Play behavior is related to the development of strength, motor coordination and physical resistance, learning of novel environmental information and creativity, and the acquisition of social skills. This enhances adaptive abilities so that they are prepared to handle future real-life situations . Juveniles play to explore their environment and experiment with a variety of strategies that may be effective in that niche . The main period of animal play coincides with the main period of physical, hormonal and social development, suggesting that play behavior can modulate developmental aspects . In a review, Fromberg  describes children’s play as symbolic, meaningful, active, pleasurable, voluntary and intrinsically motivated, rule-governed and episodic. Allen and Bekoff  argue that social play, during which players follow the implicitly agreed upon rules not to dominate, prey on, or mate with their playmates, may provide a foundation of fairness for other forms of social cooperation and discuss turn-taking during social play as a behavioral indicator of rudimentary morality. According to these authors, cooperation and fairness can be driving forces in the evolution of sociality. Since play is a pleasurable emotion that only occurs when animals are in a relaxed (positive welfare) state it might be used as an indicator of animal welfare.
Enhancement of positive welfare and adaptive abilities may be possible by means of genetic selection. Whereas artificial selection has resulted in unprecedented increases in production traits, the possibility to include behavioral traits in selection programs is rarely considered, despite its potential to improve animal production and welfare. Breeding goals have been broadened beyond production traits in most farm animal species to include health and functional traits, but opportunities exist to improve breeding indices with the inclusion of behavior . In an extensive review summarizing the estimates of genetic parameters for behavioral traits in cattle, pigs, poultry and fish, Canario et al.  indicate that the genetics of behavioral traits is understood to some extent, but is seldom accounted for in breeding programs. The rare genes identified that are associated with behavior are related to motivational processes, which clearly demonstrates that behavioral traits are important welfare indicators . Breeding for behavior presents a number of particular challenges, and breeding for welfare even more. It is difficult and time-consuming to directly measure behavior in a consistent and reliable manner, necessary to evaluate the large numbers of animals that are necessary for a breeding program . In addition, the major obstacle to overcome before genetic solutions can be implemented is determining which trait(s) to select for in order to truly improve animal wellbeing, either through direct measurements or through indirect measurements that are strongly correlated. This is particularly difficult as scientists have proposed many different conceptions and definitions of animal welfare, such as animal function, the balance of pleasure and suffering, and preference satisfaction [14, 15]. Considering play behavior as a potential indicator of animal welfare has been proposed only recently .
When play markers are considered for inclusion in the breeding goal, it is important to investigate the reliability and consistency of several measures of play behavior in different contexts. In the present study, the increase in the space allowance increased the occurrence of locomotor play in piglets, which is in accordance with results by Jensen and Kyhn  in calves. The present experimental setup investigating locomotor play resulted instantly in an obviously pleasurable and measurable response and consistency of this response was measured at different ages.
Females had higher scores of total number of joyful movements and total time expressing joyful movements than males but this, respectively, was significant and approached significance only at 41 d of age. Sex differences in social play behavior in piglets were investigated by Dobao et al. , who observed a higher social play activity in males than in females. Social play included encounters between two individuals that are involved in reciprocal contact, such as pushing, butting, biting or mounting. Their results verify analogous results described in other species, which support the idea that contact-oriented play develops in part to provide the training for combatant skills in males . Differences in play fighting between males and females have been shown to depend on the action of androgens perinatally . However, as suggested by Berger , non-social locomotor play, as investigated in the present study, is likely part of a general anti-predator strategy in both sexes. Indeed, Sachs and Harris  observed that female lambs performed more locomotor play than male lambs and no sex differences were found in locomotor play in gazelle . Newberry et al.  reported an absence of significant sex differences in the frequencies of the play markers “hop, scamper, pivot, toss head, shake object and carry object” in domestic piglets living in multi-litter groups with their dams.
In accordance with Jensen and Kyhn , in the present study, locomotor play generally did not involve physical contact and triggered locomotor play in a litter mate on few occasions only. However, the results of the present study show that the litter of origin had an effect on the measures of playful behavior. Piglets from one of the litters generally appeared to play for a particularly long time, while piglets of another litter generally appeared to play for a particularly short time. These results indicate that the litter environment or ‘a general mood’ in the litter may affect all litter mates during play; further research is needed since the present study included four litters only. In addition, it may indicate that locomotor play has a genetic component. Walker and Byers  indicated a possible genetic basis for locomotor play in house mice with a heritability of 0.55 ± 0.40. Heritability of play behavior in farm animals needs to be investigated when this trait is considered to be included in the breeding goal.
Litten et al.  observed a significant relationship between body weight and behavioral development in piglets. Piglets that are heavier at birth gain more weight and are the more successful fighters among piglets of a litter . Sundman  observed that piglets suckling at the anterior teats tended to be more playful, and piglets suckling at the rear teats less playful, than those suckling the middle teats. In the present study, body weight adjusted for sex was not significantly related to the number or time of joyful movements in any of the three tests. This may indicate that the joyful movements investigated in this study are relatively unrelated to the piglet’s social position within the litter.
When play behavior is considered as a measurement that can be included in the breeding objective it is important that a play marker is chosen that can be measured instantly and reflects the inherent play behavior of the individual when other factors are kept constant. In the present study, the phenotypic correlations between total time at 41, 44 and 48 d of age, adjusted for the effect of litter and sex, were mostly positive, although significant only between 44 and 48 d of age; the same was true for the total number of movements. These results indicate that playfulness may be a trait that is inherent to the individual but still much dependent on the day of measurement. Newberry et al.  and Jensen and Kyhn  indicated that play behavior decreased with age. Indeed, on average in the present study, the total number and total time exhibiting locomotor play was highest at 41 d of age. This may be an effect of getting familiar with the test situation also.
In the present experiment, play behavior has been investigated in piglets that were weaned at 35 d of age. Weaning at this age is not common in commercial production animals where animals are commonly weaned around 21 d of age or earlier . Under commercial conditions, piglets are regrouped and mixed with unfamiliar pigs resulting in aggressive and submissive agonistic behavior . It will be of particular interest to investigate the relationship between pre-weaning play behavior and post-weaning adaptive abilities that are required during mixing. Donaldsen et al.  investigated the effects of early play experience on play behavior of piglets after weaning where they hypothesized that play experience gained by piglets during early ontogeny would affect the ability to cope with weaning stress. Indeed, piglets that had been allowed to play before weaning regained frequency of play behavior sooner after weaning . Future research will investigate whether pre-weaning play behavior is related to learning ability and adaptability in different environments.