The Occurrence and Function of Play Behaviour in Mammals
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Play in mammals is a widely reported behaviour. It tends to occur in younger mammals and diminishes as they reach adulthood. Play in young mammals often mimics adult behaviour that is essential for survival e.g., predatory skills. This has led to the theory that the function of play is to enable young mammals to practice and develop the skills that are required for adult survival. In accordance with this theory, play is classed into locomotor play, predatory play, object play and social play, with the assumption that each class of play confers a future benefit for the survival of individuals. However, this proposed theory of the function of play is controversial and has received much criticism. The main criticism is the lack of evidence to support that play has any particular function. Studies on play have relied on anecdotal evidence or on experiments that lack appropriate controls. In addition, there is a competing theory that advocates mammalian play is just a behavioural by-product lacking any particular function. Several studies have addressed this issue but they also have lacked the appropriate controls and have therefore also been unsuccessful in determining if play has no function. In order to determine the role of play, future studies need to address the problems of past studies by incorporating better-designed experiments with adequate controls. This will help to increase our knowledge of play in mammals and help to determine whether play has any adaptive function in mammalian life.
One of the most noticeable behaviours of many species of mammals is that they engage in acts of play. Never is this more evident than in humans. Humans spend most of their childhood engaged in playful behaviour with toys or with other children. However, playful behaviour is also seen in other mammals, such as kittens and puppies. They spend huge amounts of time and energy playing with objects and engaging in pretend fighting with each other usually involving feigned bites and scratches. Play, however, is not the reserve of domesticated mammals-it is often manifested in many species of wild mammals, such as dolphins, primates, ungulates and a variety of other wild mammals.
An interesting question about play behaviour is whether it serves any particular function. There are many experiments that indicate play in humans is important in developing children's social skills that might be important for adult life. These might be things such as learning about fairness or deception (e.g., see Smith 2005) One reason why experiments on human play are so revealing is that language allows the experimenter to extract specific details or experiences from the subjects. However, when it comes to non-human mammals (herein mammals) this is obviously not the case. Therefore, the issue whether play in mammals has a function is a contentious one, with some arguing that it has a function and others arguing that it does not. The literature on this debate will now be reviewed.
The study of play in mammals has been active for many decades and one fundamental problem was how to define play behaviour. Ethnologists have argued this issue for years with lengthy papers and books being published about the problem (e.g. Smith 1984). However, Bakeoff & Byres' (1998) definition appears to be favoured by most ethologists. They define play behaviour as a repeated behaviour, lacking an obvious purpose, initiated voluntarily when the animal is unstressed.
One aspect of play in mammals is that it often involves immediate costs to the individual. In terms of energy costs, it is estimated that between 5-20% of the surplus energy, i.e., energy not involved in growth or metabolism, is used during play activity (Manning & Stamp-Dawkins 1998). Play can also prove costly by increasing the risk of injury. For example, young desert bighorn sheep live in a harsh environment and are often pierced by cactus thorns during playing, whereas young gelada baboons on steep slopes have been recorded falling several meters during play resulting in injury, such as lameness (Manning & Stamp-Dawkins 1998). Another significant cost of play occurs in many prey mammals because play results in less attention being paid to potential predators. Harcourt (1991), for example, found that 86% of young fur seals were killed by predators when the seals were engaging in play.
Many researchers argue that play has evolved because it has some adaptive purpose. In other words, it has evolved because it confers some benefit to the survival of individuals. At first this theory seems unlikely when the above costs of play are considered. However, it is argued that the benefits of play are not immediate, but delayed. So, although play has no immediate function and can be a costly behaviour, it is in fact the long-term benefits of play that are an advantage to the mammal. And it is argued that these future benefits of play outweigh its immediate costs, allowing the behaviour of play to be adaptive (e.g., Smith 1984). However, it is worth noting that such arguments do not explain Harcourt's (1991), finding in which 86% of young fur seals were killed by predators when the seals were engaging in play. It is obvious that in this situation future benefits of play behaviour are non-existent.
The occurrence of play in mammals appears to reveal two specific trends. First, the most consistent finding of play research is that mammals play more when their quality of life is at high levels. This includes abundant food resources, warm weather and a safe environment. Burghardt (2005) termed this finding the Surplus Resource Theory and claims the occurrence of play can be predicted depending on the state of individuals' environment. Second, play behaviour invariably occurs in young mammals and tends to diminish as mammals get older. This has therefore led many to suggest that play is important for the development of adult behaviour in mammals (e.g., Smith 1984). This fits in well with the theory that play benefits individuals in the future. Consequently, the categorisation of play is based on the assumption that play confers future benefits. On this basis play in mammals has been categorised into locomotor play, predatory play, object play and social play (Burghardt 2005). Each category is suggested to function differently in aiding the survival of individuals by providing future benefits:
Locomotor play consists of juvenile mammals carrying out movements similar to the physical actions of adults, but with no immediate or obvious end goal. Examples of locomotor play include running, leaping, heel kicking and whirling around. Ungulates are the main participants in this type of play. It is suggested that locomotor play appears to function as motor skill training for mammals that rely on agility and speed to avoid predation. Not surprisingly, running is the most common aspect of locomotor play observed. However, this escape-oriented play is not limited to ungulates. For example, the young in many primate species chase each other and it is hypothesised that this might also act as training for emergency situations, such as when predators threaten them.
Predatory play involves actions that mimic real predatory behaviour such as stealth and pouncing actions. The premise of this type of play is that it enables young mammals to practice and finely tune their hunting skills, which allows them to become successful hunters during adulthood. Unsurprisingly, this type of play-behaviour is found predominantly in predatory mammals such as felids and canids.
Object play has obvious overlaps with predatory-play, such as when a kitten chases and attempts to catch an object. However, non-predatory object play has been reported in mammals. The main proponents of this type of play are primates especially our closest relatives, the great apes. The increased complexity and dexterity of the hands of these species, as well as longer periods of maturation and increased cognitive abilities, may explain this trend. Chimpanzees, for example, often use tools in their daily lives, such as fishing termites out of holes with sticks. It is therefore suggested that object play is important for developing manual dexterity. Young chimpanzees often play around with sticks before learning their termite-fishing skills that manifest in older chimpanzees (Burghardt 2005).
Social play mainly exists in group-living mammals. Many species of young mammals have been reported to engage with their peers in bouts of wrestling and other physical activities. Elephants, for example, slap each other with their trunks, apes wrestle and giraffes use their long necks to spar gently (Burghardt 2005). These behaviours appear similar in many ways to competitive and often aggressive adult contests, but without the actual violence. It suggested that social play allow infants to learn how to deal with other group members. In particular, researchers suggest that social play may safely teach infants the skills they will later use in aggressive social competition. Moreover, social play may also strengthen social bonds between group members. This is thought to reduce the amount of actual aggression between group members thereby reducing injury or death that can occur during inter-group conflict.
An interesting characteristic of social play is that it is usually preceded by the occurrence of play signals, such as "play invitations," "switch signals," or "play faces". Bruner (1972), for example, hypothesises that the function of such signals is to indicate that play is about to start and this prevents any initial play actions being mistaken, by the receiver, for more serious actions such as true fighting.
Although some researchers believe that play in mammals has specific useful functions that benefit the survival of individuals, there are many who argue against such a theory. One of the main criticisms is that most of the reported cases of play rely on anecdotal evidence. This usually involves one individual seeing a behaviour that they deem is play and reporting it as such. The problem with anecdotal evidence is that any single report or story is irrelevant in the face of scientifically-collected data. For example, one person may deem a particular behaviour as play whereas another person may not see the same behaviour as play. Scientific studies help to reduce some of this type of ambiguity and are therefore needed to increase our understanding of play. However, it is reasonable to assume that anecdotal evidence of play has a role in initiating scientific studies, e.g., a reported anecdote of unusual or revealing cases of play can be followed up with scientific research.
The study of play behaviour has also come under criticism because of its reliance on the adaptionist approach, which is often used to argue the evolutionary explanation of play, i.e., that play is an adaptive trait that has evolved to increase the fitness of individuals. Poirier (1982), for example, suggests that play would not be so widespread among mammals were it not adaptive, and Lewis (1982) states that it seems reasonable to assume that play, like all other behaviour, has evolved for some reason (often assumed to be an adaptive reason). However, there are many who argue against such adaptionists' theories, suggesting that play is just a by-product of evolution and has no particular function (e.g., Fagen 1981). Some researchers take this view and believe that play might just feel good and that is why individuals engage in such behaviour (Fagen 1981). In addition, Gould and Lewontin (1979) argue a good theoretical case for the evolution of by-products that serve no adaptive function but are mistakenly believed to have evolved for the purpose. It is therefore possible that play has no adaptive function.
Another problem about the hypothesis that play functions to benefit individuals in the future is that there seems to be little solid evidence to support it. For example, Fagen (1981) reviews substantial evidence showing that, instead of preceding the mastery of many skills, play follows it in many cases. In addition, in spite of considerable research, the evidence for the delayed benefits of play is sparse. For example, Lee (1983) studied vervet monkeys in East Africa over several years. Drought occurred for the duration of some juveniles' lifetime and observations showed that these individuals did not engage in play, instead they concentrated all their energies on searching for food and water. Therefore, Lee proposed that many juveniles went into adulthood without ever experiencing play. Lee compared these individuals with other group members who had not grown up in the drought periods and had been observed experiencing play. Lee found no differences in the adult behaviour of monkeys who had experienced play and those that did not experience play. However, it should be noted that Lee did not observe the drought mammals 24 hours a day over their juvenile period, so it is impossible to be certain that play was completely absent from their behavioural repertoire.
It appears the only way to study if play functions to benefit individuals in the future is to do controlled experiments. However, experiments involving depriving individuals of play and then comparing their behaviour with other individuals who have not been deprived of play should be avoided. Although, such experiments would ensure that individuals had not engaged in play, (e.g., 24 hour camera monitoring) it is virtually impossible to deprive play without depriving them of other variables, such as social contact. Many studies show that denying mammals social contact has drastic results on their development (Manning & Stamp-Dawkins 1998). Therefore, such experiments are unlikely to shed any light on the function of play.
Recently, some researchers hypothesise that behaviour might have an immediate function. This has led to several controlled experiments to test the theory. Interestingly, the design of such experiments do not involve depriving subjects of play, so reduce the problems of other variables. Palagi et al (2004), for example, studied the occurrence of play in relation to immediate benefits in captive chimpanzees housed in a zoo. They found that peak play occurred in adults and juveniles just prior to their daily feeding times, which happen at the same time each day. The authors suggest that play therefore gives immediate benefits to individuals because play just prior to feeding times functions to reduce tension between group members. Feeding times are periods of high tension as group members compete for food, and play is suggested to reduce this tension. Although this hypothesis might be feasible, it also possible that other factors are involved. It can be imagined that when the chimpanzees anticipate feeding times they become excited and this excitement might be a trigger for the onset of play. In this situation play might just be a consequence of excitement and not as a function of reducing tension between group members during feeding time. There seems to be no literature on the relationship between excitement and play, but this theory can not be ruled out in Palagi et al's study. Maybe the experiment would have benefited from having a condition in which an unfamiliar feeding time was presented to the apes. In this condition it would be expected that the apes would not anticipate the arrival of food and therefore not increase their play before the food was given. This would mean the apes would compete for food without having prior peak-play periods. The apes' behaviour during feeding, such as conflict frequency, could then be compared to prior peak-play to see if there is any effect of play in reducing conflict.
Although play behaviour is exhibited in a wide range of mammalian species it tends to be a behavioural characteristic of younger mammals and tends to diminish as they enter into adulthood. Play in young mammals appears to mimic types of adult behaviour that is essential for survival. This has led to the development of one theory that play functions to aid individuals' future survival. However, the study of play is one of the most controversial areas of animal behaviour. Whilst there are many argue that play has a function, others argue that it has no particular function. The main claim of the supporters of the play-function hypothesis is that play is such costly activity and therefore is likely to have benefits that outweigh the costs. However, there is very little convincing evidence that play has a function and confers future benefits to individuals. However, there is also a lack of convincing evidence to support the alternative hypothesis that play is just a behavioural by-product without any particular function. To date, most studies that test the function and non-function of play tend to lack proper controls or rely on anecdotal evidence or theoretical explanations. What are needed are better designed experiments that have adequate controls to tease out the role of play and make our understanding of mammalian play clearer.
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