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 1,9503,445 m (6,39811,302 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.
Eggs are darkly pigmented and laid in 1 or 2 strands (1 per ovary, the oviducts fuse just before the cloaca in toads), however egg masses may fold during deposition into radiating clusters 45 eggs wide. 10002000 eggs beaded in two envelopes are laid by females in shallow pools, tangled in vegetation. Individual eggs are 2.1 mm wide on average, and 4.1 mm including the two envelopes.
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 1,0002,000 eggs. Eggs hatch after 12 weeks, and the length of time depends heavily on ambient temperature and fluctuations in temperature. Females generally breed once and leave after 23 days, while males remain for 12 weeks.
Subadult and adult habitat utilization patterns are poorly understood. Metamorphs appear to move away from breeding ponds soon after transformation, however they probably overwinter nearby in stream channels and associated vegetation (willows, sedges, and grasses). Many juveniles (1+ years in age) probably disperse farther upland into adult foraging habitat, especially by mid-summer of their second year, but they can also be found nearby breeding ponds. Adult upland foraging habitat tends to be covered in seeps and springs, willows, tall forbs, granitic boulders, or (at lower elevation) forest clearings. Rodent burrows play an essential role in providing shelter from predation and weather, as do willows, logs, and rocks. Overwintering habitat is also includes the burrows of rodents such as pocket gophers, voles, and Belding's ground squirrels, along with willow root tangles, which all probably keep an optimal thermal and mesic environmental for hibernating toads.
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 chalcid wasps (2 months post-metamorphosis), to eating mostly hymenopterans, mostly ants (juveniles), to eating 80% hymenopterans, consisting of bees and wasps (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 have several apparent adaptations to high elevation. Males live to be at least 12 years, and females until at least 15 years. Their longevity probably helps them outlast years of low snowpack that cause poor breeding conditions, and thus low metamorphic recruitment. They are largely diurnal in contrast to the majority of anurans, probably owing to the cold mountain temperatures. Aligning their activities with diel peaks in warmth allows them to absorb solar energy to catch and biosynthesize food. The high level of melanism in eggs and tadpoles (and possibly in adult females), as well as the tendency for tadpoles to congregate in the warm shallows, probably serves the same purpose. Similarly, the selection of shallow breeding sites by adults, and shallow water margins by tadpoles, probably reflects the intense pressure to metamorphose in a short season, and hence the importance of using high temperature for rapid development. The marked dichromatism between males and females is still an evolutionary mystery. One possible explanation sexual selection. Females could be sexually selecting for lighter coloration in males, as some kind of proxy for male fitness, or males could be signaling their maleness to attract females and ward off over-zealous males. Males of the Yosemite toad and many other bufonid species change into a lighter color during breeding (e.g. Western toads, American toads), and other notable high elevation bufonids outside North America are highly sexually dichromatic (e.g. golden toads, yellow toads, marbled toads). Another more likely hypothesis is that males and females occupy largely different habitats, and evolution has de-coupled camouflage between the sexes. Males spend disproportionately more time in shallow, brown, silty breeding ponds, where they are highly exposed to predators. In contrast, females quickly leave pond habitat for rocky upland habitat where disruptive coloration may be more suitable. More than 60% of adults toads found in upland habitat during the late summer are females, whereas less than 10% are males. In lowland breeding habitat, this pattern is reversed: 54% of adult toads are males, and only 19% are females. Regardless of which hypothesis is correct, their diurnal habits make color and pattern subject to increased selection.