Play functions and hypotheses 33!

Play can have multiple ascribed functions with the suggested principal function differing from species to species (Martin & Caro, 1985). Researchers have long debated the possibilities of an all-encompassing function of such a seemingly purposeless behavior, and long lists have been developed in effort to establish all of the potential benefits arising from this behavior (functional review in Baldwin & Baldwin, 1977; general play review in Fagen, 1981). Proposed functions have been broadly categorized into three main classes: play as motor training, play as socialization, and play as cognitive or sensorimotor training (Martin & Caro, 1985). In turn, these three classes have shaped the series of hypotheses that have given play behavior research a more empirical course.

5.3.1. Physical training

The “physical training” and “motor training” hypotheses have been suggested to address the physical aspects of play. Both hypotheses theorize that play behavior functions to increase and maintain physical fitness or motor performance. Through engagement in repeated physical motions during social locomotory play, the partners involved increase the fitness of their physiological systems (Byers & Walker, 1995; Fagen, 1981; Smith, 1982). This includes the strengthening of cardiovascular, muscular, and respiratory systems vital for daily biological functioning. Similarly, the “exercise hypothesis” proposes that play increases the Darwinian fitness of the individual through enhanced

physical fitness, and it is through the strengthening of the physiological whole that fecundity and survival are concurrently improved (Martin & Caro, 1985). Support for a physically based function of play has been empirically demonstrated in five arboreal ceboid monkeys (Alouatta palliata, Ateles geoffroyi, Cacajao calvus, Cebus capucinus, and Saimiri boliviensis). The use of suspensory postures over non-suspensory postures during arboreal play was observed to increase the flexibility of the joints and thus promoted an enhanced maneuverability on unstable substrates (Fontaine, 1994). Such physical flexibility would allow for more skillful navigation through the environment and in particular, the ability to navigate more efficiently through unpredictable environments.

5.3.2. Social skills

In addition to a physical acclimation to exercise, play also establishes important social relationships between an animal and its peers. The “social skills hypothesis” states that play is not vital for the accrual of adult social skills per se, but rather that play enriches the behavioral repertoire of the animal and therefore refines its social skills while also increasing behavioral flexibility (Baldwin & Baldwin, 1974; Brown, 1988; Fagen, 1984). This behavioral flexibility equips an animal with a coping strategy for novel environments, interactions, and partners. The social skills hypothesis predicts that play will occur more frequently with an evenly matched partner and especially with a peer akin to those that are likely to be encountered in adulthood. This naturally varies between species according to adult social structures.

The majority of play behavior in the great apes is dyadic social play displaying intense locomotive sequences and rough-and-tumble behaviors. Social play among captive

western lowland gorillas (Gorilla gorilla gorilla) depends on specific social factors such as the age of the gorilla and age and sex of the partner (Brown, 1988). Younger gorillas have been shown to play more frequently with peers of a similar age, and the general frequency of play typically declines as gorillas mature (Brown, 1988). Partner preference has also been shown based on sex, and male-male and male-female play dyads typically occur more frequently than female-female dyads (Brown, 1988; Palagi et al., 2007). These play dyad structures are reasonable observations when considered from a gorilla social structure standpoint. Both male and female gorillas emigrate from their natal group, and while females form strong social bonds with the adult males of their new group, they do not tend to form strong social bonds with the other adult females (Harcourt, 1979; Watts, 1996). As adults, the male-male bond in gorillas is central, which differs from other primate species that have a strong matrilineal influence. Male gorillas may also spend part of their life living in all male bachelor groups; therefore, male-male and male-female playing would be more frequently observed than female- female dyads. These results are consistent with other studies that examine sex differences in play among gorillas (Maestripieri & Ross, 2004; Palagi et al., 2007) and remain consistent with the predictions of the social skills hypothesis.

5.3.3. Cognitive skills 5.3.3.1. Play signals

Play may also be an important arena for learning the ability to detect and react to the intentions of other individuals through the recognition of social cues that correlate with the initiation, progression, and outcome of the interaction (Bekoff, 1972, 1997; Palagi et al., 2007). Play-specific signals allow an individual to distinguish between playful and

aggressive interactions. These cues can inform a recipient about the actor’s mood and interaction intentions and can also subsequently influence the recipient’s own mood and intentions (Pellis & Pellis, 1996, 1997). The well-noted examples of play-specific signals are the “play bow” in canids (Bekoff, 1995 & 1997) and “play face” in primates5 (Aureli & Whiten, 2003; Chevalier-Skolnikoff, 1977; Fagen, 1981; Flack et al., 2004; Parr, 2005; Pellis & Pellis, 1997; Preuschoft, 2000; Smith, 1978, 1984; Van Hooff, 1973; Waller & Cherry, 2012; Waller & Dunbar, 2005). It is commonly accepted in the literature that when such play signals are used, they aid in the initiation of play (“I want to play”), establish and maintain a playful context (“I still want to play”), and punctuate the play interaction at variable temporal intervals. These specific behaviors reaffirm that in general, all of the behaviors that are performed are playful although they can possibly be conceived as aggressive within an alternate context (Bekoff, 1995). This is especially important for more dangerous behaviors, such as biting, grappling, slapping, or actions directed toward sensitive areas, namely the face, head, and genitals.

An analysis of the frequency of play in captive gorillas (Gorilla gorilla gorilla) and bonobos (Pan paniscus) observed differences between the play frequencies of two differing social conditions: food distribution and space availability (Palagi et al., 2006 & 2007). Primarily, the mean frequency of play was highest immediately before food distribution and then dropped once food had been introduced to the enclosure. Secondly, the incidence of play face signaling during play fighting increased when partners were in

5 _{Pellis & Pellis (2011) remark that not all primate species use the “play face” to initiate play, citing one} species, the black-handed spider monkey (Ateles geoffroyi), that uses head-shaking instead. Pellis & Pellis (1997) and Pellis et al. (2011) further comment that the primate play face, although accepted as an unambiguous marker of play within the research, is actually a poor, true play signal, because the open mouth is a precursor to biting behavior, which is common in primate play fighting.

a smaller indoor enclosure than one that provided more room. Both studies illustrate how play can be used flexibly to address two different potentially tense social scenarios.

The higher incidence of play before food distribution may possibly have been a mechanism to ease social excitement or stress associated with feeding times. The higher incidence of play signaling may have been flexibly employed to appease conspecifics during play fighting while in more confined quarters (Palagi et al., 2006 & 2007). The risk of play fighting escalating to aggression increases as the enclosure size decreases and the ability to escape unscathed diminishes. Similar play signaling studies of social play in canids show that the play bow frequency increases as the intensity of play increases (Bekoff, 1995). Therefore, increased play-specific signaling may work to keep the interaction benign even if the social conditions change. Another example of play-specific signaling may be the high frequencies of chest-pounding in gorillas that occurs in conjunction with social play (Brown, 1988; Maestripieri & Ross, 2004). The chest- pound may act as a behavior to solicit play from a partner while simultaneously informing the partner that the behaviors that follow will be playful. However, chest- pounding is also used within aggressive displays, so more general contextual signals may accompany this ambiguous behavior to signal the difference between a playful and aggressive display.

5.3.3.2. General contextual signals

In addition to play-specific signals, other more general context signals may also inform partners about the more immediate actions or intentions, such as approaches, retreats, and attentional states. This social information may be based in macro-cues, such as body

movements, body postures, and head orientations, and face-specific cues such as eye gaze. An investigation of general context signals is advantageous for determining what cues influence the behaviors of others and are available for predicting the behaviors of others, both vital for navigating social interactions. Whereas play-specific signals are often limited to the play context, general context signals may work to emphasize the social information contained in play-specific signals.