.g. [22],
[23]), suggesting that yawn contagion
allows individuals to automatically mimic and
synchronise facial expressions and movements
with others, and consequently converge
behaviourally and emotionally [24].
Research to uncover the underlying
mechanisms of CY has suggested that it may be
linked to and modulated by empathy (e.g.
[4], [7], [9],
[10], [11], [13],
[24], [25]), represent a case of
non-conscious mimicry (the 'chameleon effect')
[27] or a fixed action pattern
[1]. Empathy refers to a spectrum of
interacting emotional and cognitive reactions to
the experiences of others, and is mediated by at
least two separate systems: the ability to feel
and to imagine others' emotional experiences.
The ability to feel others' emotional
experiences ('affective empathy') derives from a
phylogenetically old social contagion system,
whereby one is viscerally affected by another's
emotional or arousal state (e.g. [28]).
Affective empathy is a largely automatic process
and may come about through emotional contagion,
whereby the perception of expressive behaviour
transfers emotional states from one individual
to another (i.e. the tendency to 'catch'
emotions from observed emotional states of
others: [29], see also [30],
[31]). In contrast, the 'cognitive
empathy system' (also termed empathetic
perspective-taking: [28]) entails
imagining another's emotional experience. This
system emerges phylogenetically and
ontogenetically with other 'indicators of mind'
and requires a capacity for self-other
differentiation, perspective-taking and mental
state attribution, without necessarily resulting
in emotional matching [28],
[32]&endash;[37]. There is ample
evidence that the affective and cognitive
components of empathy dissociate in humans and
have different developmental trajectories, and
that affective empathy precedes cognitive
empathy, ontogenetically and phylogenetically
[33],
[38]&endash;[40].
In human adults CY correlates with
self-reported measures of empathy and is
positively related to visual self-recognition
and performance on theory-of-mind tasks
[4], abilities considered constituent
parts of cognitive empathy (e.g. [36]).
Furthermore, research has indicated that CY is
reduced in individuals with empathy-related
disorders (e.g. schizotypy [4] and
autism [2], [41], [42]).
Recent evidence suggests that affective and
cognitive empathy may dissociate in
psychopathological populations, such as
individuals with autism spectrum disorder (ASD),
who seem primarily impaired in cognitive, but
not affective, empathy [38],
[39]. Since autistic children have
reportedly failed to evince CY [2],
[41], [42], this has contributed
to the view that CY is linked to cognitive
empathy and theory-of-mind capacities (e.g.
[4], [43]). Recent research has,
however, shown that when children with ASD have
been instructed to fixate on a yawner's eyes,
they yawn contagiously with a frequency equal to
that of typically developing children
([44] see also [45]). The
previously found association between yawn
contagion, autism, and failure on tests of
cognitive empathy and theory-of-mind may
therefore be one of correlation, rather than
causation, and rely on differences in
attentional states. Moreover, while the
association between mental state attribution and
CY in adults [4] and the temporal
emergence of CY in children (around 4 years
[3], [42], [46]) is
consistent with the suggestion that CY shares a
basis with cognitive empathy and theory-of-mind
[4], [43], evidence of CY in a
number of non-human species
[6]&endash;[14], not typically
associated with cognitive empathy, suggests that
the phenomenon is underlain by lower-level
processes.
Alternatively, CY may be unrelated to
empathy per se, but rely on non-conscious
mimicry, also termed the 'chameleon effect'
[27]. The 'chameleon effect' refers to
an individual's tendency to mimic a social
partner's behaviours (postures, gestures and
facial expressions etc.) without either
individual's awareness or intent [47].
The 'chameleon effect' has affective and
behavioural consequences for the subsequent
interactions of the involved individuals in
terms of increased levels of affinity, liking,
empathy and prosocial behaviours
[48]&endash;[50]. Moreover,
conversely, in humans social motivations, such
as the desire to affiliate or bond with another,
modulate non-conscious mimicry [47].
Studies thus converge to suggest that mimicry
serves a prosocial function (to smooth social
interaction), and that the relationship between
non-conscious mimicry and affiliation is
bidirectional: non-conscious mimicry fosters
affiliation, and affiliation fosters
non-conscious mimicry [51]. There is
evidence that the 'chameleon effect' is not
limited to humans, as capuchin monkeys (Cebus
apella) affiliate more with humans, who have
previously imitated them [52]. Moreover,
the 'chameleon effect' operates in children as
young as 18 months of age, for whom being
mimicked increases pro-social behaviour
[50]. CY however, does not emerge in
children until around 4 years of age, suggesting
that the 'chameleon effect' is unlikely to
underlie contagious yawning per se. The
'chameleon effect' may nonetheless modulate CY,
once the capacity is in place.
An even lower level mechanism, a fixed
action pattern [53] has been suggested
to underlie CY [1]. According to this
hypothesis, CY relies on a specific, fixed and
unlearned behavioural action pattern, for which
the releasing stimulus is another's yawn. The
hypothesis is supported by the fact that yawning
follows a stereotyped pattern [1] and
may be triggered by multiple (minimal) forms of
stimuli independently (e.g. observing, hearing
and thinking about others yawning
[1]&endash;[5]). While fixed
action patterns may have moderators, evidence
that CY is modulated by a social variable
(familiarity/social bonding [7],
[9], [10], [13],
[26]), has led to the suggestion that CY
requires more complex mechanisms. Nonetheless,
the fixed action pattern, empathy and 'chameleon
effect' hypotheses are not mutually exclusive
explanations for the mechanisms underlying
CY.
Adult humans, chimpanzees, bonobos and
gelada baboons have shown a social modulatory
effect on the strength of contagion. For humans
cross-cultural observational data have shown the
CY effect to be stronger in response to the
yawns of kin, then friends, then acquaintances,
and lastly strangers [26]. Similarly,
for bonobos and gelada baboons, CY correlates
with social bonding [9], [10].
Studies of chimpanzees present a more complex
picture. Chimpanzees yawn contagiously in
response to videos of yawns by familiar, but not
unfamiliar conspecifics (i.e., non-group members
[7]). In contrast to bonobos
[9], baboons [10] and humans
[26]), chimpanzee susceptibility to yawn
contagion does, however, not appear affected by
relationship quality with familiar conspecifics
(as indexed by grooming and proximity patterns
[8]). While this may suggest that yawn
contagion in chimpanzees is influenced by
familiarity (group membership), but not
relationship quality (with in-group members),
the methodologies used may account for the
differences found across species. While the
positive effect of relationship quality on CY in
bonobos, baboons and humans has been established
through observational studies of spontaneous
yawns, the negative findings for chimpanzees
derived from projecting videos of yawning
group-members on a wall. Videos were presented
to multiple individuals simultaneously,
including sometimes to the individual depicted
in the video. The results may thus have been
influenced by the medium, the attentional states
of observing chimpanzees, and the likelihood of
others yawning to the stimuli. In contrast the
study, in which chimpanzees were found to yawn
contagiously to familiar, but not unfamiliar,
conspecifics, presented video stimuli
individually, in a context where attentional
focus was ensured [7]. The order of
presentation of the in- and out-group chimpanzee
yawn stimuli may, however have produced
carry-over effects, as all subjects first viewed
videos of familiar in-group members yawning,
meaning that less attention may have been paid
to the out-group yawn stimuli, viewed at a later
point. The difference in CY to familiar and
unfamiliar conspecific yawns may thus lie in
attentional states, and the results await
additional analyses or replication. Ascertaining
whether CY in chimpanzees is influenced by
relationship quality, or only by a less
fine-grained in-group/out-group effect requires
further research.
The literature on a potential social
modulatory effect on CY in dogs is also
contradictory, with the authors of one study
suggesting that dogs exhibit auditory CY to the
sound of only familiar yawns [54] (for a
criticism of the study, see [12]), while
other studies have demonstrated CY to strangers
during live interactions [11],
[12], and found no evidence of
familiarity-biased contagion in young
[12] and adult dogs ([55], for a
methodological criticism of this study, see
[13]). A recent study, in which dogs
were tested under low-stress conditions, however
suggests that for adult dogs CY is correlated
with level of emotional proximity to the model
[13].
Overall, as predicted by the
perception-action model [33], studies
suggest that familiarity increases
state-matching [33], [56] and
for primates, contagious yawning [7],
[9], [10], [26]. While,
however, the familiarity effect on CY has
broadly been interpreted as support for the
hypothesis that CY is modulated by empathy, the
studies reviewed cannot distinguish whether the
modulating variable for CY is empathy
(operationalised as degree of social bonding
with the original yawner) or the 'chameleon
effect' (non-conscious imitation to smooth
social interaction and cement
relationships).
Thus far, social modulation of yawn
contagion has only been demonstrated in adult
individuals. In neither human children
[46], nor young domestic dogs
[12] is the emergence, or strength of
yawn contagion influenced by familiarity with
the yawning model, which has lead to the
suggestion that the social modulatory effect in
adult primates (including humans) only emerges
at later stages of development ('developmental
hypothesis of empathy modulation of CY'
[12]).
Humans and dogs show a developmental trend
in susceptibility to yawn contagion, which does
not become prominent until around 4 years of age
in humans [3], [42],
[46] and 7 months in dogs [12].
Naturalistic observations suggest a similar
developmental trend in gelada baboons, for which
a study found that four infants exhibited few
yawn responses to nearby adults' yawning
[10]. In humans CY emerges
developmentally at the time when also
cognitive-empathy-related behaviours begin to
clearly manifest, and children begin to show an
increase in the ability to correctly identify
the emotions of others (e.g.
[57]&endash;[61]). Thus far,
only chimpanzees above the age of 10 yrs. have
been explicitly tested for yawn contagion, with
some suggestion of a potential age effect. This
indication derives from a study, in which
videoed stimuli of conspecific yawns elicited CY
in only the two oldest individuals tested (26
and 27 yrs.), while not in four younger
individuals (21&endash;26 yrs.), nor in any of
three infants accompanying their mothers to the
test [6].
All previous tests of CY in chimpanzees have
deployed videoed [6]&endash;[8]
or computer animated stimuli of conspecific
yawns [62]. There is, however, some
indication that the medium may mask the
'message' for younger and non-human subjects.
For example, live models have elicited CY in
four-year-old children (35% of 4 yr. olds tested
[42]), while neither videos, nor
stories, in which the protagonist repeatedly
yawned, have evoked CY in children below 5 yr.
[3]. Moreover, while dogs have shown CY
in three of four experiments involving live
models [11], [12], [13],
[55], they have failed tests involving
videoed (conspecific and human) yawn stimuli
[55], [63].
Some authors have suggested that differences
in susceptibility to yawn contagion may owe to
an attention bias, whereby observers pay closer
attention to affiliated familiar individuals
than non-affiliated ones [27]. While
attention differences have been proffered to
account for the apparent association between
yawn contagion and empathy (operationalized as
social bonding, relationship quality and
familiarity: [7], [9],
[10], [13], [26]), it
also applies to findings of a developmental
progression of susceptibility to CY in children
[3], [42], [46] and dogs
[12]. Young individuals may simply pay
less attention to others' physical and emotional
states, than older individuals do.
In this study we examined the extent to
which two factors affect chimpanzees'
susceptibility to yawn contagion: ontogeny
(their age, infants and juveniles), and
emotional closeness to the yawning model. While
attention levels are difficult to certify and
quantify, we ensured that yawns were
perceivable, by presenting them dependent on the
chimpanzees' attentional focus (and repeating
yawns if initially presented outside the
chimpanzees' field of vision). We sought to
increase ecological validity and the chance of
evoking CY in younger subjects by using live
rather than videoed models. For practical
reasons we consequently used human models, and
the study thus represents the first test of
interspecies yawn contagion in primates. We
hypothesised that, if CY is related to the
development of empathy and emotional
understanding in humans, a similar developmental
effect might be found in chimpanzees. We
therefore predicted that (1) juveniles would be
more susceptible to CY than infants. As model
identity and empathy may facilitate social
behaviours, such as non-conscious facial mimicry
[64] and imitation [65]), we
presented the chimpanzees with a familiar
yawning model, that they had a strong and
positive emotional relationship with (their
surrogate human mother) and an unfamiliar model.
We predicted that (2) the chimpanzees would
be more likely to yawn contagiously to the
familiar than unfamiliar model, and that the
familiar model would evoke more CY in the
infants. We moreover predicted (3) yawn
frequency to increase in response to viewing a
human model performing repeated yawns, but
neither of two control behaviours, nose-wiping
and gaping, nor when the model performed none of
the three behaviours (baseline phase). Previous
studies have deployed various control behaviours
(gapes, laughs, smiles and coughs, see
[66]). Our key control behaviour,
gaping, however has the advantage of including
much of the motor pattern of a yawn, while
remaining an arbitrary expression. If CY is an
emotional contagion [24], reflecting
perception and internalisation of the emotion
and/or physical state that another's yawning
reflects (for anecdotal observations of this in
dogs, see [12]), only yawn stimuli
should evoke yawning. A comparison of the rate
of yawing in response to yawn and gape stimuli
thus goes some way to exclude a more motoric,
reflexive interpretation of CY.
We also examined the chimpanzees' responses
to viewing a human nose-wipe, as this is a
facially oriented action, which occurs
frequently and spontaneously in chimpanzee
behaviour. Since it can be a marker of
nervousness [67], or itchiness, it may
also correlate with potentially contagious
emotions. While there is some suggestion that
behaviours other than yawning are also
contagious (e.g. laughter in chimpanzees
[68] and humans [69], itching in
humans [70], [71] and stretching
in budgerigars [14]), there is no
empirical evidence to suggest that such
behaviours are underpinned by individual levels
of empathy, or empathy with the observed model
[71]. It is, however, conceivable that
such effects may be underpinned by lower-level
processes, such as a perception-action mechanism
[33] or proto-mimesis (the matching of
exteroception and proprioception based on
mirror-neuron systems: [72]). We
therefore remained agnostic, as to whether a
potential contagion effect for yawning and
nose-wiping would be similar or different.
Finally, we strove to minimise the possibility
of evoking 'tension yawns' by engaging the
chimpanzees in a bouts of calm play and cuddling
through the bars of their enclosure.
Discussion
The current study is the first to
demonstrate the existence of cross-species
contagious yawning in chimpanzees. It further
shows that, like humans [3],
[42] and dogs [12], chimpanzees
are subject to a developmental increase in
susceptibility to yawn contagion. Viewing a
human yawn elicited yawning in 48 per cent of
juvenile chimpanzees, while infants were immune
to contagion (no infant yawned in response to
human yawns). This age effect is consistent with
the suggestive results of a previous study
failing to elicit CY in three infant chimpanzees
accompanying their mothers, when these viewed
videoed conspecific yawns [6].
Regardless of the medium (live or videoed),
infant chimpanzees seem immune to yawn
contagion. The results, moreover, suggest that
heterospecific yawn contagion in dogs
[11], [12] is not necessarily a
function of the social domestication of this
species, but that contagion in chimpanzees and
dogs may reflect either prolonged ontogenetic
interaction with humans or general attention to
the emotional and/or physical states of others,
regardless of the species, to which they
belong.
There is increasing evidence that mirror
neuron networks may be recruited for yawn
contagion, although the extent thereof remains
debated [5], [45],
[75]&endash;[79]. While however,
children [80], chimpanzees [81]
and macaques, Macaca mulatta [82]
exhibit neonatal imitation (thought to be
underpinned by mirror neurons [83]),
young chimpanzees and children are not subject
to yawn contagion (children reach adult-like
levels only by around 12 years of age
[3]). Whether CY may be part of the
group of 'contagious' mouth behaviours that
occur during the short window of neonatal
imitation (and subsequently disappears)
nonetheless remains untested. If CY does not
appear during the restricted time span of
neonatal imitation, this suggests that CY
involves, at least in part, other mechanisms
than mirror neurons, or that the action
understanding, that mirror neurons underpin, may
improve with experience (see [84]), and
thus age.
The distribution of CY across and within
species has been suggested to be consistent with
the claim, that yawn contagion shares its
mechanism with the capacity for theory-of-mind
[4], [43]. Nonetheless, yawn
contagion in young dogs (from 7 mts of age) has
been interpreted as evidence suggesting that CY
is underlain by processes less complex than
cognitive empathy [12], which is
considered to involve theory-of-mind-like
attribution and perspective-taking. Indeed,
there is little empirical evidence of
theory-of-mind attribution in young dogs or
juvenile chimpanzees. Interestingly, there is
some evidence of perspective-taking in young
dogs at 8 mts of age [85], and in
chimpanzees the emergence of CY coincides with
that of perspective-taking (around 4.5 years, as
examined by performance on the 'guesser-knower'
paradigm [86]).
Overall, the existence of CY in non-human
species, and the development of CY in humans and
other species, is consistent with the notion
that the development of affective empathy, and
an emerging capacity for perspective-taking (as
well as increased attention to and improvement
in the identification of others' affective
states) is sufficient to explain the
distribution of yawn contagion, ontogenetically
and phylogenetically. The comparatively late
emergence of CY relative to evidence of
affective empathy in children below 4 years of
age, suggests that CY may rely on the
development of other and interacting capacities
(such as perspective-taking, attention to and
identification of others emotional states).
While CY co-emerges temporally with cognitive
empathy in children [3], [42])
and correlates with adult performance on
theory-of-mind-tasks in humans [4], CY
is not a reliable marker of the presence of
cognitive empathy in a species. A lower-level
hypothesis suggests that susceptibility to yawn
contagion relies on attentional states and
biases [27]. Experimental studies
demonstrating a developmental effect on yawn
contagion in non-human animals (the present
experiment and [12]) have, however,
controlled for the attentional focus of both
younger and older individuals.
On a proximate level, the immunity of infant
chimpanzees to CY may indicate a developmental
immaturity of socio-cognitive skills and/or
neural networks involved in processing social
information [10]. That is, it may
reflect developmental changes in
action-understanding (based on mirror neurons,
which appear to acquire their properties through
experience [86]), perspective-taking
and/or attention to and identification of
others' affective states. On an ultimate level,
there may be a less strong selective pressure
for immature individuals to synchronise and
coordinate behaviour with others, given their
lesser roles in group-decision processes.
In contrast to yawning, there was no
contagion effect for nose-wiping. Juvenile
chimpanzees were, however, more likely to wipe
their nose during the 5-min interval, where they
viewed a model nose-wipe, than in the subsequent
5 minutes (a similar, non-significant, trend was
found for yawning). There was however, a
(non-significant) trend for juvenile chimpanzees
to nose-wipe at a higher frequency across all
conditions than infants. Thus, while nose-wiping
might be (low-level) contagious in humans
(similar to itching [70], [71]),
it is not so for chimpanzees, when performed by
a human model. Moreover, in line with recent
research suggesting that human itching is not
related to individual levels of empathy
[71], we found no evidence that
chimpanzee nose-wiping was related to empathy
with the observed model (operationalized as
familiarity).
Contrary to prediction, but consistent with
findings for human children [46], young
dogs [12] and adult chimpanzees
[8], we found no evidence that emotional
closeness with the model increased the
susceptibility of young chimpanzees to CY. While
'liking' a model increases spontaneous,
non-conscious facial mimicry in adult humans
[64] and has been suggested to
facilitate imitation [65], the present
study does not provide evidence in support of
this. In contrast, Chartrand and Bargh
[47] have shown that, in humans, social
motivations, such as the desire to affiliate or
bond with another, modulate non-conscious
mimicry. The (albeit statistically
non-significant) tendency of juvenile
chimpanzees to yawn more in response to the
yawns by the unfamiliar (than familiar) model
may suggest that the 'chameleon effect'
[47] operates in heterospecific yawn
contagion contexts. Young chimpanzees may be
motivated to smooth interactions with unfamiliar
human partners. Overall, the results open for at
least three testable hypotheses: (1) A potential
social modulatory effect on CY emerges only at
later stages of development [12]. While
juvenile chimpanzees (present results) and young
dogs [12] exhibit CY, neither have shown
a social modulatory effect on CY, whereby
familiarity with a human model has influenced
susceptibility to contagion. In contrast, there
is some evidence that CY in adult members of
these species is influenced by familiarity
[7], [54], [13].
Further examination of this hypothesis might
test the effect of model familiarity on CY in
young children, that otherwise do exhibit CY
(i.e. children aged 4&endash;6 yrs.). (2)
Emotional closeness with a model does not affect
CY in chimpanzees. While one study has shown
that conspecific CY in adult chimpanzees is
influence by model familiarity (in- and
out-group membership [7]), another has
failed to evince an effect of relationship
quality with familiar conspecifics [8]
(used as a proxy for emotional closeness). A
further test of the effect of relationship
quality on chimpanzee CY might use observations
of yawns in response to spontaneous conspecific
yawns (a methodology that has shown a
familiarity effect in adult humans [26],
bonobos [9] and baboons [10]).
(3) Heterospecific yawns do not elicit a
familiarity effect on CY in chimpanzees. Adult
chimpanzees have only been tested on yawn
contagion when viewing the yawns of
conspecifics, while young chimpanzees have only
been explicitly tested with respect to
heterospecific yawn contagion. Given that
chimpanzees typically engage in competitive, or
even hostile, relationships with unfamiliar
conspecifics, but do not automatically do so
with unfamiliar humans, the familiarity effect
on yawn contagion in adult chimpanzees, may only
apply when chimpanzees view the yawns of
conspecifics. That is, adult chimpanzees may
apply 'targeted empathy' to interactions with
conspecifics, while they apply a more
generalised form of empathy to interactions with
humans, who they rarely engage in competition
with and mostly experience as cooperative (in
e.g. food provision contexts). Further research
is thus required to ascertain the mechanisms
underlying the variable results in studies of CY
across ages and species.
