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Salamander Streambeds

Salamander Streambeds Photo Credit: Bill Bouton [CC-BY-SA-2.0], via Wikimedia Commons

Sometimes, in nature, little things can make a big difference. Here’s an example from the streams of Oklahoma.


Transcript

How streambeds shape a species. I'm Bob Hirshon and this is Science Update.

Imagine if your body type depended on the pile of your carpet. That’s kind of like the life of the Oklahoma salamander, according to evolutionary biologists Ron Bonett and Paul Chippindale at the University of Texas at Arlington.

The salamanders are born in streams as fish-like larvae. Bonett says that in streams with large, loosely packed gravel, the salamanders mature and breed in this fishy form, and wriggle down to underground water during the dry summers. But other streambeds are more tightly packed.

Bonett:

So in the summertime, when the stream’s dry at the surface, there’s not really any access to any underground water.

So instead, these salamanders shed their gills and transform their bodies for a new life on land. I'm Bob Hirshon, for AAAS, the science society.


Making Sense of the Research

When Charles Darwin traveled to the Galapagos Islands, he noticed that different species of finches (small birds) that lived there had different beaks depending on their diet and habitat. For example, birds that ate bugs had long, thin beaks that could poke around in the ground for food, while birds that ate mostly seeds and leaf buds had short, strong beaks for grinding. These observations led to his theories of adaptation and natural selection: animals with traits (adaptations) that gave them a competitive advantage in their environment were more likely to survive and pass on their traits to their offspring (natural selection).

There are all kinds of environmental factors that can drive natural selection, but the salamanders’ case is striking because it’s so specific and seemingly insignificant. All of the salamanders in this study are born in streams that dry out in the summer. However, the water currents keep flowing under the surface of the streambed during the dry season. If the salamanders can get to that water, they can pretty much keep the same lifestyle and diet they had when they were born. If not, they have to either forge out onto land to find food, or starve and fry.

Whether or not the salamanders can wriggle underground depends on the size of the soil particles. To understand why, imagine draining two swimming pools and filling one with beach balls and another with sand. If there were a gold ring at the bottom of each pool, which ring would be easier to get? You can imagine that the one with the beach balls would be a lot easier to squeeze through. That’s because there’s a lot more empty space in between beach balls than there is between sand particles, which means it’s easier to push the beach balls out of the way.

To see how the soil particles affected the salamanders, Bonett’s team measured the size of the particles in 22 different streambeds and compared them to the populations of salamanders that live there. In streambeds with large, hard gravel, most of the salamanders were paedomorphic, which means that even when they mature and breed, they keep their juvenile, fish-like qualities, including gills. In streambeds with small, soft soil particles, metamorphic salamanders dominated. When these salamanders hit puberty, they shed their gills and transform into air-breathing animals with bodies more suited to walking than swimming—just in time for summer.

Bonett’s team looked at several other environmental factors, but none influenced the salamanders’ body type nearly as strongly as the soil particle size. So in this case, a very small difference in the environment led to a big difference in adaptation.

What’s not yet known is how flexible the differences are. For example, what would happen if you moved an immature, paedomorphic salamander from a big-gravel stream to a fine-soil stream? Would it be able to change like the metamorphic salamanders? The potential for a trait to adapt to an individual animal’s circumstances is called plasticity. If the way these salamanders mature is a plastic trait, it may be possible that all of the salamanders are born with the potential to transform, but only do so in dry environments. The other possibility is that a salamander’s adult body type has been pre-programmed into its genes, and only many generations of natural selection can alter the species.

Now try and answer these questions:

  1. What are the main differences between the two types of salamanders?
  2. Why do the size of the soil particles matter to their body types?
  3. Suppose you moved 900 immature metamorphic salamanders and 100 immature paedomorphic salamanders to a fine-grained streambed. What would happen during the summer if body type were a highly plastic trait? If it were not plastic?
  4. Although this particular study dealt with natural variations in habitat, what does this study suggest about the effects humans have on the environment?

You may want to check out the March 10, 2006, Science Update Podcast to hear this Science Update and the other programs for that week. This podcast's topics include: Baby's ear for language; the effects of streams on salamanders; what kids do online; how marital stress can be bad for your heart; a computer that picks perfect employees.


For Educators

In the feature Inside the Lab and Out of the Field: A Profile of Frog Researcher Dr. Tyrone Hayes, from the Exploratorium, students learn about a scientist studying how genes and hormones regulate developmental changes in frogs, and how that development can be affected by water pollution.

The printable fact sheet Global Warming: Oklahoma, from the National Wildlife Federation, contains information about the current and future effects of global warming on the people and wildlife of these salamanders’ native Oklahoma.


Related Resources

Introduction to Natural Selection
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Shyness
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