Sunday, March 21, 2010

Tadpole Tails and Predator Induced Plasticity

Last Wednesday, while doing field work near Goethe State Forest, I happened onto the paths of a couple pinewoods tree frogs (Hyla femoralis). One frog was observed practicing evasive acrobatic skills between the leaves of a saw-palmetto dominated groundcover; the second frog, taking a more leisurely approach to the day, was found lazily stretched out on mid-swamp tree branch. Because of a recent environmentally-induced neglect of this blog on my part, I thought that the two frogs would serve as a good model for a post on how tadpoles can alter their developmental physiology in response to local ecological conditions.

Like many other frogs, the pinewoods tree frog undergoes a complex life cycle which carries them from the ephemeral waters of ponds, swamps and puddles to an adulthood existence in the trees. In response to the variability of selective pressures expressed by their environments, natural selection has shaped Hyla femoralis in such away as to be flexible. One example of this flexibility is the way in which their tadpoles can alter phenotype – their morphology - in response to the presence of predators. As opposed to their developmental processes rigidly rendering tadpoles displaying uniform and unchanging morphologies, the DNA of flatwoods tree frogs has been programmed to make size, growth rate and coloration malleable characteristics. The ability of an organism to change its physical characteristics to better fit local conditions is called ‘developmental plasticity.’

In addition to normal variations encountered at the regional level, or within individual populations, the colors and shapes exhibited by tadpole tails can differ from one location to another; this is because tail characteristics can be changed in response to cues in the environment. In waters lacking abundant predators, Hyla femoralis tadpole tails are generally colorless, or are of a dull brownish-red color. In contrast to relatively safer waters, the tadpoles hosted by puddles with abundant predators (predators like dragon fly larvae, for example) are often found bearing tails with distinct red-spot markings and an enhanced, taller shape and muscular robustness. Increased tail muscle provides greater propulsion, allowing the tadpoles to employ accelerated speeds as part of their predator evasion tactics. Though, it’s still an area for inquiry, changes in the pattern and coloration of tails may provide a crypsis function by either providing improved camouflage, or by directing predatory attacks tail-ward, away from the tadpoles’ main body mass – improved survival through either concealing or revealing.

The chemical signals that switch tail enhancement into overdrive include those compounds released by other tadpoles as warning pheromones during predatory attack and those molecules discharged by the predator while digesting prey-tadpole tissues. So, in other words, a predator’s attack and digestion of a conspecific tad liberates chemicals into the water that are received by other tadpoles during development; as a result, resources are directed away from ‘normal’ growth processes and are directed to tail augmentation.

Cool stuff!

A couple quick notes:

Pinewoods tree frogs display distinct orange or yellow spots on their inner side of their thighs, while in the field these spots help distinguish Hyla femoralis from other species with similar body color patterns. Though not pictured here, the leg spots were observed during the frogs’ recent attempts at evading a certain species of primate paparazzi. Their tadpole stage lasts for about two months, and the breeding season begins in March and runs through the summer months.

Although cypress swamps and pine flatwoods are distinct natural community types, they are both occupied by the pinewoods tree frog. The first snapshot above shows one Hyla femoralis precariously perched on the stem of a saw palmetto plant just a few inches off the ground in a well-drained upland flatwoods area. The second shows another individual leisurely laying on a tree branch about five feet above the surface waters of a swamp. In Florida, these communities are often directly adjacent to each other:

LaFiandra, E., & Babbitt, K. (2004). Predator induced phenotypic plasticity in the pinewoods tree frog, Hyla femoralis : necessary cues and the cost of development Oecologia, 138 (3), 350-359 DOI: 10.1007/s00442-003-1412-3

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