focus on nature

When This Poisonous Desert Froggy Goes A-courtin', You'd Best Leave Him Be

You'd think it would have cooled off by midnight, even during a Sonoran Desert summer. But the heat of the day hangs heavy with humidity from yesterday's thunderstorm. In this suffocating atmosphere, Brian Sullivan and Keith Malmos crawl through barbed wire and step around cow pies as they lug their recording gear to the edge of a cattle tank where several dozen poisonous male Colorado River toads are "singing" as best they can. Some of these males look as if they could occupy most of a dinner plate. One bruiser sits confidently in a pool of rainwater runoff; his throat membrane expands as he periodically pumps out his species' characteristic call, a low ferryboat whistle lasting about a second. The two herpetologists point their microphones at him, recording his vocalizations for science.

According to Sullivan, a professor at Arizona State University West, Phoenix, the calls sound "positively pitiful" for a toad this big. Despite their bulk, Bufo alvarius cannot match the earsplitting continuous trills of some much smaller toads. In fact, most male Colorado River toads do not sing at all. Because only a fraction of the male population calls in this species, the great herpetologist Frank Blair wondered if the call still worked to attract females. He suspected that the call might be just a functionless holdover from the past when all adult males of the species belted out a truly Effective come-hither signal to potential mates. Sullivan and Malmos check Blair's hypothesis by bagging some females and carrying them to isolated ponds away from calling males. There they place each female in turn between two speakers. Once the toad settles down, the researchers broadcast recordings of a toad Caruso. When a lusty female Colorado River toad jumps to a calling male, he immediately scrambles onto his partner's back and holds tight while she deposits her eggs in water. He releases sperm at intervals, fertilizing the eggs, while fending off attacks by ungracious rivals among which are the silent non-calling males. These fellows try to break up pairs in amplexus (as toad matings are called by those in the know), and they also will grab a female headed to a caller, short-circuiting the female's preference. Colorado River toad males secure mates, and females lay their eggs only one or a few nights each summer, always a day or two after a substantial rain. These activities occur primarily in temporary desert ponds, rather than in the Colorado River, which marks the far western border of the toad's range.

Although some unimpressed females promptly disappear into the night, others hop eagerly toward the speaker, even jumping into the sound equipment in their enthusiasm. Thus, contrary to Blair's supposition, the calls of Colorado River toads still sound good to interested females.

In their largely dry homeland, these desert-adapted toads divide the year into two blocks. They spend the cooler months in torpid dormancy deep within a burrow. During the summertime, however, the toads get up and go looking for a nighttime romance after a monsoon storm has filled their neighborhood's ponds. They also hunt for food. Sullivan tells me that these toads will eat anything that moves, including Arizona's biggest scorpions. While on these foraging walkabouts near populated areas, Colorado River toads sometimes come into contact with humans or their pets.

If Fido or Rover finds a Colorado River toad and decides to maul it vigorously, the big amphibian does not just sit there and take it. Instead, it releases a milky white secretion from large warty glands on its neck. The fluid contains a potent chemical, a bufotoxin, which if ingested even in small amounts is guaranteed to make Fido either deathly ill or just plain dead.

So we recommend that both humans and pets avoid contact with Colorado River toads, and admire them - from a distance - for their ability to cope with a decidedly amphibian-unfriendly environment.