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Play
“We have seen that there are many, indeed innumerable, ways of characterizing and defining
play. All single-criterion definitions such as ‘play is behavior done for its own sake,’ ‘play is not
serious,’ ‘play is pleasurable behavior,’ or ‘play is preparation for the future’ fail because they
include much that clearly is not playful in any traditional sense even if the criteria could be
objectively applied.
Thus we still face the dilemma of deciding in any given example or species whether we are
dealing with a presumptive example of play. This is not just a pedantic question. In order to
try to elucidate the genesis of play, we must use the comparative method. In order to apply
the comparative method, we need to be able to know what we are comparing across species
and populations. If we are studying the ontogeny of play and its fate and transformations in
individuals, we also need to know whether we are comparing related activities or those that
just look similar.
After considering all the proposed criteria and how they might be applied to the occurrence
of play in animals never considered playful (e.g., turtles), I have now decided that five key
criteria are possible. It seems to me that applied in a careful manner, they can distinguish
play from virtually all other phenomena with which it might be confused, including exploration
and curiosity on one hand and stereotyped behavior on the other. The key here is that all five
criteria have to be simultaneously satisfied for the label of play to be attached. Equally
important is an additional proviso: When a criterion contains more than one attribute, only one
needs to apply for the criterion to be met.
1. The performance of the behavior is not fully functional in the form or context in which it
is expressed; that is, it includes elements, or is directed toward stimuli, that do not
contribute to current survival.
The critical phrase is ‘not fully functional,’ instead of purposeless, nonadaptive, or having a
delayed benefit.
Now this clearly does not eliminate all instances of nonplay behavior. We know, for example,
that many predatory chases of prey be lions are unsuccessful, as are defensive attacks by prey
and rivals. Many courtship attempts, indeed most attempts in many species, are unsuccessful.
We also can deceive animals with models, and nature does this with mimics. Thus, many
performances of behavior do turn out to be ineffective, misdirected, and unsuccessful. We
would not, for these reasons, call them play.
2. The behavior is spontaneous, voluntary, intentional, pleasurable, rewarding, reinforcing,
or autotelic (‘done for its own sake’).
Only one of these often overlapping concepts needs to apply. Note that this criterion also
accommodates any subjective concomitants of play (having fun, enjoyable), but does not
make them essential for recognizing play. Note the ‘or’ in the definition. Scientists averse to
anything tainted with anthropomorphism might not accept that a behavior is pleasurable for a
rat, but may accept that performing it may be reinforcing. Physiologists may not want to use
the concept of spontaneous because it may imply that the behavior is uncaused. Similarly,
intention (or intentionality) may be key words for those who object to voluntary.
3. It differs from the ‘serious’ performance of ethotypic behavior structurally or temporally
in at least one respect: it is incomplete (generally through inhibited or dropped final
elements), exaggerated, awkward, or precocious; or it involves behavior patterns with
modified form, sequencing, or targeting.
Notice that this is a structural and temporal descriptive criterion that acknowledges, but
does not require, that play may be found only during a limited period early in an animal’s life.
Although individual variation may be more common in play than in other behavioral contexts,
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this needs to be quantitatively assessed. The variances from normal ‘serious’ ethotypic
behaviors seen in various guises in play may offer the illusion that the play is free or creative
when it is just the shift from the expected, coupled with our own fascination, that is at work.
Also, since play may involve many kinds of different objects, many being artificial in captivity,
variability is to be expected, just as a wolf stalks a bison differently than it hunts a rabbit.
4. The behavior is performed repeatedly in a similar, but not rigidly stereotyped, form
during at least a portion of the animal’s ontogeny.
This criterion explicitly counters the apparent freedom, flexibility, and versatility of play that
have been so often noted. It is clear from the literature that repetition of patterns of
movement is found in all play and games in human and nonhuman animals. It is also useful in
distinguishing exploratory responses to novel stimuli, which typically habituate quickly, from
the play actions that may follow initial exploratory behavior. Repeated actions also facilitate
the use of play in learning or improving skill and . . . an ‘urge’ to repeat behavior (try and try
again) may be essential for play to have functional and adaptive roles in behavioral
development and psychological well-being.
Play, then, is something that is repeatedly performed, often in bouts, during a predictable
period in the animal’s life (which in some cases may be virtually life-long).
5. The behavior is initiated when an animal is adequately fed, healthy, and free from
stress (e.g., predator threat, harsh microclimate, social instability), or intense
competing systems (e.g., feeding, mating, predator avoidance). In other words, the
animal is in a ‘released field.’
One value of these five criteria is that they effectively link the different kinds of play:
locomotor, object, and social. Play manifests itself in many ways. Whether all play shares
some common phyletic history or physiological underpinnings is not the question at issue now.
For the sake of convenience, I will refer to the ‘big five’ criteria for identifying play as:
1. Limited immediate function
2. Endogenous component
3. Structural or temporal difference
4. Repeated performance
5. Relaxed field
Keeping in mind the nuances underlying each word, a one-sentence definition could then
read as follows: Play is repeated, incompletely functional behavior differing from more serious
versions structurally, contextually, or ontogenetically, and initiated voluntarily when the animal
is in a relaxed or low-stress setting (pp 68-82).
The following conclusions, while they are based on different levels of analysis, reflect the
five ethological aims discussed in chapters 1 and 5.
1. Play behavior is recognized by five criteria. Playful activities can be characterized as being
(1) incompletely functional in the context expressed; (2) voluntary, pleasurable, or self
rewarding; (3) different structurally or temporally from related serious behavior systems;
(4) expressed repeatedly during at least some part of an animal’s life span; and (5)
initiated in relatively benign situations.
2. Play is a heterogeneous category and different types of play have their own phylogenetic
and developmental trajectories.
3. The comparative evidence shows that play is not limited to some or even all placental
mammals, but is found in a wide range of animals, including marsupials, birds, turtles,
lizards, fish, and invertebrates.
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4. Animals that play often share common traits, including active life styles, moderate to high
metabolic rates, generalist ecological needs requiring behavioral flexibility or plasticity, and
adequate to abundant food resources. Object play is most often found in species with
carnivorous, omnivorous, or scavenging foraging modes. Locomotor play is prominent in
species that navigate in three-dimensional (e.g., trees, water)or complex environments and
rely on escape to avoid predation. Social play is not easily summarized, but play fighting,
chasing, and wrestling are the major types recorded and occur in almost every major group
of animals in which play is found.
5. Play may be arranged along a continuum so that much play may have no important
evolved adaptive function (primary process play), while other types of play may have
secondarily evolved important roles in maintaining or developing and enhancing behavioral
performance and cognitive and emotional well-being. Demonstrating such roles, however,
has been fraught with difficulties.
6. The genesis of play lies in primary process play with few, if any, immediate adaptive
consequences. Such playlike behavior may then be favored by natural selection and
eventually become adaptive, even essential, and incorporated into the behavioral
repertoire. Play may also represent the precocious maturation of behavior before it is
required for survival. Rich and complex play repertoires were facilitated by the evolution of
parental care. Much play may be derived from juvenile instinctive behavior that is no
longer honed by natural selection operating on its original precocial form and thus is open
to neural and behavioral reorganization.
7. Play originally arose from proximate processes such as instinctive behavior and associated
neural organization, and the performance of playful acts may be rewarding (autotelic)
regardless of their adaptiveness or function. Thus play may be derived from, as well as
incorporated into, many different behavior systems. The emotional aspects of play found
so compelling are typically found in social animals or those from a highly social ancestry.
8. The evidence for the functional role of play is limited and consequently play has not
entered the mainstream of behavioral research in biology, psychology, or the social
sciences.
9. The brain mechanisms underlying play are diverse. The neural substrates of play in
vertebrates have their origin in structures in the basal ganglia of the forebrain, the
cerebellum, and may involve projections to the limbic system (affect) and cortex, especially
the premotor and prefrontal areas (context assessment and performance skill). Midbrain
structures (e.g., thalamus and hypothalamus) are sources of the sensory and motivational
processes that underlie behavioral performance in play. Neurotransmitters (such as
dopamine and opioids) and hormones also have influences on play that are still poorly
understood, but undoubtedly important.
10. Our understanding of play will only be enhanced when its study is embedded in the normal
lives of animals and looked at from a phylogenetic perspective.
Experimental and
neuroscience studies of play in a few model species can identify some mechanisms and
developmental processes, but they will never be able to solve the conundrums of play in
isolation from evolutionary processes.
11. Play is a crucial test for the power of science to understand ‘mysteries,’ and its effective
study requires integrative and interdisciplinary work by the best scientific minds as well as
intensely focused research by specialists (pp. 382-283).
The survey of play in diverse animals and the resulting phylogenetic trees show that it is a
diverse phenomenon that evolved independently and was even secondarily reduced or lost in
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many groups of animals. A phylogeny of the major animal groups shows that the craniate
vertebrate (backboned) animals and invertebrates have long and separate lineages with many
branches. Only in animals with several life history, ecological, and physiological attributes did
play appear, become prominent, and evolve complex forms. Important adaptive roles for play
are thus derived and perhaps evolved in tandem with putative ‘plasticity genes.’
The field of molecular genetics has begun to pay attention to the interaction of specific
genes with developmental and life history processes in the expression of many traits of
animals. Underlying the grand biological diversity in the world is a surprisingly conservative
genetic toolkit. Traits that were thought to have arisen completely independently in animals
separated by hundreds of millions of years, such as eyes in flies and mice, are controlled by
similar genes; introducing the corresponding mouse gene into a fly induces compound eye
tissue, not mouse-type eye tissue. Thus although play arose many times in evolution, it may
have been the result of common environmental contexts that activated a suite of retained
homeotic hox genes which, although they may have other functions, could be repeatedly coopted in the service of playlike traits. With a conserved and widely shared genome available,
play, as surplus resource theory posits, could then readily appear whenever physiological
(including neural), life history, energetic, ecological, and psychological conditions, in
conjunction with a species’ behavior repertoire, reached a threshold level to activate genetic
processes. An important task for the future is to tease apart these conditions, including the
genes, and rigorously evaluate their contribution to both the origins and elaboration of play, as
is currently being advocated in the study of [ADHD]. This should be done in conjunction with
studies of gene activity, particularly of the neural structures underlying the behavior patterns
involved in play (pp. 381-385).”
The Genesis of Animal Play: Testing the Limits, by Gordon M. Burghardt, The MIT Press, 2005
“I completely agree with Jay Feierman’s your last comment: “The issue here is human playing
behavior.” Let me remind everyone that I began this focus on play because Jay did so in his
edited book, The Biology of Religious Behavior, and he made statements there that I
challenge. Jay said, “When behavior shows lots of variation in form when repeated by the
same or different individuals, it can only be defined by its function, although its form can still
be described. For example, imagine a child is ‘playing,’ which is a functional characterization
of the child’s behavior. One cannot define ‘playing’ by its form because there are too many
behaviors that can be used when a child is playing” (p. 75). Playing to Jay is an example of
Type II Behavior, which he says cannot have a phylogeny, an evolutionary history.
Jay is wrong about “playing” (importantly, he does not define “play” in his book, but was
quick to challenge me to do so). Human and nonhuman play has for years been defined by its
form and most researchers into play conclude that it has an evolutionary history, even though
there are numerous, competing theories regarding the adapted function of play. Furthermore,
the complexity and variability of the species-typical play behavior found in all juvenile
mammals cannot be distinguished from human behavior on this basis, that juvenile humans
show more complex patterns (or some such anthropocentric perspective). I think it is safe to
say that there are innumerable videos of humans and nonhumans at play (e.g., humans,
wolves, chimpanzees, and birds engaged in sensorimotor play; see below). It takes a really
ethnocentric perspective to call a human 2-year old play “more complex” than that of, say, a
juvenile chimpanzee or wolf (adjusting for faster development to adult stages).
First, I’ll discuss play’s form. The essence of identifying an observed behavior as being
play, or its definition, is this: the individual’s pattern of movement is observed to be “a
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seemingly ‘non-serious’ variant of functional behavior, [that is] playful behaviors resemble
serious behaviors but participants are typically more concerned with the behaviors themselves
(i.e., ‘means’) rather than the function (‘ends’) of the behavior” (Pellegrini et al 2007; see link
below).
Here is another definition: “Play is locomotor activity performed postnatally which appears
to an observer to have no obvious immediate benefits for the player, in which motor patterns
resembling those used in serious functional contexts may be used in modified terms. The
motor acts constituting play have some or all of the following structural features: exaggeration
of movements, repetition of motor acts, and fragmentation or disordering of sequences of
motor acts” (Pellegrini and Smith 2005, p. 28).
The best treatment on the subject of defining play can be found in Burghardt 2005. Here
is what he says:
“After considering all the proposed criteria and how they might be applied to the occurrence of
play in animals never considered playful (e.g., turtles), I have now decided that five key
criteria are possible. It seems to me that applied in a careful manner, they can distinguish
play from virtually all other phenomena with which it might be confused, including exploration
and curiosity on one hand and stereotyped behavior on the other. The key here is that all five
criteria have to be simultaneously satisfied for the label of play to be attached. Equally
important is an additional proviso: When a criterion contains more than one attribute, only one
needs to apply for the criterion to be met.
1. The performance of the behavior is not fully functional in the form or context in which it
2.
3.
4.
5.
is expressed; that is, it includes elements, or is directed toward stimuli, that do not
contribute to current survival.
The behavior is spontaneous, voluntary, intentional, pleasurable, rewarding, reinforcing,
or autotelic (‘done for its own sake’).
It differs from the ‘serious’ performance of ethotypic behavior structurally or temporally
in at least one respect: it is incomplete (generally through inhibited or dropped final
elements), exaggerated, awkward, or precocious; or it involves behavior patterns with
modified form, sequencing, or targeting.
The behavior is performed repeatedly in a similar, but not rigidly stereotyped, form
during at least a portion of the animal’s ontogeny.
The behavior is initiated when an animal is adequately fed, healthy, and free from
stress (e.g., predator threat, harsh microclimate, social instability), or intense
competing systems (e.g., feeding, mating, predator avoidance). In other words, the
animal is in a ‘released field.’” (pp. 69-78).
Pellegrini and Smith (2005), Chapter 3, “Social Play in the Great Apes,” by Kerrie P. Lewis,
p. 29 contains “Table 3.1. Ethogram of Social Play Behaviors in the Great Apes.” Play fighting,
for instance (from this table), has these typical characteristics: “Aggressive movement patterns
without threat gestures: wrestle; slap; jump at; jump on; play bite; cuff; pull; rough-andtumble; roll; run toward; kick; sniff; chase; attack-withdraw; spar; drag; pinch; hit.”
T.M. Caro’s research on cheetah cubs (Caro 1995; see link below) contains “Table I.
Behaviour patterns comprising different types of play and exploration of cheetah cubs in this
study.” In that table, play is defined/categorized based upon observed patterns of behavior,
such as, “bounding gait: slow run with stiff legs causing a rocking motion” (p. 335).
Thus, play can and has for years been defined by its form…and this includes human
animals. Chapter 4 in Pellegrini and Smith 2005 was written by Douglas P. Fry and titled
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“Rough-and-Tumble Social Play in Humans.” Table 4.1. “Distinguishing Features of R&T and
Aggression” is in essence an ethogram of human R&T play, a categorization along lines
distinguished by motor movements. Chapters 5 and 6 cover object play, first in Great Apes
and then humans. The observed patterns of behavior between humans and Great Apes are
consistently the same, and, more importantly, there are important similarities in the ontogeny
of the various types of play between humans and Great Apes.
I already replied to your challenge, “Please give me the evolutionary history of the human
game of playing chess [or basketball], which is a specific example of a non-verbal human
playing.” Neither is an example of the kind of play we’re talking about, and does not
fit any of the definitions of play used by serious scientists studying the
evolutionary history of play. “Playing games are not included in our discussion [of play].
Games, unlike play, require participants to follow a priori rules. That is, participation in games
requires participants to follow pre-set rules. If they do not follow the rules, they are penalized
in some form. In play, children do follow rules, but the rules are flexible and are typically
negotiated by children” (Pellegrini et al 2007).
Play in human children is thoroughly discussed in Burghardt 2005. The author
“amalgamates” the different categorization schemes of Piaget, Parten and Smilansky, first
compiling a table and then “for each type of play, [he] also mention[s] at least one possible
analogue in nonhuman play. Table 4.1. Some types of play described in human infants and
children. Sensorimotor (functional or exploratory play), Construction (building, piling,
stacking), Parallel (side-by-side, noninteractive), Pretend (pretense, make-believe, symbolic
play), Sociodramatic (role playing, fantasy), Rough-and-tumble (R&T play, physical play),
Language (babbling, repartee, jokes, humor), Rule (games with rules, either chance or skill,
pairs or teams), Ritual (repetitive social routines, can include rule play)” (p. 97).
I won’t belabor the point that play in human children, in both ontogeny and form, has
analogues if not mirror-images in nonhumans beyond citing what Burghardt says about the
first category of human play. “Nonetheless, the fact that all primates studied go through a
similar series of sensorimotor (0-2 years) and preoperations (2-6 years) stages, but stop at
different levels (e.g., sensorimotor stage 5 in the object permanence series in rhesus
monkeys) has brought renewed respectability to aspects of the recapitulation view of play
[one of the theories on the adapted function of play]” (p. 99).
Where Jay goes astray is his mistaken belief that complex patterns of behavior, those that
“show lots of variation in form,” and are therefore Type II Behavior cannot have an
evolutionary history. Human play behavior, that is play meeting the widely-accepted definition
of play (see above), is, indeed, complex and variable. Play in our closest relatives is also
complex and variable. In both, however, there are consistent patterns of behavior whose
form can and has been studied, with the consensus being that play has a rich evolutionary
history.
While studying (and continuing to do so) play behavior, I frequently run across suggestions
that many observers are anthropocentric in their view of behavior. I would caution Jay that
his perspective on play and complex behavior in general may also be anthropocentric.
References:
Burghardt 2005. Gordon M. Burghardt, The Genesis of Animal Play: Testing the Limits, MIT
Press, 2005.
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Caro 1995. “Short-term costs and correlates of play in cheetahs,” T.M. Caro, Animal Behavior,
1995, 49, 333-345
http://www.catsg.org/cheetah/05_library/5_3_publications/C/Caro_1995_Short_term_costs_an
d_corelates_of%20play_in_cheetah.pdf
Pellegrini et al 2007. “Play in Evolution and development,” Anthony Pellegrini, Danielle Dupois,
Peter K. Smith, Developmental Review 27 (2007) 261-276
http://evolution.binghamton.edu/evos/wp-content/uploads/2008/11/Pellegrini01.pdf
Pellegrini and Smith 2005. The Nature of Play: Great Apes and Humans, Edited by Anthony D.
Pellegrini and Peter K. Smith, The Guilford Press, 2005.
“In terms of play, [Tinbergen’s] four aims can be framed in the following way: What are the
internal and external processes leading to performance of playful behavior? What is the
ontogenetic path of play and its development in the life of the individual animal? What are the
consequences to an animal of performing a given playful behavior or behavioral variant? How
did play evolve from nonplay and what has been its evolutionary history?
In this book, the fourth aim is the central one. The main questions are the following:
 Is play just a trash-can concept for a motley set of behavioral phenomena that share
superficial characteristics?
 Do all kinds of play share common causal mechanisms deriving from common
ancestors?
 What factors led to play becoming prominent in the lives of so many animals, yet
absent in so many others?
 Where does playfulness first appear in animal evolution and did it evolve just once or
repeatedly?”
Gordon M. Burghardt, The Genesis of Animal Play: Testing the Limits, MIT Press, 2005, p. 11
“That play is a primary emotional function of the mammalian brain was not recognized until
recently, but now the existence of such brain systems is a certainty. For instance, juvenile
rats will exhibit roughhousing or RAT ludic behaviors (from ludare, meaning ‘to play’) even if
they have been prevented from having any prior play experiences during earlier phases of
development. Just as most young birds fly when the time is ripe, so do young mammals play
when they have come of age. Young rats start to play around 17 days of age, and if denied
social interaction throughout the early phases of psychological development (e.g., from 15 to
25 days of age), they play vigorously as soon as they are given their very first opportunity.
Thus, the impulse for RAT play is created not from past experiences but from the
spontaneous neural urges within the brain. Of course, a great deal of learning probably occurs
during the course of roughhousing play, but this is ultimately the result of spontaneously
active PLAY impulses within specific circuits of the brain, some of them in ancient parts of the
thalamus, which coax young organisms to interact in ludic ways on the field of competition. I
may well be that various neuronal growth factors are recruited during play, but evidence at
such molecular levels of analysis remains nonexistent.
Although play reflects genetically ingrained ludic impulses of the nervous system, it
required the right environment for full expression. For instance, fear and hunger can
temporarily eliminate play. In most mammals, play emerges initially within the warm and
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supportive secure base of the home environment, where parental investment is abundant” (p.
281).
Humans are a uniquely playful species. This may be due in part to the fact that we are
neotenous creatures who benefit from a much longer childhood than other species. Another
feature that adds to the complexity of our playfulness is the simple fact that our play instincts
are modified so markedly by our cognitively focused higher brain areas. Although cortical
processes surely add a great deal of diversity to our playful behaviors, especially as we
develop, it is unlikely that the primal brain ‘energy’ for playfulness emerges from those higher
brain functions. These energies probably emerge from the same ancient executive systems
that govern that govern RAT play in other species. As those primitive playful impulses
percolate through the brain, they assume new forms ranging from slapstick humor to cognitive
mirth. Indeed, the hallmark of PLAY circuitry in action for humans is laughter, a projectile
respiratory movement with no apparent function, except perhaps to signal to other one’s social
mood and sense of carefree camaraderie” (p. 287).
Jaak Panksepp, Affective Neuroscience: the foundations of human and animal emotions,
Oxford U. Press, 1998
“The central argument of this biological book on play behavior may be summarized, briefly and
simply as follows. Animals risk time, energy, and injury to play. They do so in certain
environments and because play is important for development in those environments. Play is a
behavioral mediator between the environment and the phenotype. Through play, the cerebral
cortex is stimulated to grow, to develop, and therefore to take a larger role in control of
behavior, making that behavior more flexible. Simultaneously, play experience produces
adaptive modifications of effector structures, such as muscle, bone and connective tissue,
used in that behavior. Through play, animals acquire physical ability and develop social
relationships. Such plasticity evolved because of economic tradeoffs in brain development: the
optimal balance between cortical and subcortical control of behavior depends on
environmental information, and the experience of play reliably serves to indicate that the
animal is in an appropriate environment for such responses to develop, as well as to develop
the responses themselves, and anatomical structures that support them, through repetition
and variation. How can such behavior evolve when the plasticity underlying the effects
involved is subject to manipulation by others in the service of their own selfish interests, which
may not coincide with those of the subject? The probable answer to this question of
evolutionary stability is that play occurs under circumstances in which current or future
resources cannot be competed for in this way; that such damaging behavior would be costly to
the actor as well as the recipient and therefore generally evolutionarily unstable against a
strategy of not playing at all; that social play is designed to include only messages that cannot
be faked or only stimuli that cannot be successfully manipulated to damage another; that
developmental resilience, including the ability to detect and resist attempts at damaging
manipulation through play, holds such damage to an evolutionarily tolerable (though not
necessarily negligible) level; and that social play evolved through kin selection, with close kin
favored as playmates, making cheating even less likely given the conditions for play already
described above (pp. 19-20).
2. Defining play. Like other coarse-grained behavioral categories, including aggression and
even sex, play is difficult to define. ‘Play’ is a convenient, but scientifically inexact term used to
denote certain locomotor, manipulative, and social behaviors characteristic of young (and of
some adult) mammals and birds under certain conditions in certain environments. The term
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‘play’ may be defined structurally or functionally. It may refer to a category of behavior, to a
behavioral or social relationship, or even to a mental state. Play can be a theoretical extreme
point of an abstract continuum whose opposite extreme is agonistic, predatory, or predator
avoidance behavior. The scientific study of play consists of a set of behavioral descriptions
and a set of theoretical, observational, and experimental investigations of the behaviors
described, including causation, results, and correlates. For the purposes of this book, play
includes nonagonistic fighting and chasing maintained by social cooperation; solo locomotor
and rotational movements performed in the absence of threatening predators, parasites, and
conspecifics; developing locomotor or manipulative behavior repeated with slight variation at a
previously established level of mastery; and diversive effector interactions with an inanimate
object subsequent to the termination of an initial phase of sensory and mastery activity,
including exploratory manipulation, directed toward the object. These behaviors are the
outward expressions of a set of distinct, but functionally related developmental pathways that
may run parallel to each other, merge, or branch. Therefore, the distinctness (by any arbitray
criterion) of these categories of play may depend on the species, sex, and developmental state
of the individual studied (p. 21).
4. Continuity. The natural history of play in mammals and birds is remarkably uniform.
When compared across species, behavioral content and structure of play appear to vary little if
at all around norms of nonagonistic contact, including wrestling or sparring, or nonpredatory
locomotion and body rotation, including pursuit, capture, handling, and evasion. Most
differences in play behavior between species mirror behavioral, demographic, anatomical, or
environmental constraints. However, complexity or elaborateness of play appear to increase
with relative brain size across a wide phylogenetic spectrum” (p.22).
Robert Fagan, Animal Play Behavior, Oxford U. Press, 1981
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