> The nymphs emerge in large numbers about the same time, sometimes more than 1.5 million individuals per acre (>370/m²).[14] Their mass emergence is a survival trait called predator satiation: for the first week after emergence, the periodical cicadas are an easy prey for reptiles, birds, squirrels, cats, and other small and large mammals.[5][15] Early ideas maintained that the cicadas' overall survival mechanism was simply to overwhelm predators by their sheer numbers, ensuring the survival of most of the individuals. The emergence period of large prime numbers (13 and 17 years) was hypothesized to be a predator avoidance strategy adopted to eliminate the possibility of potential predators receiving periodic population boosts by synchronizing their own generations to divisors of the cicada emergence period.[16] Another viewpoint holds that the prime-numbered developmental times represent an adaptation to prevent hybridization between broods with different cycles during a period of heavy selection pressure brought on by isolated and lowered populations during Pleistocene glacial stadia, and that predator satiation is a short-term maintenance strategy.[17] This hypothesis was subsequently supported through a series of mathematical models, and stands as the most widely accepted explanation of the unusually lengthy and mathematically precise immature period of these insects.[18] The length of the cycle was hypothesized to be controlled by a single gene locus, with the 13-year cycle dominant to the 17-year one,[19] but this interpretation remains controversial and unexplored at the DNA level.