Adaptations to an Arid Environment in the Great Basin Spadefoot (Spea intermontana)

Caroline Daggett


During the early morning hours from early April into June, one may notice small tracks in the sandy soils that surround many ponds and pools of water in central and eastern Oregon. These tracks, most likely left by the Great Basin Spadefoot, will lead from the water's edge and end in small ridges that mark the spot where the toad wiggled its way into the soil (Svihla, 1953). There, in burrows of varying depth, the Great Basin Spadefoot will spend much of the year, emerging mainly only at night or after a rainfall. This adaptation of burrowing underground, along with sporadic breeding and rapid development to metamorphosis, enables the Great Basin Spadefoot to survive in such an arid environment.

The physiographic range of the Great Basin Spadefoot in Oregon is confined to the portion of the state east of the Cascades. It is found throughout the Columbia Basin and Great Basin of central and eastern Oregon, but not in the northeast section of the state. The Great Basin Spadefoot is well adapted to the semi-arid climate of central and eastern Oregon where they inhabit open areas of bunchgrass prairie, sagebrush, or ponderosa pine (Leonard et al., 1993). The permanent and temporary water sources found in these habitats serve as the breeding grounds for the Great Basin Spadefoot.

The spadefoot gets its name from the presence of a sharp, black spade on its hind feet. The spade serves as a cutting edge that allows the spadefoot to dig through the soil rapidly. As it digs backward through the soil, the spadefoot is concealed by the loose soil falling over it and filling in its burrow. Spadefoots have also been found to utilize the burrows of other animals (Stebbins, 1951). Great Basin Spadefoots may spend up to 7 or 8 months buried in the ground in a state of dormancy, allowing them to survive the cold and dry periods common in the Great Basin (Leonard et al., 1993). They are primarily nocturnal which allows them to avoid the heat and low humidity of daytime, and consequently, the chance of desiccation. When they do emerge to feed and mate, it is almost always after rainfall.

It has been found that the spadefoot occupies burrows of varying depth depending on the time of the year. Ruibal et al. (1969) found that for a species of spadefoot in Arizona that closely resembles the Great Basin Spadefoot, winter and pre-summer burrows are on average deeper than the summer and early fall burrows. Presumably, when precipitation is high, burrows will tend to be shallower because this is the period in which the spadefoot will be active and in which the soil moisture will be at its highest. In early fall, Ruibal et al. (1969) found the average burrow depth to be 24 cm, most likely representing the period when the toads were getting ready to over-winter. Later in the fall, as the soils dry out, the animals will dig to even deeper levels where they will remain through the winter at an average depth of 54 cm. The deepest burrow they found was 91 cm. There is a case where a spadefoot was found 15 feet below the surface (Bragg, 1965). Before the rains start, studies have indicated that the spadefoots may already have begun to move up from their deep, over-wintering level.

In Oregon, where the rains come in the spring, the Great Basin Spadefoot probably begins to move up closer to the surface during February and March. They are active at times between April and June, coming out at night and after rainfall and retreating to shallow burrows (1.3cm-4.9cm deep) by day. Studies show that after they are done mating, spadefoots may leave the vicinity of water and forage at night in the grasslands and sagebrush nearby, but will still retreat to their burrows by day (Ruibal et al., 1969). During the warm and dry summers, the Great Basin Spadefoot remains underground in a state of aestivation. They may once again re-emerge after the periodic fall rains. Stebbins (1951) reported that large numbers of spadefoots were observed following heavy rainfall in August and October in Harney and Klamath Counties. They will then most likely retreat to deep burrows to hibernate during the cold winter.

Though the spadefoot has a high tolerance to water loss- up to 48.8% of its body weight, the main reason that spadefoots burrow in the soil is to prevent desiccation (Nussbaum et al., 1983). They have the ability to absorb moisture from the soil through their permeable skin, but they can also lose moisture to the soil just as they can to the air. When they begin to retreat underground for their long periods of dormancy, soil moisture is still relatively high because most likely the rains have just ended. When the soil moisture is high, the toad does not lose water to the soil because the osmotic pressure of the toad's body fluids exceeds that of the soil moisture tension. Thus, under these conditions, the movement of water from the soil to the toad can occur (Shoemaker et al., 1969). The spadefoot has a very large urinary bladder in which it can store water to save as a reserve during very dry periods. As the soil dries out over the course of the summer and winter, the soil moisture tension increases and may become greater than the osmotic concentration of fluids in the toad. To reach an equilibrium between ambient soil moisture and osmotic concentrations in the toad, the spadefoot may lose some of its stored bladder water to the soil. The toad can prevent this loss of water to the soil if it can raise its osmotic concentration by raising its urea level (Ruibal et al., 1969). High concentrations of urea can be stored in the plasma, tissues, and urine of the toad. With higher urea concentrations, the spadefoot is able to absorb water from soils that contain as little as 3% water (Duellman et al., 1986). Also, as urea production increases, the osmotic concentration of the toad increases and results in a decreased osmotic gradient between the toad and the soil which reduces the rate of water loss. Above all, to survive in a terrestrial environment, the spadefoot must prevent large amounts of water from being lost to the soil.

The breeding habits of the Great Basin Spadefoot are quite sporadic due to the fact that many of the pools of water used for breeding are only temporary. The warm, dry climate of eastern Oregon makes for high evaporation rates from these shallow water bodies. When rains do come, many small pools are created that may last for only a few days or weeks. Thus, to best take advantage of these pools and ensure the survival of the species, embryonic and larval development must be rapid.

Some experiments with spadefoots have shown that vibrations caused by falling rain will actually bring the toad to the surface. The sound of rainfall together with warmer temperatures and a drop in barometric pressure is probably the best explanation for why spadefoots emerge when they do. Bragg (1965) reported that among the spadefoots, the Great Basin Spadefoot is unique in that it utilizes the floodwaters of mountain streams for breeding even when no rain has occurred. They also use lakes and ponds that have water in them permanently. What the spadefoots in general need is a sudden appearance of water in quantity. In the arid habitat of the spadefoot, this generally occurs only after rainfall. They generally will not breed in less than 0.5 inches of rain (Bragg, 1965). In Oregon, the increase in the amount of agricultural land that is being irrigated has created additional breeding sites in places where they probably wouldn't have occurred otherwise.

Male spadefoots are the first to arrive at breeding sites where they immediately begin their loud mating calls. The call of the male Great Basin Spadefoot can be heard 100-200m away and lasts for about one second (Nussbaum et al., 1983). When repeated over and over again and coupled with about a hundred other calling males, a very loud chorus erupts that may be discernible a mile or more away. This loud chorus attracts other males to the breeding site. The louder the chorus, the more females it will attract and from farther away. Females arrive at the site simultaneously where intense competition ensues between the males to secure a mate, leaving many of the males unsuccessful (Bragg, 1965).

Females lay their eggs in many small packets containing 20-40 eggs. The packets are attached to blades of grass or other vegetation along the margins of the pond, or they may be attached to pebbles along the bottom. Females may lay as many as 800 eggs. In warm temperatures, the eggs will hatch in as little as 2-3 days. Because the eggs are deposited on the margins of ponds at night, the developing embryo will be very susceptible to the high water temperatures and decreased water levels during the day. Development must be rapid in order for the embryo to survive. Therefore, by midday when water temperatures are too warm, the embryo will already have reached a more heat tolerant stage. As newly hatched larvae, they may be able to escape the shallow periphery of the pond and retreat to deeper waters in the center (Brown, 1967).

Larval development takes about 1-2 months depending on the permanency of the water and abundance of food resources. If drying does start to occur in the pond, the tadpole is able to metamorphose at a smaller size. Likewise, if the pond level remains stable, metamorphosis can be delayed and the tadpole can grow to a larger size. During their first few days as a tadpole, spadefoots will eat whatever is available in their pool. Often this is planktonic organisms or organic matter. They also become fond of dead insect larvae and dead tadpoles. Bragg (1965) found that the growth of tadpoles was increased when they scavenged for food in the muddy bottoms of ponds and pools. He believes that tadpoles are able to utilize the decayed substances of other dead tadpoles that collect in these areas. Most likely it is the presence of substances like iodine and thyroxin that are concentrated in dead larvae that can stimulate metamorphosis in tadpoles. Therefore, even in a pool of water that was rapidly drying up, Bragg found fully metamorphosed spadefoots emerging while tadpoles of the same age in a nearby pond with plenty of water were a week away from transformation. It is not known if cannibalistic tadpoles occur among the Great Basin Spadefoot. Other species of spadefoots have tadpoles that can be both cannibalistic and normal in body form. The cannibals have much larger heads and jaws than the normal tadpoles (Nussbaum et al., 1983). Since many pools produce large amounts of larvae that are competing for limited food supplies, it is advantageous for some tadpoles to be cannibals. That way they can feed on other tadpoles and speed up their rate of development and leave the pool. Further studies need to be done to determine whether or not this occurs in the Great Basin Spadefoot.

For many, when they think of central and eastern Oregon they do not associate amphibians with such a dry environment. The fact that they do occur there given the aridity is quite remarkable and the adaptations they have evolved are fascinating. I have only covered the most obvious of these adaptations; in doing so I have simplified what isn't so simple and have left out that which is just as important. I hope that others will appreciate this creature as I do given the little amount of knowledge we truly do know about it.


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