Baby Salamanders Rush to Grow Bigger Than "Bite Size"

Scott Norris
for National Geographic News
August 27, 2007
Some salamanders take great risks in order to grow more quickly, racing to become too large for predators to eat, a new study reports.

Rather than play it safe by hiding from predators, young spotted salamanders risk their lives by foraging most actively where the risk of predation is highest.

The behavior runs counter to common expectations of predator-prey relationships.

Prey species are generally thought to be more cautious foragers when predation risk is high, even if this means obtaining less food and growing more slowly.

But for young spotted salamanders, safety lies in size rather than concealment, said study author Mark Urban, of the National Center for Ecological Analysis and Synthesis in Santa Barbara, California.

That's because the species' main predator, the marbled salamander, is limited in the size of prey it can swallow.

By living dangerously during their first weeks of life, the spotted salamanders take less time to grow larger than bite-size.

"It becomes a race to grow big enough before being eaten," Urban said. "Even if the initial predation risks are high, the rewards of rapid growth may more than make up for it."

His study appears in today's edition of the journal Proceedings of the National Academy of Sciences.

Defying Expectation

In the study, conducted on larvae found in Connecticut ponds, not all spotted salamanders turned out to be risk-takers.

Urban found that foraging activity varied greatly from pond to pond.

Where rates of predation by marbled salamanders were the highest, the spotted salamander larvae foraged more actively and showed faster growth.

Urban also collected salamander eggs from populations exposed to different degrees of predation risk and raised the larvae under uniform conditions.

Larvae that hatched from eggs taken from high-risk populations foraged more actively than those from low-risk populations, even when exposure to predators was held constant.

This showed that the risk-taking behavior is genetically controlled and has evolved independently in separate salamander populations, Urban said.

(Read related story: "Evolution's 'Driving Force' Shifts Based on Behavior, Study Says" [November 16, 2006].)

"Different feeding rates have evolved across a landscape of ponds separated by as little as 100 meters [328 feet]," Urban said.

The new findings make sense but also come as a surprise, said Andrew Hendry, a biologist at McGill University in Montreal.

"Here is one of those exciting moments in science when data overturn a seemingly inevitable and obvious generality," Hendry said.

"Prey that evolve with predators should evolve greater caution when foraging, right? Wrong—at least in Urban's system.

"The explanation for this inversion is itself obvious in retrospect," he continued.

"If predators can only eat smaller individuals, then prey should grow as quickly as possible."

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