School › Reptiles and amphibians › Ages 15–16
Out of the water: tetrapods, the amniotic egg and ectothermy
The move from water to land, the four membranes of the amniotic egg, the energy budget of an ectotherm, temperature-dependent sex in sea turtles and why amphibians are in trouble, with a quantitative water-loss practical.
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The first tetrapods appeared nearly 400 million years ago. The change from lobe-finned fishes took about 50 million years in the Devonian: Tiktaalik lived about 375 million years ago, about 10 million years before Acanthostega and Ichthyostega, which had four limbs and may have had both gills and lungs.[1]
Amphibians (about 6,770 species: about 5,965 frogs, 620 salamanders and 185 caecilians) still depend on water. Their skin is moist and permeable and is a gas-exchange surface along with the lungs and mouth cavity; most salamanders have no lungs. Larvae have gills and a lateral line, which are lost at metamorphosis.[1][3]
The amniotic egg, which appeared with the first amniotes about 340 million years ago, freed reptiles from water. The amnion cushions the embryo and keeps it hydrated, the chorion carries out gas exchange, the allantois stores nitrogenous waste and the yolk sac supplies nutrients. Skin with keratin and waxy lipids cuts water loss.[2][3]
Most reptiles are ectotherms that regulate body temperature by behaviour, basking or seeking shade. Because their main heat source is the environment, they can survive on about 10% of the energy needed by an endotherm of the same size.[2][3]
In sea turtles, nest temperature sets the sex of the hatchlings: warmer sand produces more females. On the warmer northern beaches of the Great Barrier Reef, a study found 86.8% of adult and 99.8% of sub-adult turtles were female.[4]
Permeable skin makes amphibians sensitive to their environment. Chytrid fungi that infect amphibian skin (Bd and Bsal) can drive species to extinction, and an estimated 42.5% of the world's amphibian species are threatened.[5][3]
Try it: Measuring water loss through model skins
You need
- Three identical sponges or blocks of agar (about 4 × 4 × 2 cm)
- Cling film, and a thin layer of petroleum jelly as a 'waxy lipid' coat
- Top-pan balance (0.01 g)
- Ruler
- Petri dishes or watch glasses
- Stopwatch or clock
Safety
Petroleum jelly is slippery: wipe benches and hands. Do not put the balance near water. Check for latex or skin allergies before handling coatings, and wash hands afterwards.
What to do
- Soak the three sponges and let them drip for one minute. Measure their dimensions and calculate the exposed surface area of each (top and four sides).
- Leave sponge A bare ('amphibian skin'). Wrap sponge B in cling film with the bottom open ('scaly skin'). Coat the exposed faces of sponge C with a thin layer of petroleum jelly ('waxy lipids').
- Weigh each sponge on its dish and record the starting mass.
- Leave them side by side, away from radiators. Reweigh every 10 minutes for 40 minutes (and again after 24 hours if you can).
- Calculate the mass lost, the percentage of the starting water lost, and the rate of loss in g per hour per cm² of exposed surface.
- Plot mass lost against time for all three, and estimate the uncertainty of each reading from the balance resolution and repeat readings.
What happens, and why?
Water evaporates through the exposed surface. The bare sponge loses water fastest, like permeable amphibian skin; the covered sponges lose much less, like reptile skin with keratin and waxy lipids that reduce water loss. Draughts, temperature, the open base and uneven coating all add uncertainty, so compare rates measured at the same time and place.[2][1]
Play: Amphibian, reptile or both?
Sort each fact: true of amphibians, of non-bird reptiles, or of both?
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Quiz
Other ages: Ages 6–7 · Ages 10–11