Famine reveals incredible shrinking iguanas

Marine iguanas in the Galápagos Islands are the first vertebrates shown to shorten and then regrow, say researchers in the United States and Germany.

The seagoing reptiles shrank by as much as 20 percent during a 2-year food shortage inflicted by El Ni±o, says Martin Wikelski of the University of Illinois at Urbana-Champaign. In the Jan. 6 Nature, he and Corinna Thom of the University of Würzburg in Germany describe observations of some 6,000 Amblyrhynchus cristatus during four El Ni±os. They report reversible shortening in more than 100 individuals.

People certainly shrink as aging bones lose calcium, and doctors would love to reverse this symptom of osteoporosis. Iguanas might hold that secret, according to Wikelski. The size of the reptilian changes that he observed requires that the bones themselves shrink and then lengthen, he calculates. If connective tissue alone shriveled, the animals could shorten only by about 10 percent.

He first dismissed the apparent shrinkage as goofs in measurement. “We didn’t believe our own data for 18 years,” Wikelski says. He reconsidered his position when the researchers noticed that the length decreases correlated with El Ni±os, ocean warmings that thin out marine algal beds where the iguanas swim and graze. Moreover, the length of some iguanas’ bodies, the herpetologists’ traditional “snout-to-vent length,” dropped as much as 6.8 centimeters.

“That is way beyond any measurement error,” Wikelski claims.

The risk of error has stymied other researchers who have wondered whether animals shrink, according to Judy Stamps of the University of California, Davis. She recalls data on lizards and salamanders that seemed to suggest shrinkage. “Most people have seen it, and most people have ignored it as a measurement mistake,” she says. However, the iguana changes look too big to ignore, she adds.

Ken Nagy of the University of California, Los Angeles echoes Stamps. In desert reptiles, “I see hints of de-growing, but usually it’s so small,” he says. Species littler or wigglier than marine iguanas haven’t made good test cases. “It’s always pretty iffy to measure the length of a snake,” Nagy notes.

Howard L. Snell of the University of New Mexico in Albuquerque says he finds growth anomalies in Galápagos land iguanas but remains cautious about shrinkage. “I have to admit that I’m not completely convinced,” Snell says. “Martin’s interpretation of the observations could be correct,” he adds but suggests that only X rays will resolve the question.

Shortening could help iguanas eke out an existence in El Ni±o years, Wikelski speculates. During 1992-1993, he found that among big adults, the iguanas that shrank more lived longer.

Nagy hesitates to call shrinkage adaptive. He’s found that marine iguana adults expend less than 10 percent of their daily energy diving for food. So, he questions whether shrinking would boost foraging efficiency and trim nutritional needs enough to matter. Shrinking is just a result of starvation, he speculates.

The big males may be shrinking most because of their mating habits, suggests Brian Henen of California State University, Northridge. The real hulks spend several weeks defending territories where females cruise. Defense doesn’t allow big guys much, if any, chance to swim away to feed. Small males don’t bother with real estate and spend more time eating.

The question of shrinking might never have gotten serious thought if Wikelski’s team hadn’t built up so many years of data, muses Gordon M. Burghardt of the University of Tennessee at Knoxville. He grumbles, “Such studies are typically not valued highly enough to fund by governments or foundations, but they have the potential to unlock many secrets.”