LIVING things, once they have grown, generally have a habit of staying grown. Normal, healthy adults of any species are certainly not expected to shrink – and then to grow again soon afterwards.
So when ecologist Martin Wikelski of Princeton University noticed that the Galapagos marine iguanas he’d been studying for nearly 20 years seemed to be changing size from one year to the next he didn’t quite believe his own results.
At first he put the anomaly down to measurement error, but the results kept rolling in, some showing shrinkage of 20 per cent. “I can see that we wouldn’t be able to distinguish a few millimetres here and there from measurement error,” says Wikelski. “In some cases, it was a full 6 centimetres. You simply can’t make an error that big.”
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Since the changes in body length are too large to be explained simply by weight loss, Wikelski is convinced that he’s found the first evidence of shrinking in a healthy adult vertebrate. He also thinks he knows why their size fluctuates so much. His work could eventually be useful to bone researchers at NASA and to those studying osteoporosis.
Marine iguanas vary considerably in size depending on which of the 13 principal islands in the Galapagos they live on. Body length, not including the tail, ranges from a maximum of just 28 centimetres on the smallest island, Genovesa, to 59 centimetres on the largest, Isabela. This variation is thought to be because the larger, more westerly islands are exposed to nutrient-rich upwelling currents that provide ideal conditions for green and red algae, the iguanas’ main food source.
Wikelski’s team had already found that when it comes to sexual selection, size definitely matters. On all islands, larger males are most likely to be chosen as mates by females, and larger females are more likely to lay more eggs.
This success comes at a price. The biggest iguanas are worst affected in years when food is scarce, particularly during years of the climate phenomenon El Niño, which occurs approximately every three to seven years. El Niño causes unusually high rainfall, and a rise in sea temperatures from an average 18 °C to a maximum of 32 °C as cold currents and upwellings are disrupted. This is too hot for green and red algae to grow and only brown algae survive, which iguanas find difficult to digest. With less food around, the lower energy requirements of smaller iguanas mean they do better.
Wikelski’s team noticed that shrinkage seemed to be more pronounced in El Niño years. Perhaps, they thought, shrinkage is triggered by adverse conditions. That way, even the heftiest iguanas could become small enough to survive the El Niño, then regrow when food became plentiful, re-establishing themselves at the top of the hierarchy.
The results so far seem to support this idea, since shrinking iguanas on all islands do survive better under harsh conditions. What isn’t known is how they shrink and regrow. Wikelski speculates that bone absorption accounts for most, if not all, of the shrinking. He suggests that the change in diet might trigger the release of hormones that instruct the bone to either grow or degrade.
Although he has not been able to confirm this yet, what is clear is that the shrinkage is not caused by changes in the intervertebral tissue, that lies between the spinal vertebrae. Intervertebral tissue is made up of cartilage and fluid, which is softer and more easily compressed than bone. Since wearing of tissue between the vertebrae causes stooping and reduced height in older humans, some researchers have suggested that compression could be behind the iguanas’ knack for changing size. However, this tissue makes up no more than 10 per cent of the spine’s length, so it couldn’t account for the scale of shrinkage recorded by Wikelski. Compacted intervertebral tissue would also restrict the animals’ movement, but Wikelski says that although the shrunken iguanas look a bit disproportional and frog-like, they can move perfectly well, albeit more slowly than usual, perhaps to conserve energy.
Despite the lack of a clear mechanism, the idea that an animal can lose and regrow bone is attracting interest from researchers in other fields. “When astronauts go into space from a 1 g environment, that has a huge impact on the structure, form and function of their bone,” says Dr Ruth Globus, a bone scientist and endocrinologist at NASA’s Ames Research Center in California. She suggests that if the iguanas are spending more time underwater looking for food during El Niño events, there may be similar changes in the mechanical loads placed on their skeletons, which could trigger the process that leads to shrinkage.
“Working out how the iguanas shrink, then regrow to their original size could lead to new ways to reverse human bone loss”
Researchers studying the way that osteoporosis leads to bone loss have also shown interest in the work, although as yet no one knows whether there is any similarity between the iguanas’ shrinkage and the way that microgravity, osteoporosis and long periods of bed rest cause human bone loss. There are some intriguing similarities: Wikelski’s studies have shown that in El Niño conditions the iguanas show elevated levels of the steroid stress hormone corticosterone. Cortisol, a similar hormone in humans, is elevated in astronauts in microgravity, and in people confined to bed for long periods. People on long-term steroid medication are also at risk of developing osteoporosis. But that’s where the trail ends.
“My difficulty is that I don’t see the mechanism,” says John Currey, a bone-breakage researcher at the University of York, UK, who has an interest in osteoporosis. “The vertebrae would have to shorten, not just get more porous, as in osteoporosis.”
Perhaps even more important than understanding how the iguanas shrink is working out how they reverse the process to grow back to their original size, and sometimes even larger, within a year. Getting a handle on this could lead to new ways to reverse human bone loss, helping people with osteoporosis and making long-term manned space missions more feasible.
An El Niño event is looking unlikely this year but Wikelski plans to return to the Galapagos in January, armed with a portable X-ray unit. Working with a team of bone researchers he hopes to take X-rays over the next five years to compare iguanas in normal years to future El Niño years. The team hopes to confirm that the shrinkage is indeed caused by bone loss, and to convince others to study the mechanisms behind the phenomenon.
It remains to be seen whether or not Wikelski’s discovery will find an application in human medicine, but if further studies can isolate the mechanism the iguanas use for bone shrinkage and re-growth, and if these can be adapted for human physiology, the potential for understanding more about how bone tissue behaves under different conditions could be enormous. Meanwhile there are tantalising hints that the iguanas may not be alone. Since Wikelski’s paper, other researchers have taken a second look at what they initially assumed to be measurement errors in snakes and even tortoises. There are no clear answers yet, but perhaps the incredible shrinking reptiles will turn out to be more common than we thought.
