NEW SCIENTIST (London, UK) 28 August 07 Greedy larvae too much of a mouthful for predators (Roxanne Khamsi)

Gluttony may protect certain species of prey from predators, suggests a new study. Some salamander larvae seem to have evolved such that they actively overeat to avoid becoming a meal themselves, say researchers.

Once the larvae reach a certain size, they no longer fit in the mouths of their predators. And the new study found that the larvae were mostly likely to engage in this overeating behaviour in ponds where they faced the greatest number of predators.

Mark Urban of the National Center for Ecological Analysis and Synthesis in Santa Barbara, California, US, spent three years collecting data from 10 ponds in the northeast of the country.

All of the ponds contained the spotted salamander Ambystoma maculatum, but only some of them were also home to its primary predator, the bigger marbled salamander Ambystoma opacum.

Marbled salamander larvae can comfortably gulp down prey smaller than 3.3 millimetres. And, notably, at three weeks the body of a typical spotted salamander larva measures about this size in diameter at its thickest point, making it relatively easy to swallow.

Scientists have shown, however, that spotted salamander larvae in ponds with many marbled salamanders measure about 3.8 mm in diameter – they believe that these prey bulk up to avoid becoming dinner.

Exactly how these larvae plumped up faster remained unknown, however. So Urban took spotted salamander eggs from various ponds back to the lab and waited until the eggs hatched. Over the course of a month, he recorded how often the resulting larvae dined on zooplankton – tiny invertebrates that float in the water.

An analysis of his observations revealed that the spotted salamander larvae from ponds with many predators snapped at zooplankton at a rate of 1.8 times per minute. By comparison, larvae from ponds without any predatory salamanders attempted to eat at the slower rate of 1.4 times per minute.

Urban suggests that the larvae from ponds with high numbers of predators have evolved to eat at a faster rate because this helps them achieve the bulk they need to avoid death. Traditionally, he says, increased rates of foraging are associated with higher rates of predation, but it seems the race to increase in size can be worth the risk where you have a gape-limited predator.

The biological mechanism driving these larvae to eat more remains a mystery, Urban notes. "The behavior could be determined by genes that could control something as simple as how hungry they are."

Journal reference: Proceeding of the National Academy of Sciences (DOI: 10.1073/pnas.0704645104)
Greedy larvae too much of a mouthful for predators

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