Play
Jeff Schank
ANB 218a 2013

• Criteria for play
1. Incompletely functional;
2. spontaneous, pleasurable, rewarding, or voluntary;
3. differs from other more serious behaviors in form
(e.g., exaggerated) or timing (e.g., occurring early in life before the more serious version is needed);
4. is repeated, but not in abnormal and unvarying stereotypic form (e.g., rocking or pacing); and
5. is initiated in the absence of severe stress.

• Solitary locomotor-rotational play
– Vigorous motor acts, typically performed alone (e.g., playful running and twisting in ungulates , and somersaulting in monkeys )
• Object play
– Involves the playful use or manipulation of inanimate objects
(e.g., a dog retrieving a stick or a cat batting a ball )
• Social Play
– Social play involves two or more players that are usually, but not always conspecifics .
– Typical movement patterns involve chasing , wrestling , and tailpulling, and even a form of peek-a-boo
– Of all social play patterns, rough-and-tumble play (R&T) , or playfighting, is most frequently studied in animals

• Surplus Energy
– Animals play when the have surplus energy and they are in good health—Why play?
• Instinct-Practice
– Instinctive behavior requires practice to become optimal (assumption) and play is a form of practice—
Why and how is play practice?
• Recapitulation
– Play is a vestigial relic from past evolution—Why is play so common?

• One problem with the older theories is that each was viewed as an independent theory
• However, aspects of each are relevant to the modern functional view
• Functional view
– Play likely has multiple functions: motor training, practice, and socialization
– Very difficult to test the functions of play because presumably the fitness benefits are delayed till adulthood

• Motor training
– Physical exercise: No evidence
– Cerebellar synaptogenesis: play occurs after most of cerebellar synaptogenesis is complete
• Training for unexpected events
– Play allows an animal to acquire kinematic and emotional skills required for dealing with unexpected events
– Predicts play should be more frequent in changing environments
– Can’t explain why play is less frequent is poor (food shortages, stressful, challenging) environments

• Practice
– Little evidence that play facilitates learning corresponding adult behaviors
• Social benefits
– Enhancing social skills
– Strengthening social bonds
– Reducing aggression
– Refining social assessment
– Learning and promoting cooperative behavior
• sharing
• Reciprocity
• Altruism
• Fairness
• Little evidence to support any of these theories since some animals deprived of social play still develop adult social competencies
• Playing animals seem to learn fairness and cooperation, and may even punish cheaters by not playing with them
• I’ll come back to social play later

• Costs
– Injury from falls or aggressive retaliation
– Reduced time spent in survival behaviors (e.g., foraging)
– Expenditure of energy
– Increased predation risk as a result of reduced vigilance and the conspicuousness of play to predators
• Delayed Benefits
– Nunes (2004) found that social play in female
Belding’s ground squirrels increased reproductive success during their first breeding season

• Examples of interspecies play
– Lion and Tiger
– Wolf, Bear, Human
– Cat and Owl
– Rat and Cat
– Kangaroo and Lemur
– Dog and Deer
– Polar Bears and Dogs
– Cat and Dolfins
– Monkeys and Dogs
– Humans playing with Stingray?
• How do we explain interspecies play?
• Does it tell us anything about social play?
• Does it tell us anything about the evolution of social play?

• It seems plausible if not obvious that social play could derive its fitness benefits by learning the skills necessary for adult cooperation
• The evolution of cooperation, however, is hard to explain, especially at low frequencies
• Suppose a social play gene is introduced into a population of non-cooperators at low frequencies
– How do juveniles with the play gene find others to engage in social play?
– If they do, and learn to cooperate, adults would be at a disadvantage in a population of mostly non-cooperators
• It would appear that the evolution of social play by facilitating adult cooperation is improbable at best

• Sometimes intuitions can be misleading!
• Let’s consider agents with generic biological properties characteristic of animals that engage in social play and adult cooperation
• Agents can
– Movement and Aggregation
– Development (juvenile  adult)
– Learning
– Reproduction
– Parental investment
– Life span
– Foraging for resources

Prisoner’s Dilemma: T > R > D > S (e.g., T = 5 > R = 3 > D = 1 > S = 0)
Stag Hunt: R > T ≥ D > S (e.g., R = 2 > T = 1 ≥ D = 1 > S = 0)

Rescorla-Wagner learning model
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Bouts*of*Scoical*Play*
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Figure 7 . Snap shots of the evolution of social play and adult cooperation over 100,000 rounds for weak learning ( a
= .1), very low probability of reciprocating social play by juvenile agents with the social play gene ( r = .025), but high parental investment ( p
I
= .75).
The likelihood of The inclusion of a social play component allowed cooperation to evolve, particularly when group selection processes were operating. As would be expected, higher rates of learning (α) and play reciprocation ( r ) were associated with a higher probability that cooperation would evolve—though even a reciprocation rate of zero in combination with a relatively low level of α was sufficient for cooperation to evolve (albeit with low probability). In contrast, cooperation did not evolve in the no-play control condition, where agents cooperated if and only if they had inherited the “social play” gene without any social play experience required.
This may seem counter-intuitive, as one might expect cooperation to evolve more readily in a situation when only one condition must be met (having the social play gene) for cooperation
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• Social play could evolve facilitating the learning of adult cooperative skills
• How does it work?
– Sewall Wright’s shifting balance theory
– Effectively neutral mutations in low frequency
– Parental investment: inheriting wealth
• There is much to be done to understand play
– None of Tinbergen’s 4 questions have been adequately developed