Large size is also favoured in males because larger males tend to be more successful at reproducing, both because of their size advantage in endurance rivalry and their advantage in sperm competition because larger males are able to produce more sperm. One reason that males are so much smaller in "Eunectes" is that large males can be confused for females, which interferes with their ability to mate when smaller males mistakenly coil them in breeding balls; as a result, there is an optimum size for males where they are large enough to successfully compete, but not large enough to risk other males trying to mate with them. During the mating season female anacondas release pheromones to attract males for breeding, which can result in polyandrous breeding balls; these breeding balls have been observed in "E. murinus", "E. notaeus", and "E. deschauenseei", and likely also occur in "E. beniensis". In the green anaconda "(E. murinus)" up to 13 males have been observed in a breeding ball, which have been recorded to last two weeks on average. In anaconda breeding balls, several males coil around one female and attempt to position themselves as close to her cloaca as possible where they use their pelvic spurs to "tickle" and encourage her to allow penetration. Since there are often many males present and only one male can mate with the female at a time, the success of a male often depends on his persistence and endurance, because physical combat is not a part of the "Eunectes" mating ritual, apart from firmly pushing against other males in an attempt to secure the best position on the female. Cannibalism is quite easy in anacondas since females are so much larger than males, but sexual cannibalism has only been confirmed in "E. murinus". Females gain the direct benefit of a post-copulatory high-protein meal when they consume their mates, along with the indirect benefit of additional resources to use for the formation of offspring; cannibalism in general (outside of the breeding season) has been confirmed in all but "E. deschauenseei", although it is likely that it occurs in all "Eunectes" species. Although sexual reproduction is by far the most common in "Eunectes", "E. murinus" has been observed to undergo facultative parthenogenesis. In both cases, the females had lived in isolation from other anacondas for over eight years, and DNA analysis showed that the few fully formed offspring were genetically identical to the mothers; although this is not commonly observed, it is likely possible in all species of "Eunectes" and several other species of Boidae.