The Yosemite toad (Anaxyrus canorus, formerly Bufo canorus ) is a species of true toad in the family Bufonidae. Endemic to the Sierra Nevada of California, the species ranges from the Alpine County to Fresno County. Yosemite toads are only found in the montane to subalpine elevational zone of 1,950–3,445 m (6,398–11,302 ft) asl. The Yosemite toad is similar to the nearby Western toad, but in many ways adapted to a high elevation lifestyle. It was initially described during the Grinnell Survey of California, by an undergraduate student of Joseph Grinnell named Charles Camp.
Yosemite toads show high site fidelity to both breeding and upland foraging habitat, often using the same breeding pools and refuges in subsequent years. After breeding, adults forage in upland riparian areas of the breeding meadows, or (less commonly) disperse into the surrounding forest or subalpine environmental matrix. Although adult toads are capable of significant dispersal away from breeding pools, they are generally found within 90 m of permanent water; occasionally they are found in xeric forest habitat. Female toads disperse farther than males, and have been found to move up to 1.26 kilometers from their breeding grounds in one season. However, dispersal between meadows appears to be very low, estimated to be about 2% between 400 m meadows.
This is possibly due to unfavorably dry habitat between meadow complexes. Overwintering sites are rodent burrows or willow thickets (see "Habitat Utilization"), and the first freezing nighttime temperatures seem to cue adult toads to seek hibernacula. Hibernation usually begins in the late summer or early fall, between September and October.
Like other toads, Yosemite toads are ambush predators. They lunge at prey and open their mandibles, causing their sticky tongue to unfold, flip downward, and pull the animal into their mouths. Adult stomach contents have included: tenebrionid beetles, ladybird beetles, weevils, craneflies, mosquitos, caterpillars, carpenter ants, dragonfly naiads, centipedes, julid millipedes, and spiders. Juvenile stomach contents have included: ants, spiders, and wasps. Metamorph stomach contents have included: owl flies, flies, springtails, spider mites, and spiders. There appears to be an ontogenetic shift from eating mostly spider mites (metamorphs), to eating a mix of spider mites, spiders, and tiny wasps (2 months post-metamorphosis), to eating mostly larger hymenopterans, mostly ants (juveniles), to eating 80% hymenopterans, consisting of bees and wasp (adults). Tadpoles are grazers on detritus and algae, however it remains unclear whether they ingest those items, bacteria, rotifers, or something else. Tadpoles are also known to opportunistically scavenge conspecific tadpoles, Sierra chorus frog tadpoles, Belding's ground squirrels, and predaceous diving beetle larvae, as well as graze on lodgepole pine pollen grains.
Yosemite toads are explosive breeders (breeds within a short time period), migrating to breeding pools and flooded areas in late spring while snowbanks still veil the frozen meadows. They have been termed the "toad that stays on its toes" or "tiptoeing toad" due to their habit of crossing snowdrifts without touching their abdomen to the cold snow. Breeding time varies greatly with elevation and yearly snowpack (April to July), and depends on timing of snowmelt. Males arrive to breeding ponds synchronously when the meadow is ca. 50% covered in snow. Depending on the population density they will either join a breeding chorus by making an advertisement call to females, or will actively search for them. Their vocalization is a high-pitched, sonorous trill lasting an average of 2.6 seconds and repeated frequently. Males intermittently call from pool margins, under logs, or inside willows to attract females. When females arrive, they are immediately grasped in amplexus by one or multiple males as the males fight for a limited number of mating opportunities. Breeding sex ratios can be very skewed toward males since females breed less frequently than males, although both sexes typically do not breed in consecutive years. While in amplexus, females will lay one clutch of 1,000–2,000 eggs. Eggs hatch after 1–2 weeks, and the length of time depends heavily on ambient temperature and fluctuations in temperature. Females generally breed once and leave after 2–3 days, while males remain for 1–2 weeks.
Yosemite toads are reported to have declined substantially in distribution and abundance across their restricted range. Although threats to persistence of the species are less understood than in other California amphibians such as the Sierra Nevada yellow-legged frog, the two most significant appear to be drought (increasing with climate change) and disease (primarily chytridiomycosis). Interactions between the two are not well understood. The species is listed as "Endangered" by the International Union for Conservation of Nature (IUCN), "Threatened" by the US Fish & Wildlife Service (USFWS) under the ESA, and "California species of special concern" by the California Department of Fish & Wildlife (CDFW).
