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My Fear Is Not, and Never Will Be, Your Fear: On Emotions and Feelings in Animals



Do nonhuman animals (henceforth, animals) have emotions, and if so, are these similar to ours? This opinion piece aims to add to the recent debate about this question and provides a critical re-evaluation of what can be concluded about animal and human emotions. Emotions, and their cognitive interpretation, i.e., feelings, serve important survival functions. Emotions, we believe, can exist without feelings and are unconsciously influencing our behavior more than we think, and possibly more so than feelings do. Given that emotions are expressed in body and brain, they can be inferred from these measures. We view feelings primarily as private states, which may be similar across closely related species but remain mostly inaccessible to science. Still, combining data acquired through behavioral observation with data obtained from noninvasive techniques (e.g., eyetracking, thermography, hormonal samples) and from cognitive tasks (e.g., decision-making paradigms, cognitive bias, attentional bias) provides new information about the inner states of animals, and possibly about their feelings as well. Given that many other species show behavioral, neurophysiological, hormonal, and cognitive responses to valenced stimuli equivalent to human responses, it seems logical to speak of animal emotions and sometimes even of animal feelings. At the very least, the contemporary multi-method approach allows us to get closer than ever before. We conclude with recommendations on how the field should move forward.
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Affective Science
My Fear Is Not, andNever Will Be, Your Fear: On Emotions andFeelings
MariskaE.Kret1,2,3 · JorgJ.M.Massen4· FransB.M.deWaal4,5
Received: 19 May 2021 / Accepted: 17 December 2021
© The Author(s) 2022
Do nonhuman animals (henceforth, animals) have emotions, and if so, are these similar to ours? This opinion piece aims to add
to the recent debate about this question and provides a critical re-evaluation of what can be concluded about animal and human
emotions. Emotions, and their cognitive interpretation, i.e., feelings, serve important survival functions. Emotions, we believe,
can exist without feelings and are unconsciously influencing our behavior more than we think, and possibly more so than feelings
do. Given that emotions are expressed in body and brain, they can be inferred from these measures. We view feelings primarily
as private states, which may be similar across closely related species but remain mostly inaccessible to science. Still, combining
data acquired through behavioral observation with data obtained from noninvasive techniques (e.g., eyetracking, thermography,
hormonal samples) and from cognitive tasks (e.g., decision-making paradigms, cognitive bias, attentional bias) provides new
information about the inner states of animals, and possibly about their feelings as well. Given that many other species show
behavioral, neurophysiological, hormonal, and cognitive responses to valenced stimuli equivalent to human responses, it seems
logical to speak of animal emotions and sometimes even of animal feelings. At the very least, the contemporary multi-method
approach allows us to get closer than ever before. We conclude with recommendations on how the field should move forward.
Keywords Emotion· Feeling· Consciousness· Anthropomorphism· Evolution
Various Positions onAnimal Emotions
Two decades ago, a symposium was held in Amsterdam
with many luminaries of affective science. The symposium’s
title, Feelings and Emotions, generated lively debate about
the definition of both concepts. Some speakers considered
them merely two sides of the same coin, whereas others
saw a sharp contrast. Damasio (2004, pp. 50, 52) stressed
how emotions are “bioregulatory reactions” that prepare the
organism for adaptive behavior (cf. Frijda, 2010), whereas
feelings are the mental representations of the physiological
changes that occur during an emotion. In the current arti-
cle, we adhere to these two definitions. Most contemporary
researchers do not deny the existence of emotions in ani-
mals. Disagreements mostly concern feelings. It is important
therefore, to discuss the terminology to describe research
findings and how far researchers may go in interpreting their
data (LeDoux, 2017; Mobbs etal., 2019). Since there is lit-
tle consensus in the literature, we will start off reviewing
the views of some of the key researchers. See Table1 for a
summary of the terminology used in this paper.
Panksepp’s (2011) view on animal emotions was that
all mammals share neural pathways that are linked to emo-
tions. Therefore, advances in the study of neurobiology and
neuroscience will help in understanding the biology and
psychology of emotion. According to him, the awareness
Handling Editor: Ralph Adolphs
* Mariska E. Kret
1 Cognitive Psychology Unit, Institute ofPsychology, Leiden
University, Leiden, TheNetherlands
2 Comparative Psychology & Affective Neuroscience Lab,
Cognitive Psychology Department, Leiden University,
Leiden, TheNetherlands
3 Leiden Institute forBrain andCognition (LIBC), Leiden,
4 Animal Behaviour andCognition, Department ofBiology,
Utrecht University, Utrecht, TheNetherlands
5 Psychology Department, Emory University, Atlanta, GA,
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Table 1 Clarification of our terminology
Terms How the terms are used in this article Species-specific aspects
Animals Humans
Emotion primitives Functional and adaptive properties, such as approach
or avoidance behaviors which are observable, and
which are expressions of internal brain (or emotion)
Simple organisms can show emotion primitives as
these have clear survival functions
Also humans can have emotion primitives
Survival functions A subcortical defensive survival circuit centered on
the amygdala which initiates defensive behaviors in
response to threats
Present in all vertebrates
Emotions Emotions are “bioregulatory reactions” that prepare
the organism for adaptive behavior. They can exist
without feelings and are unconsciously influencing
behavior more than we think, and possibly more so
than feelings do
Via observation of the expressions or by taking physi-
ological measures, animal emotions can be inferred
Without being aware of it, emotions can have a large
impact on humans behavior and decisions
Feelings Feelings are the mental representations or cognitive
interpretations of the physiological changes that
occur during an emotion
We suggest experiential similarity between related
species, but for the moment animal feelings remain
Generally seen as what people verbally report as their
subjective experience of emotion, although, for rea-
sons outlined in the text, this may not be accurate
Evolutionary parsimony When related species show similar behavior under
similar circumstances, this is likely driven by simi-
lar psychological processes
A difficulty is that there is no hard line or golden
standard that states when species are closely related
Cognitive parsimony The simplest possible cognitive process should be
assigned to observed behaviors
This is applied most often when interpreting the
behavior of nonhuman animals
This is seldomly applied when interpreting human
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of both one’s own and others’ emotions1 is an intrinsic
function of the brain, shared homologously by all mamma-
lian species. We can study these functions in other animals
via clever experimental paradigms using brain measures
(Panksepp, 2005).
Anderson and Adolphs (2014) define emotions as inter-
nal brain states and view emotional behaviors as a class of
behaviors that express these internal emotion states. These
states exhibit general functional and adaptive properties,
called “emotion primitives,” that apply across species.
These properties can be studied even in evolutionarily dis-
tant organisms such as the fruit fly, allowing functional dis-
section of their mechanistic bases and tests of their causal
relationships to behavior. The authors’ empirical approach
circumvents the question of whether or not animals have
Barrett (2006) states that top-down conceptual knowledge
is essential to shape emotions in the social world. She makes
a distinction between hominids (great apes and humans)
and the rest of the animal kingdom. Great apes have dense
interconnectivity between subcortical and cortical areas and
among cortical areas (but see Stacho etal., 2020, for simi-
lar findings in birds), the functional consequences of which
should not be ignored. To Barrett (2012), emotions are not
mechanistically present: They can be expressed in different
ways and don’t have distinct neurophysiological correlates.
Barret’s approach sheds light on the meaning-making pro-
cess, with a research focus on humans and the way feelings
are being constructed. Since psychological research relies
mostly on self-report, its findings concern, in our terminol-
ogy, feelings rather than emotions. Accordingly, construc-
tional theories, such as Barrett’s, mostly concern feelings.
LeDoux is hesitant in using interpretive terms such as
“emotions” and “feelings” and advocates “survival circuits”
both as a term (LeDoux, 2017) and as a focus in animal
research (LeDoux, 2014). Animals may have emotions
the way Anderson and Adolphs (2014) describe them, but
due to unique features of the human brain (e.g., Koechlin,
2011; Preuss, 2011; Semendeferi etal., 2011), other species
lack the same consciousness and linguistic distinctions as
humans, making it questionable whether they can have feel-
ings like us. LeDoux does not say that animals cannot have
feelings. According to him, emotions are not the cause of
feelings. Rather, survival circuit activity occurs in parallel as
a correlate to, rather than as the cause of conscious feelings
(LeDoux, 2021). This makes it impossible to know about
feelings without asking about them directly.
In biology, finally, views on emotions go back to Darwin’s
(1872) The Expression of the Emotions in Man and Ani-
mals, which stressed continuity between humans and other
species. As exemplified by de Waal (2011, 2019), emotions
are considered species-typical and placed in an evolution-
ary light as they prepare the organism for adaptive behavior.
Everything about them, including the possibility of feelings,
is assumed to be similar between related species even if the
focus of the behavioral biologist is always more on emo-
tional behavioral expression than on internal states. Thus,
de Waal (2011, p. 201) takes a position like Anderson and
Adolphs (2014), saying about animals that similar feelings
may be assumed but that the actual experiences of animals
remain mostly inaccessible for the time being.
In sum, most researchers agree that animals show
responses to certain stimuli that are adaptive and based on
internal states that may or may not be referred to as emo-
tions. In contrast, there is considerable discussion about
whether animals have conscious awareness of these emo-
tions – feelings – and how important feelings are in relation
to said emotions.
Our View onEmotions andFeelings
inHumans andOther Animals
Our position is that the exclusion of subjective experiences
of emotions in other animals is highly unreasonable (for sim-
ilar positions, see, e.g., Burghardt, 2019; Bekoff & Pierce,
2017; Paul etal., 2020). Importantly, subjective experiences
probably differ between species and also between individu-
als within a species. Every species has evolved under spe-
cific environmental selection pressures and has a body and
a brain that is unique in its form and output. Some species,
however, such as chimpanzees and humans, are relatively
similar, since they share a long evolutionary history. The
way emotions manifest themselves in humans and apes is
also very similar, including homologous facial expressions
that activate a facial musculature that is nearly identical
between humans and chimpanzees (Burrows etal., 2006;
Parr etal., 2007). Logically, given the similarity in body and
brain, the same may hold for how emotions are being experi-
enced as feelings by the members of closely related species.
As for feelings, we know introspectively that we experi-
ence them ourselves. However, this is the only direct evi-
dence that we have. Feelings are not visible from the out-
side, which is why they are oftenbeing denied in nonverbal
organisms. Remember that there was a time when people
believed the same about the feelings of human neonates. As
a consequence, neonates were, for example, operated upon
(e.g., circumcision) without anesthesia. Nowadays, most
people would agree that this was an incorrect assumption
even though the body and brain of a young infant are very
1 Panksepp used the word “emotional feelings.” In his view, core
emotional feelings may reflect a variety of extended emotional action
systems—including seeking, fear, rage, lust, care, panic, and play. In
the current article, we try to keep the number of different terms used
in the literature to a minimum.
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different from those of adults, sometimes even more so than
those of closely related but different species. If we could
ask a naïve extraterrestrial which two out of three individu-
als, human adult, human infant, and adult chimpanzee are
most similar, we are not sure that the alien would point at
the two humans in this trio. There exist numerous areas in
which the differences between humans (e.g., young vs. old
or brain-damaged vs. neurotypical) are larger than those
between closely related species, but we don’t deny any
humans feelings.
We not only believe that various animal species expe-
rience feelings but also that they may experience unique
feelings due to their unique evolutionary background.
Feelings are most likely similar if the evolutionary path-
ways overlap. Similar to the unique evolutionary path-
ways of different species, individuals within a species all
have their own developmental pathways that shape body
and brain in form and output. Consequently, we expect
intraspecific variation in how feelings manifest them-
selves. Hence our title that “My fear is not, and never will
be, your fear.” We will never know what it is like to be a
bat (Nagel, 1974), but neither will we know exactly what
it is like to be our neighbor.
Feelings are hard to demonstrate in other species since
they cannot verbally report their inner states. However, it
would be a mistake to think that humans always know what
they feel. Many people visit a therapist to figure that out.
Further, people likely respond in ways they deem socially
desirable (e.g., “I’m fine”), which makes it hard to measure
actually felt feelings in humans.
Our View onMeasuring Emotions
First and foremost, when aiming to investigate emotions or
feelings, the terms should be clearly defined and operation-
alized. In the relevant literature, different terms are being
used to refer to emotion. Some researchers choose to use
certain terms exclusively for humans, while other words are
mostly used for other animals. Andrews (2020) stresses that
if we invent new words for other species while keeping old
words for human beings, we are throwing up unnecessary
semantic barriers to comparing humans with other species.
Interpreting behavior as associated with a particular feeling
in humans and interpreting the same behavior as something
else in animals thwarts comparative research, and conse-
quently the progress of fundamental research into the proxi-
mate and ultimate causations of emotions.
Increasingly, researchers approach emotions as
multifaceted states that include physiological, behavioral,
and cognitive components that are measurable (Mendl
etal., 2010; Paul etal., 2005; Massen etal., 2019). We
believe that insight in emotion states can be gathered
with various methods.
Numerous examples could be given, but just to make
our case, we will cover a few here that combined relatively
noninvasive methods in various species to compare behav-
ioral and physiological measures of emotions. In a study
using thermography, Nakayama etal. (2005) demonstrated
a decrease in nasal skin temperature in rhesus macaques in
response to a human dressed in a lab coat and holding a
catching net. Along with this temperature drop, the monkeys
frequently showed a silent bared-teeth face, staring open-
mouth face, and lip-smacking, all expressions of negative
emotions. Another example is the finding that in a touch-
screen task, bonobos had an attentional bias towards emo-
tional expressions of conspecifics. In addition, more “nose-
wipes” were observed during trials where an emotional
image had to be approached rather than avoided, indicative
of emotional arousal (Kret etal., 2016). In an experiment
with dogs, it was shown that when separated from their
owner, dogs were more alert. They stood up, walked, or ran
around and especially towards the door, while barking and
whining. On the physiological level, a detailed analysis of
their heart rate suggested a negative emotion (Katayama
etal., 2016). Another study by combining behavioral and
cardiac measurements suggests that sheep have negative
emotions following negative events, and positive emotions
following positive situations (Reefmann etal., 2009). Car-
diac activity and salivary cortisol concentration were com-
bined in a study with horses (Janczarek etal., 2019). The
horses showed negative emotions in response to the presence
of an audience in the arena. By measuring psychophysiologi-
cal reactions, hormone levels, cognitive bias tasks or behav-
ioral observations, emotions can be inferred.
Emotions are contagious: i.e., they easily spread through-
out a social group. Various studies have shown basic forms
of empathy in social species, from rodents to primates, such
as mimicry of expressions of emotion, matching another’s
emotional state, and responding to the distress of others
with reassurance behavior or helping actions (reviewed by
Preston & de Waal, 2002; de Waal & Preston, 2017). Some
animals, such as ravens, not only match conspecifics’ emo-
tions on a behavioral level (e.g., Osvath & Sima, 2014)
but also match their judgement bias, which is interpreted
as an emotional state, after having witnessed a conspecific
react with apparent frustration to a negative manipulation
(Adriaense etal., 2019). Chimpanzees show jealous reac-
tions when their own valuable social bonds are under threat
(Webb etal., 2020); long-tailed macaques relax (i.e., show
a decrease in circulating cortisol) while cooperating with
a friend (Stocker etal., 2020), and several species consider
“the glass half full rather than half empty” in judgement
or cognitive bias tasks (Paul etal., 2020). In line with
Panksepp’s argument, recent research incorporating the
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behavioral, physiological, and cognitive components of
emotions, isthus suggesting that not only do animals show
emotional behavioral responses, they also seem to experi-
ence them as well as those of conspecifics (Kret etal., 2020;
Nieuwburg etal., 2021).
Emotions are embedded in a complex network of brain
structures including both cortical and subcortical areas acti-
vated in close interplay with the body (Prochazkova & Kret,
2017). Recent neuroimaging research taps into emotions and
their cognitive interpretation and shows that it’s too simple
to state that “feelings are cortical.” Using ecologically valid
paradigms involving risky decisions or social dilemmas to
induce strong emotions, these studies have shown the piv-
otal role of ancient brain structures in human feelings of
social exclusion, depression, and even suicidal tendencies
(e.g., Cáceda etal., 2020). Other studies have demonstrated a
close connection between the body and the brain. For exam-
ple, a study put participants under high levels of stress while
measuring bodily responses (heart rate, skin conductance,
cortisol), self-reported stress levels, and brain activity via
fMRI. Clear relationships were observed between the neural
responses on the one hand and bodily responses and self-
reported stress on the other (Orem etal., 2019).
Emotions even recruit the most ancient neural structures.
The spinal cord rapidly activates in response to emotional
stimuli (Smith & Kornelsen, 2011), and classical work by
Hohmann (1966) has shown that a spinal cord lesion drasti-
cally impacts the feelings reported by patients. We are not
saying here that all animals with a spinal cord have feelings.
But the involvement of such structures, which are highly
conserved among vertebrates (Leung & Shimeld, 2019),
casts doubt on the emphasis on consciousness, language,
cultural construction, and human uniqueness. Even the evo-
lutionarily more “recent” neural architecture of humans is
mostly shared with mammals and birds (e.g., Stacho etal.,
2020). The human brain is hardly categorically distinct from
other brains. That said, we don’t deny that the human brain
may have features that other species lack (e.g., Koechlin,
2011; Preuss, 2011; Semendeferi etal., 2011) and that these
unique structures may alter emotional experiences (LeDoux,
2017). At the same time, this is also true for other species
since all species have unique brains. The difference is that
there is a lot more unknown about the role of other species’
brain structures in emotions. Demonstrating parallel neural
mechanisms involved in the emotions of humans and other
animals, Panksepp (2011) saw no reason to postulate differ-
ent emotional experiences, and we tend to agree with him
on this point.
But even if we do put an emphasis on consciousness:
Recent studies on mirror self-recognition (as a proxy for
self-awareness), theory of mind, metacognition, and plan-
ning for the future (reviewed by de Waal, 2016) do sug-
gest self-reflective capacities in animals, even if some
other scientists remain skeptical (e.g., Heyes, 2017; Pov-
inelli, 2020). For example, rhesus monkeys have “mem-
ory awareness” in that they know what they know or don’t
know (Smith etal., 2013; Templer & Hampton, 2012);
capuchin monkeys, California scrub jays, and Eurasian
jays seem to not only grasp what others know, but also
what others desire (Hattori, 2012; Ostojić etal., 2017),
and chimpanzees take (false) beliefs of conspecifics into
account (Krupenye etal., 2016). Given how ill-defined
consciousness is and how widespread advanced cognitive
traits seem to be in other species, we deem it premature to
assume that said species have no consciousness of their
emotions. From an evolutionary perspective, it is more
logical to assume humanlike consciousness in species
related to us rather than deny it, which means that we
best adopt the former as a working hypothesis (de Waal,
Cognitive andEvolutionary Parsimony andEthical
Imagine an animal that backs away from a harmful stimu-
lus. We see a chimpanzee who, while staring at a snake and
uttering soft alarm calls, carefully and slowly moves out
of the way. The starting point of some scientists, for exam-
ple, LeDoux, is that such behavior should be assumed to
be unconsciously controlled and devoid of feelings unless
proven otherwise. The rule of cognitive parsimony is applied
here, which postulates the simplest possible cognitive pro-
cess when it comes to interpretating behavior. Cognitive par-
simony is important to consider when interpreting behaviors
of many animal species. If in the example above we would
have described the behavior of a fruit fly instead of a chim-
panzee, we agree that this approach would be most correct.
However, the example is about humans’ most closely living
relative, a species that is well-studied and one that we have
a lot of information on.
It is important to realize that the rule of cognitive parsi-
mony is rarely applied to human behavior. When researchers
suggest that human emotions rely on higher-order conscious
cognitive processes, since humans verbally report their emo-
tions, they risk postulating processes that may be unneces-
sary, hence violating Occam’s razor. Setting aside the dis-
cussion about the validity of self-report, feelings are best
considered the consequence rather than the cause of emo-
tional states (Anderson & Adolphs, 2014). Consequently,
we think that feelings cannot inform us about the cognitive
processes and complexity of the emotional state itself. In
our view, this chimpanzee in the above example is feeling
scared and is acting deliberately cautiously to minimize
potential harm. This does not mean that that interpretation
is all that should be reported. To get the complete picture, it
should be accompanied with an objective description of the
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behavior so that this data remains accessible and open for
future interpretation.
By looking to preserve cognitive parsimony at all costs,
comparative psychologists may be disregarding evolution-
ary parsimony, which dictates that we should offer explana-
tions that posit the fewest possible changes in the phyloge-
netic tree. Although cognitive parsimony can be important,
we here would like to emphasize the biological stance of
evolutionary parsimony, stating that when related species
show similar behavior under similar circumstances, these
are likely driven by similar psychological processes (de
Waal, 1999). Until the contrary can be demonstrated, we
must assume that similar behavior in these species is paired
with similar emotions and in some cases similar feelings.
This position is, of course, not entirely new. One of the first
to advocate cross-specific uniformity in behavioral expla-
nations was philosopher David Hume (1739, p. 226), who
formulated the following touchstone well before we had a
theory of evolution:
Tis from the resemblance of the external actions of
animals to those we ourselves perform, that we judge
their internal likewise to resemble ours; and the same
principle of reasoning, carry'd one step further, will
make us conclude that since our internal actions
resemble each other, the causes, from which they are
deriv'd, must also be resembling. When any hypoth-
esis, therefore, is advanc'd to explain a mental opera-
tion, which is common to men and beasts, we must
apply the same hypothesis to both.
Hume’s stance raises the question how far we can stretch
the concept of similar states and how related two species
should be for them to experience similar feelings. Whereas,
given the ancient structures and mechanisms involved, we do
expect some sort of emotional states in all animals, we are
agnostic about feelings in distantly related species, such as
invertebrates. Conversely, we argue that it is unreasonable to
exclude the possibility of feelings in all animals and specifi-
cally in those that are closely related to us, hence similar in
body and brain. We, furthermore, embrace the idea that feel-
ings may have evolved convergently in multiple lineages and
that the comparative study of these taxa will help us shed
light on the selection pressures that may have shaped the
evolution of both emotions and feelings (Fitch etal., 2010;
Massen, 2020).
Finally, we would like to include a warning. Those who
do not set emotions apart from feelings, and doubt the
latter’s existence in animals, have a special obligation to
produce convincing evidence when they deny the exist-
ence of animal feelings. This Cartesian position carries
ethical implications. Humans experience a different sense
of obligation towards entities with or without feelings,
which is why the question of animal sentience is central
to every current debate about the humane treatment of ani-
mals. This means that we need to proceed with the utmost
care in this domain so as to avoid giving fodder to those
who consider animals unworthy of moral consideration.
We (including the authors of this opinion piece) have an
obligation to be clear about what is a mere assumption and
what is fact when it comes to animal feelings. At the same
time, scientific evidence of animal emotions is needed to
create a better understanding of the depth of their emo-
tional lives. To that extent, the section below lists some
important steps to be taken.
Future Steps
Where toGo fromHere?
First, we should shift the focus from things we cannot meas-
ure to things that we can measure (Adolphs & Andler, 2018).
For example, technological advances allow us to measure
animals’ emotional facial expressions (e.g., the chimpan-
zee’s and other species’ FACS of Waller etal., 2020). We
can also noninvasively measure bodily expressions and
physiological arousal by using thermography, pupillometry,
heartrate measurements, hormone levels, and measurements
of neural activation (reviewed by Nieuwburg etal., 2021).
Similarly, we study emotionally biased decision-making and
the perception of emotions in experimental paradigms with
techniques such as touchscreens and eyetrackers (e.g., Parr
& Heintz, 2009; Kret etal., 2016). We acknowledge that the
associated feelings stillremain inaccessible but note that this
also largely holds for humans. For this reason, our recom-
mendation to focus on emotions’ measurable aspects extends
to modern psychological research on humans, which thus
far has concerned itself more with feelings than emotions.
Doing so will facilitate comparisons between humans and
other species and move us away from the unreliability of
introspection (Baumeister etal., 2007). This is not to deny
the importance of feelings, but there is debate (see above)
about how essential they are to the way emotions work.
Second, the aim of human and comparative psychology is
to understand psychological states or traits through experi-
mentation, observation, and interview. We seek to under-
stand behavior and assign meaning to what we see, often
using hypotheses based in physiology, neuroscience, and/or
evolutionary theory. Whether postulated intervening vari-
ables are knowable or unknowable is not always the issue.
We don’t ask astronomers not to invoke gravity, which is
invisible, to explain planetary movements, or biologists not
to invoke shared evolution, which is also invisible, to explain
why chimpanzee hands are so strikingly similar to those of
humans. Science is full of postulated intervening variables
to make sense of observed phenomena. In the same way,
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the invisibility of animal feelings is not a good argument
against them.
A third step forward, in our view, is to try to take the per-
spective of animals more when asking questions and design-
ing studies. If we take a typically human phenomenon and
ask the question whether, say, chimpanzees show it too, it
is more likely that this behavior characterizes us better than
them. The animal behavior literature is full of examples
where we have misjudged animals based on human testing
biases (de Waal, 2016). These biases often dictate the search
for humanlike traits in animals, especially in those that are
closely related to us, and in doing so overlooks the unique-
ness of other species. We have trouble seeing a chimpanzee
the way a conspecific does. Rather than focusing on human-
like emotions, we should consider the species-specific emo-
tions of other animals as they have evolved in line with that
species’ specific needs. We need a bottom-up approach that
does not necessarily focus on predefined human emotions.
To conclude, in our view, if a species shows behavioral,
neurophysiological, hormonal, or cognitive responses to
valenced stimuli, we can speak of emotions until proven oth-
erwise. In some instances, we might even speak of feelings.
We advocate a multi-method rather than a single method
approach and believe that the variety of species that we can
study, with their unique brains and bodies, can give us new
insights into emotions and feelings.
Acknowledgements This research was supported by the Templeton
World Charity Organization (TWCF0267) and European Research
Council (Starting grant # 804582) to MEK. We also thank Joseph
LeDoux, one other referee, and the editors for constructive feedback
on our manuscript.
Additional Information
Conflict of Interest The authors declare no competing interests.
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Adolphs, R., & Andler, D. (2018). Investigating emotions as functional
states distinct from feelings. Emotion Review, 10(3), 191–201.
Adriaense, J. E., Martin, J. S., Schiestl, M., Lamm, C., & Bugnyar, T.
(2019). Negative emotional contagion and cognitive bias in com-
mon ravens (Corvus corax). Proceedings of the National Academy
of Sciences, 116(23), 11547–11552.
Anderson, D. J., & Adolphs, R. (2014). A framework for studying
emotions across species. Cell, 157(1), 187–200.
Andrews, K. (2020). How to study animal minds. Cambridge Univer-
sity Press.
Barrett, L. F. (2006). Are emotions natural kinds? Perspectives on Psy-
chological Science, 1(1), 28–58.
Barrett, L. F. (2012). Emotions are real. Emotion, 12(3), 413.
Baumeister, R. F., Vohs, K. D., & Funder, D. C. (2007). Psychology
as the science of self-reports and finger movements: Whatever
happened to actual behavior? Perspectives on Psychological Sci-
ence, 2(4), 396–403.
Bekoff, M., & Pierce, J. (2017). The animals' agenda: Freedom, com-
passion, and coexistence in the human age. Beacon Press.
Burghardt, G. M. (2019). A place for emotions in behavior systems
research. Behavioural Processes, 166, 103881.
Burrows, A., etal. (2006). Muscles of facial expression in the chimpan-
zee (Pan troglodytes): Descriptive, comparative, and phylogenetic
contexts. Journal of Anatomy, 208, 153–168.
Cáceda, R., James, G. A., Stowe, Z. N., Delgado, P. L., Kordsmeier, N.,
& Kilts, C. D. (2020). The neural correlates of low social integra-
tion as a risk factor for suicide. European Archives of Psychiatry
and Clinical Neuroscience, 270(5), 619–631.
Damasio, A. R. (2004). Emotions and feelings: A neurobiological per-
spective. W: ASR Manstead, N. Frijda, A. Fischer (red.), Feelings
and Emotions (s. 49–57).
Darwin, C. 1998 [1872]. In P. Ekman (Ed.), The expression of the
emotions in man and animals (3rd ed.). Oxford University Press.
de Waal, F. B. M. (1999). Anthropomorphism and anthropodenial:
Consistency in our thinking about humans and other animals.
Philosophical Topics, 27, 255–280.
de Waal, F. B. M. (2011). What is an animal emotion? The Year in
Cognitive Neuroscience, Annals of the New York Academy of
Sciences, 1224, 191–206.
de Waal, F. B. M. (2016). Are we smart enough to know how smart
animals are? Norton.
de Waal, F. B. M. (2019). Mama’s last hug: Animal emotions and
what they tell us about ourselves. Norton.
de Waal, F. B. M., & Preston, S. D. (2017). Mammalian empathy:
Behavioral manifestations and neural basis. Nature Reviews.
Neuroscience, 18(8), 498–509.
Fitch, W. T., Huber, L., & Bugnyar, T. (2010). Social cognition and
the evolution of language: Constructing cognitive phylogenies.
Neuron, 65(6), 795–814.
Frijda, N. H. (2010). Impulsive action and motivation. Biological
Psychology, 84(3), 570–579.
Hattori, Y. (2012). Food-related tolerance in capuchin monkeys
(Cebus apella) varies with knowledge of the partner's previous
food-consumption. Behaviour, 149, 171185.
Heyes, C. (2017). Apes submentalise. Trends in Cognitive Sciences,
21(1), 1–2.
Hohmann, G. W. (1966). Some effects of spinal cord lesions on expe-
rienced emotional feelings. Psychophysiology, 3(2), 143–156.
Hume, D. A. Treatise of human nature (1739; reprint, Harmonds-
worth, :1985).
Janczarek, I., Wilk, I., Stachurska, A., Krakowski, L., & Liss, M.
(2019). Cardiac activity and salivary cortisol concentration of
leisure horses in response to the presence of an audience in the
arena. Journal of Veterinary Behavior, 29, 31–39.
Katayama, M., Kubo, T., Mogi, K., Ikeda, K., Nagasawa, M., &
Kikusui, T. (2016). Heart rate variability predicts the emotional
state in dogs. Behavioural Processes, 128, 108–112.
Affective Science
1 3
Koechlin, E. (2011). Frontal pole function: what is specifically
human? Trends in Cognitive Sciences, 15(6), 241.
Kret, M. E., Jaasma, L., Bionda, T., & Wijnen, J. G. (2016). Bonobos
(Pan paniscus) show an attentional bias toward conspecifics
emotions. Proceedings of the National Academy of Sciences,
113(14), 3761–3766.
Kret, M. E., Prochazkova, E., Sterck, E. H., & Clay, Z. (2020). Emo-
tional expressions in human and non-human great apes. Neuro-
science & Biobehavioral Reviews, 115, 378–395.
Krupenye, C., Kano, F., Hirata, S., Call, J., & Tomasello, M. (2016).
Great apes anticipate that other individuals will act according
to false beliefs. Science, 354(6308), 110–114.
LeDoux, J. E. (2014). Coming to terms with fear. Proceedings of the
National Academy of Sciences, 111(8), 2871–2878.
LeDoux, J. E. (2017). Semantics, surplus meaning, and the science
of fear. Trends in Cognitive Sciences, 21(5), 303–306.
LeDoux, J. E. (2021). What emotions might be like in other animals.
Current Biology, 31(13), R824–R829.
Leung, B., & Shimeld, S. M. (2019). Evolution of vertebrate spinal
cord patterning. Developmental Dynamics, 248(11), 1028–1043.
Massen, J. J. M. (2020). Studying the evolution of cooperation and
prosociality in birds. Ethology, 126(2), 121.
Massen, J. J. M., Behrens, F., Martin, J. S., Stocker, M., & Brosnan,
S. F. (2019). A comparative approach to affect and cooperative
decision-making. Neuroscience & Biobehavioural Reviews, 107,
Mendl, M., Burman, O. H. P., & Paul, E. S. (2010). An integrative
and functional framework for the study of animal emotion and
mood. Proceedings of the Royal Society B, 277, 2895–2904.
Mobbs, D., Adolphs, R., Fanselow, M. S., Barrett, L. F., LeDoux, J.
E., Ressler, K., & Tye, K. M. (2019). Viewpoints: Approaches
to defining and investigating fear. Nature Neuroscience, 22(8),
Nagel, T. (1974). What is it like to be a bat? The Philosophical Review,
83(4), 435–450.
Nakayama, K., Goto, S., Kuraoka, K., & Nakamura, K. (2005).
Decrease in nasal temperature of rhesus monkeys (Macaca
mulatta) in negative emotional state. Physiology & Behavior,
84(5), 783–790.
Nieuwburg, E., Ploeger, A., & Kret, M. E. (2021). Emotion recognition
in nonhuman primates: How experimental research can contribute
to a better understanding of underlying mechanisms. Neuroscience
and Biobehavioral Reviews.
Orem, T. R., Wheelock, M. D., Goodman, A. M., Harnett, N. G., Wood,
K. H., Gossett, E. W., … Knight, D. C. (2019). Amygdala and
prefrontal cortex activity varies with individual differences in the
emotional response to psychosocial stress. Behavioral Neurosci-
ence, 133(2), 203.
Ostojić, L., Legg, E. W., Brecht, K. F., Lange, F., Deininger, C., Mendl,
M., & Clayton, N. S. (2017). Current desires of conspecific
observers affect cache-protection strategies in California scrub-
jays and Eurasian jays. Current Biology, 27(2), R51–R53.
Osvath, M., & Sima, M. (2014). Sub-adult ravens synchronize their
play: A case of emotional contagion. Animal Behavior and Cog-
nition, 2, 197.
Panksepp, J. (2005). Affective consciousness: Core emotional feel-
ings in animals and humans. Consciousness and Cognition, 14(1),
Panksepp, J. (2011). The basic emotional circuits of mammalian brains:
Do animals have affective lives? Neuroscience & Biobehavioral
Reviews, 35(9), 1791–1804.
Parr, L. A., & Heintz, M. (2009). Facial expression recognition in
rhesus monkeys. Macaca Mulatta. Animal Behaviour, 77(6),
Parr, L. A., Waller, B. M., Vick, S. J., & Bard, K. A. (2007). Classify-
ing chimpanzee facial expressions using muscle action. Emotion,
7(1), 172.
Paul, E. S., Harding, E. J., & Mendl, M. (2005). Measuring emotional
processes in animals: the utility of a cognitive approach. Neurosci-
ence and Biobehavioral Reviews, 29, 469–491.
Paul, E. S., Sher, S., Tamietto, M., Winkielman, P., & Mendl, M. T.
(2020). Towards a comparative science of emotion: Affect and
consciousness in humans and animals. Neuroscience & Biobe-
havioral Reviews, 108, 749–770.
Povinelli, D. J. (2020). Can comparative psychology crack its toughest
nut. Animal Behavior and Cognition, 7(4), 589–652.
Preston, S. D., & De Waal, F. B. (2002). Empathy: Its ultimate and
proximate bases. Behavioral and Brain Sciences, 25(1), 1–20.
Preuss, T. M. (2011). The human brain: rewired and running hot.
Annals of the New York Academy of Sciences, 1225(Suppl 1),
Prochazkova, E., & Kret, M. E. (2017). Connecting minds and sharing
emotions through mimicry: A neurocognitive model of emotional
contagion. Neuroscience & Biobehavioral Reviews, 80, 99–114.
Reefmann, N., Wechsler, B., & Gygax, L. (2009). Behavioural and
physiological assessment of positive and negative emotion in
sheep. Animal Behaviour, 78(3), 651–659.
Semendeferi, K., Teffer, K., Buxhoeveden, D. P., Park, M. S., Bludau,
S., Amunts, K., … Buckwalter, J. (2011). Spatial organization
of neurons in the frontal pole sets humans apart from great apes.
Cerebral Cortex, 21(7), 1485–1497.
Smith, S. D., & Kornelsen, J. (2011). Emotion-dependent responses
in spinal cord neurons: a spinal fMRI study. NeuroImage, 58(1),
Smith, J. D., Coutinho, M. V. C., Church, B. A., & Beran, M. J. (2013).
Executive-attentional uncertainty responses by rhesus macaques
(Macaca mulatta). Journal of Experimental Psychology: General,
142(2), 458–475.
Stacho, M., Herold, C., Rook, N., Wagner, H., Axer, M., Amunts, K.,
& Güntürkün, O. (2020). A cortex-like canonical circuit in the
avian forebrain. Science, 369(6511).
Stocker, M., Loretto, M. C., Sterck, E. H., Bugnyar, T., & Massen, J.
J. (2020). Cooperation with closely bonded individuals reduces
cortisol levels in long-tailed macaques. Royal Society Open Sci-
ence, 7(5), 191056.
Templer, V. L., & Hampton, R. R. (2012). Rhesus monkeys (Macaca
mulatta) show robust evidence for memory awareness across mul-
tiple generalization tests. Animal Cognition, 15(3), 409–419.
Waller, B. M., Julle-Daniere, E., & Micheletta, J. (2020). Measuring
the evolution of facial ‘expression’ using multi-species FACS.
Neuroscience & Biobehavioral Reviews, 113, 1–11.
Webb, C. E., Kolff, K., Du, X., & de Waal, F. (2020). Jealous behav-
ior in chimpanzees elicited by social intruders. Affective Science,
1(4), 199–207.
... There is no common agreement on what constitutes animal emotions (see Paul & Mendl, 2018, Kret et al., 2022. However, emotions are often described as internal states which are expressed in physiological, cognitive and behavioral changes (Anderson & Adolphs, 2014). ...
... The assessment of pain and emotions in mammals is much less explored than in the human domain, due to the difficulties regarding ground truth and subsequent lack of large databases mentioned above. The pressing need for these assessments in animal health and welfare evaluations, has therefore made researchers resort to addressing measurements of physiological, behavioral, and cognitive components of affective states, which can be measured objectively, and even in many cases automatically (Paul et al., 2005;Kret et al., 2022). This involves physiological (such as heart rate, hormone levels, body temperature) and behavioral parameters (vocalisations, facial expressions, body postures). ...
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Advances in animal motion tracking and pose recognition have been a game changer in the study of animal behavior. Recently, an increasing number of works go ‘deeper’ than tracking, and address automated recognition of animals’ internal states such as emotions and pain with the aim of improving animal welfare, making this a timely moment for a systematization of the field. This paper provides a comprehensive survey of computer vision-based research on recognition of pain and emotional states in animals, addressing both facial and bodily behavior analysis. We summarize the efforts that have been presented so far within this topic—classifying them across different dimensions, highlight challenges and research gaps, and provide best practice recommendations for advancing the field, and some future directions for research.
... We end by briefly considering whether emotional states that we measure in animals are consciously felt. Whilst feelings remain inaccessible to direct scientific investigation, we agree with recent papers by de Waal and Andrews (2022) and Kret et al. (2022) that it is unreasonable to assume that only humans consciously experience emotions, and that indirect evidence is accumulating for sentience in a growing range of species. We discuss this in the last section of the paper. ...
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Our experiences of the conscious mental states that we call emotions drive our interest in whether such states also exist in other animals. Because linguistic report can be used as a gold standard (albeit indirect) indicator of subjective emotional feelings in humans but not other species, how can we investigate animal emotions and what exactly do we mean when we use this term? Linguistic reports of human emotion give rise to emotion concepts (discrete emotions; dimensional models), associated objectively measurable behavioral and bodily emotion indicators , and understanding of the emotion contexts that generate specific states. We argue that many animal studies implicitly translate human emotion concepts , indicators and contexts , but that explicit consideration of the underlying pathways of inference, their theoretical basis, assumptions, and pitfalls, and how they relate to conscious emotional feelings , is needed to provide greater clarity and less confusion in the conceptualization and scientific study of animal emotion.
... The pressing need for these assessments, be it pain, stress or positive emotions in animal health and welfare evaluations, has therefore made researchers resort to addressing measurements of physiological, behavioral, and cognitive components of affective states, which can be measured objectively, and even in many cases automatically [7,54]. This involves physiological (such as heart rate, hormone levels, body temperature) and behavioral parameters (vocalisations, facial expressions, body postures). ...
Full-text available
Advances in animal motion tracking and pose recognition have been a game changer in the study of animal behavior. Recently, an increasing number of works go 'deeper' than tracking, and address automated recognition of animals' internal states such as emotions and pain with the aim of improving animal welfare, making this a timely moment for a systematization of the field. This paper provides a comprehensive survey of computer vision-based research on recognition of affective states and pain in animals, addressing both facial and bodily behavior analysis. We summarize the efforts that have been presented so far within this topic -- classifying them across different dimensions, highlight challenges and research gaps, and provide best practice recommendations for advancing the field, and some future directions for research.
... The task is easier for subjects to learn compared to other methods, such as judgement bias tasks, and may therefore be a useful tool with which to identify individual differences in susceptibility to threatening stimuli [54]. Most significantly, authors have recently argued [62] that animal species responding to valenced stimuli allows for the attribution of emotions to these species-a notable advancement in the study of animal emotion. We believe we have shown evidence for meaningful responses to valenced stimuli here, thereby validating the assumption that gorillas are emotional beings. ...
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We adapted the emotional Stroop task developed for primates to test whether gorillas would show response slowing for images of ‘negative’ compared to images of ‘positive’ items placed within previously reinforced borders. Three zoo-housed male gorillas participated in six phases of an emotional Stroop paradigm. In Phase One, they learned to select blue borders over yellow borders in a forced choice task presented on the touchscreen. In Phase Two, neutral yellow or blue two-dimensional shapes were placed within the borders. On congruent trials, blue images were presented within both blue and yellow borders. On incongruent trials, yellow images were placed within both blue and yellow borders. We continued to use these trials as control trials in subsequent phases. We predicted that response latencies would be slower and accuracy would be lower on incongruent trials. Although the gorillas responded more quickly to incongruent trials, in contrast to predictions, they were more accurate on congruent trials, consistent with predictions. Therefore, we proceeded with Phase Three in which photographs of images assumed to have positive and negative valences for the gorillas were placed within the borders. On test trials, the same positive or negative image was placed within both borders. In Phase Four, a positive image was paired with a negative image on each trial and the positive image appeared in either the blue (congruent trials) or yellow border (incongruent trials). Phases Five and Six replicated Phases Three and Four with images of novel positive and negative items. The gorillas responded more quickly on congruent trials compared to incongruent trials on test trials but not on control trials throughout Phases 3–6. These findings provide some validation for the emotional Stroop task to test attentional shift with emotionally valenced items.
Researchers have studied non-human primate cognition along different paths, including social cognition, planning and causal knowledge, spatial cognition and memory, and gestural communication, as well as comparative studies with humans. This volume describes how primate cognition is studied in labs, zoos, sanctuaries, and in the field, bringing together researchers examining similar issues in all of these settings and showing how each benefits from the others. Readers will discover how lab-based concepts play out in the real world of free primates. This book tackles pressing issues such as replicability, research ethics, and open science. With contributors from a broad range of comparative, cognitive, neuroscience, developmental, ecological, and ethological perspectives, the volume provides a state-of-the-art review pointing to new avenues for integrative research.
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Recognising conspecifics’ emotional expressions is important for nonhuman primates to navigate their physical and social environment. We address two possible mechanisms underlying emotion recognition: emotional contagion, the automatic matching of the observer’s emotions to the emotional state of the observed individual, and cognitive empathy, the ability to understand the meaning and cause of emotional expressions while maintaining a distinction between own and others’ emotions. We review experimental research in nonhuman primates to gain insight into the evolution of emotion recognition. Importantly, we focus on how emotional contagion and cognitive empathy can be studied experimentally. Evidence for aspects of cognitive empathy in different nonhuman primate lineages suggests that a wider range of primates than commonly assumed can infer emotional meaning from emotional expressions. Possibly, analogous rather than homologous evolution underlies emotion recognition. However, conclusions regarding its exact evolutionary course requires more research in different modalities and species.
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What is the likelihood that humans will ever determine if other animals engage in higher-order thinking? In examining what has happened in the twenty years since the publication of our book, Folk Physics for Apes, I conclude that comparative psychologists, the academic stalwarts charged with making progress on this front, are stuck in a series of intractable, and largely unacknowledged, conceptual problems. Because higher-order mental states depend on the existence of first-order, perceptually-based representations of objects and events, and because those first-order representations are necessary and sufficient to explain current experimental and observational results, the approaches deployed by comparative psychologists are doomed to failure. I examine this Asymmetric Dependency Problem in detail and show how the failure to confront its implications leads to viciously circular arguments that cannot be fixed within the current paradigm of research. Next, I offer a seven-step method for isolating the common structural flaw in any given experiment, and work through several examples. Finally, I examine the central claims that my colleagues and I made in Folk Physics through the lens of the Asymmetric Dependency Problem and current research trends. Although the optimism we expressed that experimental approaches could implicate the presence of higher-order thinking in animals requires considerable dampening, the challenges we isolated remain as vital today as they were twenty years ago.
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Many animal species cooperate with conspecifics in various social contexts. While ultimate causes of cooperation are being studied extensively, its proximate causes, particularly endocrine mechanisms, have received comparatively little attention. Here, we present a study investigating the link between the hormone cortisol, cooperation and social bonds in long-tailed macaques (Macaca fascicularis). We tested 14 macaques in a dyadic cooperation task (loose-string paradigm), each with two partners of different social bond strength and measured their salivary cortisol before and after the task. We found no strong link between the macaques' cortisol level before the task and subsequent cooperative success. By contrast, we did find that the act of cooperating in itself led to a subsequent decrease in cortisol levels, but only when cooperating with closely bonded individuals. Two control conditions showed that this effect was not due to the mere presence of such an individual or the pulling task itself. Consequently, our study shows an intricate way in which the hypothalamic–pituitary–adrenal axis is involved in cooperation. Future studies should reveal whether and how our findings are driven by the anxiolytic effect of oxytocin, which has been associated with social bonding.
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Humans and great apes are highly social species, and encounter conspecifics throughout their daily lives. During social interactions, they exchange information about their emotional states via expressions through different modalities including the face, body and voice. In this regard, their capacity to express emotions, intentionally or unintentionally, is crucial for them to successfully navigate their social worlds and to bond with group members. Darwin (1872) stressed similarities in how humans and other animals express their emotions, particularly with the great apes. Here, we show that emotional expressions have many conserved, yet also a number of divergent features. Some theorists consider emotional expressions as direct expressions of internal states, implying that they are involuntary, cannot be controlled and are inherently honest. Others see them as more intentional and/ or as indicators of the actor's future behavior. After reviewing the human and ape literature, we establish an integrative, evolutionary perspective and provide evidence showing that these different viewpoints are not mutually exclusive. Recent insights indicate that, in both apes and humans, some emotional expressions can be controlled or regulated voluntarily, including in the presence of audiences, suggesting modulation by cognitive processes. However, even non-intentional expressions such as pupil dilation can nevertheless inform others and influence future behavior. In sum, while showing deep evolutionary homologies across closely related species, emotional expressions show relevant species variation.
In this My word, Joseph LeDoux explores what the emotional lives of other mammals might be like. He proposes that better understanding of the brain mechanisms of emotional consciousness in humans might shed light on the kinds of conscious capacities that might be possible in non-human primates and non-primate mammals, given the kinds of brains they possess.
Despite increasing interest in animal emotions, jealousy has rarely been directly addressed in comparative research, except for studies of human-pet interactions. Jealous behavior emerges when a valuable social bond is threatened by a third-party, prompting aggression or intervention attempts to direct the partner’s attention away from the rival. Emotional reactions that protect relationships are expected in species in which social relationships are important for fitness, including primates. Previous primate studies have alluded to this ultimate function, but never explicitly tested predictions corresponding to a proximate jealousy mechanism. We demonstrate jealous behavior in a long-established colony of chimpanzees (N = 17) during a socially disruptive period due to group introductions, which provided an ideal experimental opportunity to test predictions of a jealousy hypothesis. Specifically, we found that negative reactions (agonism and intervention attempts) towards social closeness between two groupmates were generally more common when the aggressor/intervener had a valuable relationship to one (as compared with both or neither) of the dyad’s members, indicating that the other partner represented a potential social rival. In line with this suggestion, we found that negative reactions most often targeted dyads containing newly introduced individuals, especially when the social conditions for jealousy were met, and in particular during the socially unstable introduction period. Results underscore the potential adaptive role of jealousy in protecting fitness-enhancing relationships from social interlopers, by extension indicating that this emotion likely evolved in diverse animal societies.
Basic principles of bird and mammal brains Mammals can be very smart. They also have a brain with a cortex. It has thus often been assumed that the advanced cognitive skills of mammals are closely related to the evolution of the cerebral cortex. However, birds can also be very smart, and several bird species show amazing cognitive abilities. Although birds lack a cerebral cortex, they do have pallium, and this is considered to be analogous, if not homologous, to the cerebral cortex. An outstanding feature of the mammalian cortex is its layered architecture. In a detailed anatomical study of the bird pallium, Stacho et al. describe a similarly layered architecture. Despite the nuclear organization of the bird pallium, it has a cyto-architectonic organization that is reminiscent of the mammalian cortex. Science , this issue p. eabc5534
Comparative psychology, the multidisciplinary study of animal behavior and psychology, confronts the challenge of how to study animals we find cute and easy to anthropomorphize, and animals we find odd and easy to objectify, without letting these biases negatively impact the science. In this Element, Kristin Andrews identifies and critically examines the principles of comparative psychology and shows how they can introduce other biases by objectifying animal subjects and encouraging scientists to remain detached. Andrews outlines the scientific benefits of treating animals as sentient research participants who come from their own social contexts and with whom we will be in relationship. With discussions of science's quest for objectivity, worries about romantic and killjoy theories, and debates about chimpanzee cognition between primatologists who work in the field and those in the lab, Andrews shows how scientists can address the different biases through greater integration of the subdisciplines of comparative psychology.
Darwin observed that form, and in his view, meaning, of facial behaviour (observable changes in the appearance of the face, often termed facial 'expression') is similar between a wide range of species and concluded that this must be due to a shared ancestral origin. Yet, as with all social behaviours, exactly how to define similarity and determine homology is debated. Facial behaviour is linked to specific facial muscle movements, so one important factor in determining homology is the anatomical basis of facial behaviours that appear similar in both appearance and social function. The Facial Action Coding System (FACS) was developed for the scientific measurement of human facial behaviour and is based on individual facial muscle movements (Ekman and Friesen, 1978). FACS has since been modified for use with various non-human primate species (chimpanzees, macaques, hylobatids, orangutans) and domestic species (dogs, cats, horses). These FACS can be used to trace continuity of form in facial behaviour across species and build a better understanding of the evolution of facial communication in mammals.