Rooting and the development of male sexual behavior in two inbred mouse strains

The sexual development of C57BL 6/J and SEC/IReJ male mice was studied during twice-weekly pairings with estrus females from 5 weeks of age. SEC were far slower to begin copulating than C57 and their responses to the female included a much higher frequency of rooting. Rooting, where the male's snout is pushed below the female's flank and jerked upwards, declines over tests as the full copulatory pattern develops. SEC also spent a higher proportion of their time in close contact with the female and groomed her more than C57. Both strains showed more rapid development of copulation if testing was delayed until 10 weeks of age, especially if they had exposure to female scent during this maturation period. The frequency of rooting and the speed of development are inversely related in these strains, but in segregating hybrid populations no such relationship is maintained. It is argued that the slowness of SEC's development cannot be ascribed to low sexual motivation. The significance of rooting remains obscure, but it seems possible that it may represent an aspect of exploratory behavior that is highly developed in SEC and some other strains.