"The results were unequivocal: snake body size at birth tightly matches the size of prey available on each island," Aubret said.
As predicted, where prey animals were bigger, newborn snakes were bigger and they grew up to be bigger adults. Where prey animals were smaller, newborn snakes followed suit, leading to smaller adults.
A New Dimension to the Island Rule?
Ecologists have long been interested in the peculiarities of island animals. Observations of pygmy elephants and giant rats led a biologist named J. Bristol Foster to propose what became known as the Island Rule. In general, Foster surmised, big animals on islands tend to get smaller than mainland counterparts because of limited access to food. Small animals tend to get larger because islands tend to have fewer predators. Since it was proposed in the 1960s, numerous exceptions to Foster's rule have been noted, and scientists now agree that the ecological factors that influence island body size are far more complex than Foster had imagined.
Aubret's findings add yet another dimension.
"Mean adult body size has always been used as a traditional measure in the literature," he writes. "On the other hand, patterns of variation for body size at birth in island populations have received, to my knowledge, no attention at all."
Aubret's work shows that selection isn't necessarily acting on adult body size.
"This study confirms that adult size variations on islands may be a nonadaptive consequence of selection acting on birth size," he said. "Animals may become either giant or dwarf adults on islands for the simple fact that they were
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