length. They are characteristically grey in colour due to digested bones, or have bone fragments included. Owen and Pemberton believe that the relationship between Tasmanian devils and thylacines was "close and complex", as they competed directly for prey and probably also for shelter. The thylacines preyed on the devils, the devils scavenged from the thylacine's kills, and the devils ate thylacine young. Menna Jones hypothesises that the two species shared the role of apex predator in Tasmania. Wedge-tailed eagles have a similar carrion-based diet to the devils and are regarded as competitors. Quolls and devils are also seen as being in direct competition in Tasmania. Jones believed that the quoll has evolved into its current state in just 100–200 generations of around two years as determined by the equal spacing effect on the devil, the largest species, the spotted-tail quoll, and the smallest species, the eastern quoll. Both the Tasmanian devil and the quolls appears to have evolved up to 50 times faster than the average evolutionary rate amongst mammals. Reproduction Females start to breed when they reach sexual maturity, typically in their second year. At this point, they become fertile once a year, producing multiple ova while in heat. As prey is most abundant in spring and early summer, the devil's reproductive cycle starts in March or April so that the end of the weaning period coincides with the maximisation of food supplies in the wild for the newly roaming young devils. Occurring in March, mating takes places in sheltered locations during both day and night. Males fight over females in the breeding season, and female devils will mate with the dominant male. Females can ovulate up to three times in a 21-day period, and copulation can take five days; one instance of a couple being in the mating den for eight days has been recorded. Devils are not monogamous, and females will mate with several males if not guarded after mating; males also reproduce with several females during a season. Females have been shown to be selective in an attempt to ensure the best genetic offspring, for example, fighting off the advances of smaller males. Males often keep their mates in custody in the den, or take them along if they need to drink, lest they engage in infidelity. Males can produce up to 16 offspring over their lifetime, while females average four mating seasons and 12 offspring.