Increased playfulness expressed by adult domesticated
animals is generally believed to be the result of paedomorphism, i.e. retention of juvenile traits
into adulthood (Goodwin et al. 1997; Dobney and Larson 2006; Kukekova et al. 2006; Himmler
et al. 2013). However, to confirm that increased playfulness in domesticated animals is caused
by paedomorphism, the ontogenetic stage when differences between wild and domesticated
animals occur must be identified (Pongrácz et al. 2010). Thus, by detecting a shift in the
expression of play behaviour in wolves and dogs at 12 and 16 weeks of age, with wolves
becoming less playful or dogs becoming more playful, in Paper III, we provide evidence
supporting the hypothesis that increased playful behaviour in dogs is caused by
paedomorphism. While wolves did not differ in playful behaviour from dogs at eight weeks,
they did express large variation. Specifically, three eight weeks old wolves scored similarly to
the most playful dogs in retrieving a small ball for a stranger (i.e. human-directed
playfulness). Such a variation can target a behavioural trait for selection, and thus Paper I, II
and III together demonstrate that playfulness could have been an important component in the
domestication of dog
