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Spring Singers

Spring -- a rapid and sudden emergence. A burst of life is synonymous with the season. Warm rains and daylight are key ingredients, but it is the amphibians that act as harbingers. Male upland chorus frogs (Pseudacris triseriata), pickerel frogs (Rana palustris), spring peepers (Hyla crucifer), and wood frogs (Rana sylvatica) sing their harmonious nuptial displays.

Frogs are sleek, large-mouthed amphibians with two powerful, oversized back legs. With this design, frogs have a means to eat and to escape being eaten. For 350 million years -- predating dinosaurs -- frogs have endured mass extinctions, ice ages, and the shifting of continents.

Frogs provide a major transfer of invertebrate and plant energy to animals higher up the food web -- to many fish, reptiles, birds, and mammals that are unable to access these food sources directly. They expend little energy for bodily maintenance because they are ectothermal vertebrates, highly efficient in biomass conversion. Unlike endotherms, they lose very little energy to heat. Most species mature slowly compared to endotherms of similar size, and are long-lived. In many ways, they are like plants, acting as energy reserves within ecosystems.

Most frogs lay their eggs in water. Embryos develop in the fertilized eggs and after about a month, they break out of the jelly as small tadpoles that make their own way to adulthood. In many habitats, life as a free swimming tadpole may become sufficiently uninviting to favor strategies that bypass this stage and provide a more direct development from egg to frog. Brooding equates to a more direct development that, once a frog happened upon it, would quickly increase its reproductive success. Genes encoding this behavior then spread through the population.

Nature has not disappointed and various frogs have evolved styles of parental care with a range of startling adaptations. Frogs display the greatest variety of reproductive modes in vertebrates. Parental care is pervasive in frogs and includes guarding of eggs and nest sites, creating foam nests, tadpole herding, transport of eggs and larvae, feeding of young, pouch brooding, vocal sac brooding, and stomach brooding. The leaf-folding frog (Afrixalus delicates) is an African frog that constructs an over-water nest. While the pair is engaged in a copulatory embrace, known as amplexus, they use their hind limbs to draw the margins of a leaf together, creating a basket in which to deposit their eggs. An oviducal secretion from the female acts as the glue to hold the nest together. After the tadpoles develop, the nest breaks down and they fall into the water below. Female flaming poison-arrow frogs (Dendrobates pumilio) of the Neotropics transport their tadpoles from forest floor nest sites to water-filled hollows in the leaf axils of bromeliads and other plants, where they complete their tadpole stage. At intervals of several days, the tending females will return to the natal sites and feed their offspring one to five unfertilized eggs from their own reproductive tracts. Males of the midwife toad (Alytes obstetricans) wrap strings of eggs about their legs and carry them in tow.

Female Ecuadorean frogs (Gastrotheca rioambae), found in Andean valleys, develop pouches on their backs with an opening near the rear that extends internally to their head. Males place fertilized eggs in her pouch, where they develop under her skin for five to six weeks before emerging as late-stage tadpoles. In an Australian frog (Assa darlingtoni), the males develop pouches on their undersides with an opening near the hind legs and extending towards the front legs. Females lay eggs in the leaf litter. A male will then place himself in the middle of the egg mass just as they hatch and either coat himself with jelly from the spawn or secrete a slippery substance himself. Tadpoles spring sideways and forward by bending their heads towards their tails, migrating over their father's slippery body and entering the brood pouch under their own steam.

In a more intermediate mode towards direct development, some frogs brood their young internally using structures already available for other purposes. In a Chilean frog (Rhinoderma darwini), the male ingests fertilized eggs and broods them in a large throat pouch that in most frogs is reserved for the earlier act of courtship. Evolution seizes its opportunities; a vocal pouch is roomy and available. It is almost inevitable, in a context of strong brooding pressure, that a lineage would overcome the behavioral obstacles to use this possibility. Stomachs provide the only other large internal pouch with an ingress and egress of sufficient size. Stomachs, however, have an additional problem not inherent in vocal sacs and novel pouches: their normal function works against the care and protection of their young. Gastric juices would quickly begin the digestion of egg and tadpole. If stomach brooding were discovered in vertebrates, it would be of immediate interest to evolutionary biologists to decipher just how such an adaptation would evolve.

In 1973, Rheobatrachus silus, a small frog living under stones, in rock pools of shallow streams, and in small stream channels, was discovered in the Conondale Range of southeastern Queensland, Australia. The next year, a second species, R. vitellinus, was discovered in Eungella National Park in central coastal Queensland. Two scientists observed that upon transfer from one tank to another, an R. silus ejected almost 15 juveniles. Believing it was a male ejecting young from his vocal sac, they dissected the frog to find it had no vocal sac and it was a female. It took until 1979 to officially record a natural birth, where fully formed frogs struggled out of a female's mouth. These frogs, of great theoretical interest, represented the only known vertebrates that swallowed their young and used their stomachs as brooding chambers. This adaptation was selected naturally, within an ancient lineage, to address a common problem among frogs.

In nature, the fortuitous presence of structures and possibilities, evolved for other reasons, often forms the foundation for new evolutionary trends. A female R. silus swallows her fertilized eggs out of hunger. Vertebrate stomachs contain natural stretch receptors that tell an organism when to stop eating by imposing a feeling of satiety. The batch of swallowed eggs has this effect. A chance substance in the eggs halts the release of hydrochloric acid and stops the passage of eggs into the intestine. After much research, prostaglandin E₂, a hormone-like substance formed throughout the body and serving many functions, was discovered in the eggs and in tadpole excretions. Prostaglandin, surely present as a biological byproduct of egg formation, turns out to suppress digestion. One female gains a reproductive edge and passes the trait onto her offspring.

New directions often derive from fortuitous prerequisites and add an unpredictable character to life's history. Many of these new directions fizzle out and lead to only a few rare species. Sometimes major trends in the order of life begin in such a modest fashion. This particular innovation seems destined to end with only a couple oddities. Rheobatrachus silus has not been seen since 1981. A series of dry summers and late rains had restricted the range of this aquatic frog; it was declared extinct in 1990. R. vitellinus populations are declining significantly.

In western North America alone, research has documented a major decline in population sizes and geographic ranges of western toads (Bufo boreas), Yosemite toads (Bufo canorus), Wyoming toads (Bufo hemiophrys baxteri), Arroyo toads (Bufo microscaphus), northern leopard frogs (Rana pipiens), Chiricahua leopard frogs (Rana chiricahuensis), lowland leopard frogs (Rana yavapaiensis), plains leopard frogs (Rana blairi), spotted frogs (Rana pretiosa), cascades frogs (Rana cascadae), mountain yellow-legged frogs (Rana muscosa), foothill yellow-legged frogs (Rana boylii), Tarahumara frogs (Rana tarahumarae), and red-legged frogs (Rana aurora). Some species have completely disappeared from historical localities. Such losses form part of a disturbing and unexplained pattern in amphibian populations throughout the world.

Frogs are good bio-indicators of ecosystem health because of their life cycle, absorptive surfaces, exposure to ultraviolet light, food habits, susceptibility to cold and drought, fragmented distributions, sequestered tissue contaminants in relation to metamorphosis, and breeding cycle. Their recent disappearances could signal a change in the order of life or could be indicative of cumulative local anthropomorphic impacts, global pollution, and climate changes. A spring evening silence is spreading as frogs sense the changes caused, at least in part, by our activities. It is time we listen to their plight. After all, humans and frogs are part of the same fabric.