There is growing recognition that the welfare of animals
in our care depends not only on their being kept free of
pain, discomfort and other negative experiences, but also
on their ability to experience positive emotions (e.g., Fraser
and Duncan, 1998; Boissy et al., 2007; Yeates and Main,
2008). Play behaviour has been suggested as one
potentially useful measure of positive welfare (Jensen
et al., 1998; Yeates and Main, 2008)