Chapter 4

Gray treefrog tadpoles. Tadpoles of the gray treefrog that live without predators exhibit high activity and develop relatively small tails that are drab. In contrast, tadpoles raised with predators exhibit low activity and develop large, red tails.

Photo by John I. Hammond.

Every spring the female gray treefrog (Hyla versicolor) must choose where she will lay her eggs. The treefrog is a medium-sized frog that lives throughout much of eastern North America and through the central United States to the Gulf Coast of Texas. As adults, they spend most of their time in forests, feeding on insects in trees. However, in the spring the male and female frogs move to the water to breed. Under ideal conditions, females lay their eggs in ponds that remain free of predators throughout the two months it takes for them to hatch into tadpoles and then metamorphose into frogs. Unfortunately, the females have no way to predict whether a pond will contain predators in the weeks ahead. Their offspring, however, have evolved an amazing ability to adjust to a wide range of different predator environments.

After a female frog lays her eggs, the embryos experience rapid growth and development; in just a few days they are ready to hatch. The timing, however, can change depending on the presence of predators, such as crayfish, that commonly consume frog eggs. Embryos of gray treefrogs, like many species of frogs, can detect the presence of predators by sensing chemical cues that the predators produce. When the embryo detects a nearby predator, development accelerates and it hatches into a tadpole sooner than it would normally, thereby reducing the risk of predation as an embryo. Although it survives the egg predator, it emerges smaller than it would have been if it had remained an embryo for a longer period.

The gray treefrog has also evolved an ability to respond to changing environmental conditions after eggs hatch into tadpoles. Like the embryos, gray treefrog tadpoles can sense predators in the water through chemical cues. When they detect the presence of a predator, tadpoles hide at the bottom of the pond, become less active, and start to change shape. Within a few days of first sensing a predator, the tadpoles develop big red tails. While the reason for the red color remains a mystery, the large tails improve a tadpole’s ability to escape from predators since they serve as a large sacrificial target that can be lost to a predator and regrown. However, the energy required for rapid development of a large tail is so great that other body parts cannot grow as fast. Consequently, tadpoles with large tails have smaller mouths and shorter digestive tracts, which limits their ability to eat and grow.

In short, the presence or absence of predators influences the tadpole’s phenotype. In an environment without predators, it becomes highly active, small-tailed, and fast growing. In the presence of predators, it becomes inactive, large-tailed, and slow growing. But the flexibility does not end there. Tadpoles not only detect the presence of predators but also distinguish different species of predators. This allows them to distribute the use of their energy according to the level of risk; they adjust their defenses most strongly to the most dangerous predators, and produce more modest defenses against less dangerous predators. This strategy has the advantage of allowing the tadpole to use its energy where it will make the most difference to its survival.

Additional studies have also shown that tadpoles can even detect what a predator had for lunch. When a predator habitually feeds on tadpoles, the treefrog tadpoles spend more time hiding and undergo changes in shape as we have noted. However, if the predator is feeding on something else, such as snails, the treefrog tadpoles spend less time hiding and only undergo small shape changes. In essence, the tadpoles detect that they are in more danger from predators feeding on tadpoles than predators feeding on other prey, and they defend themselves accordingly.

Gray treefrog tadpoles also respond to other environmental conditions, including the presence of intraspecific and interspecific competitors. In ponds without predators, many tadpoles survive and compete for algae, the food on which they depend. In response to the relative scarcity of food, tadpoles develop larger mouths and longer intestines. The larger mouths contain wider rows of tooth-like projections, and these improve the ability to scrape algae from rocks and leaves. Longer intestines enable a more efficient extraction of energy from the limited amount of available algae. To grow a large body, however, the tadpole must divert energy away from the formation of its tail. As a result, tadpoles that live in an environment with high competition have smaller tails. There is a cost associated with the phenotypic adaptation best suited to survive high competition; if a predator does show up, the tadpoles with smaller tails will be more vulnerable because they do not have the proper phenotype.

The story of the gray treefrog represents a situation in which a species can experience a tremendous amount of environmental variation within and across generations. In response to this variation, the gray treefrog has evolved a wide range of strategies that help to improve its fitness. The responsiveness of the gray treefrog tadpole represents just one example of how organisms have evolved to respond to future variation in their environment. In this chapter, we will explore the wide range of environmental variation and look at how species have evolved the ability to alter phenotype in response to changing environments.

In Chapters 2 and 3 we discussed the range of environmental conditions in aquatic and terrestrial environments and the many adaptations that organisms have evolved to handle these conditions. However, the environmental conditions an organism faces can also vary considerably over time and among different places. In this chapter we will look at environmental variation and the adaptations that organisms have evolved to respond to these changing environments.