Tadpoles and Toads in the Sanddunes
The plentiful August rains brought an incredible phenomenon to the sanddunes near Monahans. Bill Loos, one of the Sibley Nature Centers board of directors, visits the sanddunes almost every weekend. He hikes across the dunes as part of an exercise regimen. In over 13 years of visitng the dunes, he has never seen such an explosion of tadpoles in rainwater pools in the dunes. He is a superb observer and examined the toads and tadpoles closely.
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In between the larger dunes, at the bottom of the lowest areas, rainwater pooled up and stood for several weeks. The greenery is a species of sedge, that must have plentiful water to germinate and grow.
A person can see that the sedge has a triangular stem. The redstemmed plant is unknown to the Sibley staff. The mass of black in the water is thousands of tadpoles. The white sand in the water is covered with green algae, the food of the tadpoles.
The Sibley staff believe that this toad is a Woodhouse's Toad. It has a light line down its back, and dark around the spots. Its eye has a horizontal pupil. Over the years, Sibley staff has caught thousands of toads to feed hognose snakes, and Woodhouse's have not been found in Midland County (sixty miles east of Monahans) in over 40 years. Woodhouse tadpoles take over 40 days to grow from egg to toadlet, and rainwater pools rarely last that long.
How many tadpoles are in this picture? Why do they pack themselves together so closely? Is the "herding" behavior protection against the bright sun? Or is it a defense against a predator that might come and start eating them (like a night heron or green heron?) One tadpole has died and is floating sideways, its body slowly turning white. It might have died from a parasite, Some species of toads have a portion of the population of their tadpoles turn cannibalistic if the pond is near drying up. Cannibalism helps the individual grow faster, so at least a few might survive.
Looking closely at the tadpoles, it appears that all are the same species. Notice the slight sheen to the water - is that a result of tadpole poop? A few bubbles can be seen, too. Tadpoles at this stage still breathe through gills, so the bubbles are not the result of exhalation.
What are the white spots floating in the water? Is it flecks of sand stirred up from the tadpoles wriggling? Is it tiny pieces of tadpole droppings? The green alga on the sand is also visible in this photo.
In this closeup of just one tadpole, the feet are visible. At this stage, the tadpole begins to breathe through its mouth, and the mouth changes from a round hole that scrapes algae from the sand to a mouth that is wide and can eat insects. Within a day or two, this toadlet will leave the water.
A backswimmer swam near the tadpoles. It eats tadpoles by biting them and sucking out the body juices. The dead one seen in a previous photo may have died from a backswimmer. Backswimmers also eat water crustaceans like fairy shrimp and daphnia.
In a very shallow puddle the tadpoles were crammed together and parts of the bodies exposed to the open air. The oily sheen (of their wastes) discolored the water. A strong "tadpole" smell was also evident.
This appears to be a Texas toad. There is no dorsal line of white, as in the Woodhouse's Toad. There are no dark edges to the spots on its back. After toads lay eggs, they eat termites and other invertebrates for several days and then dig back into the ground. Mr. Loos found a number of toads the morning that he hiked in the dunes. It had rained the evening before, so the toads visible on his walk may have emerged from deep underground in response to the rain a second time within a week or two. It is unknown (as far as the Sibley staff can determine) if toads will mate and lay eggs two times within a few weeks.
Notice the horizontal pupil to the eye, and the large parotoid gland. Its "ear" is below the parotoid - the slightly depressed circle. Toads probably only hear other toads of its own species. Toad eyes do not have much depth perception, so they mostly react to movement around them. Small moving objects might be food, so they come towards the small objects, but a large object might be a predator, so they hop away.