<P> Although puppies do not have the urge to procreate, males sometimes engage in sexual play in the form of mounting . In some puppies, this behavior occurs as early as 3 or 4 weeks - of - age . </P> <P> Dogs reach sexual maturity and can reproduce during their first year, in contrast to wolves at two years - of - age . Female dogs have their first estrus ("heat") at 6 to 12 months - of - age; smaller dogs tend to come into heat earlier whereas larger dogs take longer to mature . </P> <P> Female dogs have an estrous cycle that is nonseasonal and monestrus, i.e. there is only one estrus per estrous cycle . The interval between one estrus and another is, on average, seven months, however, this may range between 4 and 12 months . This interestrous period is not influenced by the photoperiod or pregnancy . The average duration of estrus is 9 days with spontaneous ovulation usually about 3 days after the onset of estrus . </P> <P> For several days before estrus, a phase called proestrus, the female dog may show greater interest in male dogs and "flirt" with them (proceptive behavior). There is progressive vulval swelling and some bleeding . If males try to mount a female dog during proestrus, she may avoid mating by sitting down or turning round and growling or snapping . </P>

Which of the following has not been proposed as a function of dog play