Western toad (Bufo boreas) COSEWIC assessment and status report: chapter 8
Reproduction, growth and survivorship
Adult Western Toads congregate at breeding sites in the spring. In south-central British Columbia they breed in May (Poll et al. 1984, Gyug 1996) but in southern regions, such as California, breeding may begin as early as January (Erhardt 1996). In the Okanogan Highlands, Gyug (1996) found toad breeding to coincide with the time when average daily minimum and maximum temperatures rose above freezing and 10°C respectively. Tadpoles of this species are often found at high elevation and latitude in early fall, where little time remains before the onset of winter snows (Kinsey and Law 1998, E. Wind, pers. obs.).
There is disagreement as to whether or not Western Toads produce an advertisement call. In many parts of the range, researchers have observed males actively searching for females (e.g., Black and Brunson 1971, Nussbaum et al. 1983, Olson et al. 1986). They will clasp anything that remotely resembles a female, including rocks, sticks, other frogs, and male toads. As a result, male toads utter a release call, a weak bird-like ‘chirp’, when grasped by other males whereas females do not. Lone males may also produce a different sound than the release call (F. Cook, pers. com., L. Dupuis, pers. com., C. Davidson, pers. com.), indicating that their vocalizations have more than one function. Male Western Toads in the interior of B.C. (Hengeveld 2000, Kinsey and Law 1998, C. Houwers, pers. com.), in Alberta (E. Wind, pers. obs, W. Roberts, pers. com., F. Cook, pers. com.), Washington (K. Richter, pers. com.), and in California (J. Crayon, pers. com., C. Davidson, pers. com., S. Morey, pers. com.) have been heard calling. Differentiation between calling and non-calling population have not been investigated.
Western toads reach sexual maturity in two to six years (Porter 1972, Green and Campbell 1984, Olson 1988, Carey 1993), with a life expectancy of nine (Campbell 1970) to 11 years (Olson 1988, Carey 1993). Toad aggregations at breeding sites tend to be male biased (Blaustein et al. 1995), with the number of males exceeding females by up to 20 to 1 (Leonard et al. 1993). Blaustein et al. (1995) studied Western Toad populations at sites in central Oregon for almost 20 years. They found that the minimum breeding age for males is three years, with even proportions of males in age classes four to seven years old reproducing. In contrast, females reached sexual maturity at four to five years of age, and few older females (i.e., > 4 to 5 years) were observed at the sites. Toads are ‘explosive’ breeders, with breeding periods lasting only one to two weeks per year (Olson et al. 1986). Although the majority of individuals at each breeding site mate only once per season (Olson et al. 1986), breeding schedules differ between the sexes. Males may mate more than once per season and in consecutive years, whereas females appear to skip 1 to 3 years between breeding sessions (Olson 1988). Only 5% of female toads were observed at breeding aggregations more than once in five years, which, when combined with their reduced longevity compared to males, suggests that few females breed more than once within their life time due to the high energy expenditure required for reproduction (Olson 1988). Males persist at the site for several weeks after breeding has been completed (Jones 1999a, Davis 2000).
Amplexus with a female is generally tenacious and prolonged. Each female produces a double-strand of black ova, which are generally intertwined amongst other eggs and submerged vegetation. In shallow pools without vegetation, the eggs lay unattached at the bottom. Females produce clutches of 5,000 to 15,000 eggs (Blaustein et al. 1995). The eggs are black and about 1.5 to 1.7 mm in diameter (Blaustein et al. 1995). Embryos develop and hatch at approximately ten mm in total length (Nussbaum et al. 1983) within three to twelve days, depending on water temperatures (Leonard et al. 1993, Hengeveld 2000). Western toad tadpoles are highly gregarious, feeding and swimming in synchronized schools. By mid-summer, transforming tadpoles and toadlets aggregate on the water’s edge, potentially for thermoregulation (Black and Black 1969). Their transformation into ten to 12-mm long toadlets takes roughly six to eight weeks (Green and Campbell 1984). Metamorphosis is usually complete within 3 months of egg laying (Stebbins 1951).
Toadlets form post-metamorphic aggregations (PMAs) throughout the species’ range (Livo 1998). These aggregations are hypothesized to be the result of either a deteriorating larval environment, an inability to disperse, protection from desiccation, and/or selfish herding behaviour that invokes predator saturation. In Colorado, Livo (1998) found that lack of inlet and outlet channels and associated moisture, as well as desiccating soils surrounding natal ponds appeared to limit the ability of toadlets to disperse. In July 1998, a large PMA two to three inches deep died along a section of the Stewart-Cassiar Highway in northern B.C., where the road bisects a large wetland (R. Pojar, pers. com.). The reason for the massive toadlet death remains unclear, but seems to be related to a heat wave. Also, as PMAs increase in size, and the longer they last, there is an increased potential for individuals at the bottom to become injured, and for dispersal to become ineffective, rendering PMAs lethal in some instances.
Like other aquatic-breeding amphibians, Western Toad populations fluctuate extensively from year to year. Mortality is greatest during the larval and juvenile stage, but slight thereafter (Campbell 1970). In some regions, reproductive success appears to be related to the amount of snow pack and rate of snowmelt, which determines the persistence of breeding pools (Black and Brunson 1971). The rebound of Western Toad populations at sites in Oregon provides some evidence that Western Toads can recover after years of depression (Olson 2001).
Movement and dispersal
In contrast to jumping frogs, Western Toads move overland primarily by climbing and crawling. On Vancouver Island, radio-tracked toads moved mostly at night, starting approximately four hours after dusk and peaking at midnight (Davis 2000). Although toads are most active at night at lower elevations, they can often be seen during the day at higher elevations (Carey 1978, Russell and Bauer 1993). Toads may be diurnal in mountain habitats in response to the activity patterns of their principal food sources (Carey 1978). In some cases, activity levels appear to be related to seasonal changes in temperature, where toads are nocturnal during mid summer, and diurnal in spring and fall (Sullivan 1994).
Many Bufo species have distinct home ranges that include aquatic breeding sites, summer ranges encompassing a variety of upland and riparian areas, and hibernacula. Because female Western Toads do not breed every year they tend to move farther upland than males after breeding, up to 400 to 600+ m away from breeding sites (Jones 1999a). On Vancouver Island, Davis (2000) found toads to have small, distinct home ranges of about 0.1 ha. In Colorado, toads moved an average of 11.83 m per day, within an average home range of 46,185 m², including the breeding site (males = 72,869 m²; females = 34,325 m²), and 43,558 m², excluding the breeding site (males = 70,296 m²; females = 31,675 m²; Jones and Goettl 1998). Home range size varies according to the condition of the habitat (Campbell 1970). Males may be territorial, especially where breeding sites are scarce (Campbell 1970). Toads repeatedly return to particular microsites within their home range (Carpenter 1954, Davis 2000, Jones and Goettl 1998, Bartelt and Peterson 1994). Davis (2000) observed that toads frequently returned to small moist soil patches that they had created, and moved little the majority of the time, but that occasional long-distance excursions were made. Western toads are capable of moving over 5 km between breeding sites (Corn and Muth, unpublished data in Roelle 2000). Davis (2000) commonly found adult toads one to two kilometres away from breeding sites on Vancouver Island during the spring and summer, with some toads moving as much as 7.2 km. Movements appeared to be directional but their significance was undertermined (Davis 2000).
Western toads exhibit strong annual site fidelity. During spring migration to breeding ponds, American toad individuals are capable of well-oriented movements as large as 595 m (Oldham 1965). They will return to their traditional breeding grounds even when other potential sites are available. Davis (2000) found that Western Toads were highly philopatric and homed to their original location up to one kilometre away. Tracy and Dole (1969) found Western Toads homing to breeding ponds when displaced 200 m away. Oldham (1965) determined that toads rely on topographic gradients more so than olfactory, auditory, hygrotactic or visual cues to orient themselves. Tracy and Dole (1969) found that anosmic displaced toads were more disoriented than blind toads.
Toadlets spend the first period of their terrestrial life within the riparian area, eventually dispersing upland. Little is known of the mechanisms that determine the direction and magnitude of their dispersal. Davis (2000) found toadlets at least 300 m from breeding sites on Vancouver Island. Nothing is known of the effect of habitat features on toadlet dispersal. For example, toadlets that emerge from breeding sites within clearcuts may be seriously disadvantaged by extreme climatic conditions and increased predation. Smaller toads heat up and cool down more rapidly than larger toads (Carey 1978). The ability of toadlets to disperse may also be impacted by the persistence of PMAs, especially in riparian areas which receive a lot of trampling by people and/or cattle and near roads where there is a high risk of injury or mortality.
Diet and predation
The diet of tadpoles is comprised of filamentous algae and organic detritus, but they will also scavenge carrion. Adult toads wait for their prey on the surface of the ground or in burrows. They feed on a wide variety of invertebrates, including worms, spiders, bees, beetles, ants, and arachnids (Leonard et al. 1993, Sullivan 1994). They also eat crayfish, sow bugs, grasshoppers, trichopterans, lepidopterons, and dipterans (Stebbins 1951, Verner and Boss 1980). In urban areas, adults toads feed on cuculionids (weevils and billbugs), however, analysis of fresh fecal pellets revealed that the billbugs survive through the digestive tract, so little, if any nutrition is derived from the bugs (Ehrhardt 1996). It takes an average of three to four days for toad prey to pass through the digestive tract at body temperatures fluctuating between five and 30°C (Carey 1978).
Although juvenile and adult toads secrete a mild white poison from their parotoid glands and warts to deter predators, they are preyed upon by coyotes, raccoons, skunks, foxes, corvids, and garter snakes (Olson 1989, Leonard et al. 1993, Jones et al. 1999). Olson (1988) witnessed high predation of adult toads by ravens in Oregon. D. Low (pers. com.) found decapitated Western Toads, along with spadefoot toads, Spea intermontana, in burrowing owl holes in the interior of B.C. Many species eviscerate toads to avoid their toxic skin. The highest predation pressure on adult toads comes during the breeding season when they are exposed and vulnerable in the shallow water margins of lakes and ponds.
Western toad tadpoles and metamorphs are particularly vulnerable to snakes and birds during the period of transformation and migration, when they are not adequately suited to life in either environment (Huey 1980). Gyug (1996) witnessed high predation of newly metamorphosed young by ravens at artificial ponds in clearcuts. Garter snakes, Thamnophis sirtalis, often feed to satiation on maturing tadpoles and newly emerged toadlets (Davis 2000, E. Wind, per. obs.). Devito et al. (1998) found that metamorphic toadlets emerged sooner, in higher levels of aggregation, and more synchronously in the presence of T. sirtalis, regardless of whether or not they had completed tail resorption and were thus physically suited for terrestrial life. This altered behaviour in the presence of T. sirtalis may result in smaller size at emergence, which could have long-term effects on survival (Bellis 1962, Berven 1990) especially in clearcuts where reptiles are abundant (Raphael 1991). Synchronous metamorphosis and aggregation may function as an antipredator adaptation by swamping predators during this vulnerable period.
Western toad tadpoles are eaten by ravens, crows, ducks, herons, garter snakes, backswimmers, and giant water bugs (Olson 1989, Leonard et al. 1993, Jones et al. 1999). They are unpalatable to fish and newts. In experiments, tadpoles did not demonstrate anti-predator behaviour when exposed to trout or newts but did when exposed to aquatic invertebrate and snake predators (Kiesecker et al. 1996). Livo (1999) studied the impacts of various predators on Western Toad tadpoles in Colorado. She found that predacious diving beetle larvae, genus Dytiscus, had the greatest impact on Western Toad larvae compared to adult beetles, tiger salamander larvae, Ambystoma tigrinum, or western terrestrial garter snakes, Thamnophis elegans. The life history of the beetle larvae and of Western Toad tadpoles overlaps significantly, in terms of the onset and duration of the aquatic life phase, the relative size of the predator and prey during this life phase, and daily activity patterns as both are diurnal (Livo 1999). Boreal toad tadpoles may be susceptible to predation by these larvae throughout their entire aquatic life phase. Species of Dytiscus are wide spread throughout North America (Livo 1999).
Like all amphibians, Western Toads are ectothermic, relying on movement between habitat types to thermoregulate. Unlike other smooth-skinned amphibians, toads and newts depend largely on their lungs for thermoregulation (Noble 1954). The range of voluntary thermal minima (3.0°C) and maxima (29.5°C) of Western Toads from southern latitudes is relatively wide compared to other amphibian species (Brattstrom 1963). Davis (2000) fitted toads on Vancouver Island with data loggers and found that their temperatures fell exactly within this range, even though surface temperatures fluctuated more extensively. This tolerance partially explains why Western Toads are commonly found in clearcuts, where temperatures fluctuate extensively compared to areas with forest cover. In spring and autumn, toads were able to maintain higher temperatures in openings than under forest cover (Davis 2000). Carey (1978) calculated an energy budget for toads and found that they could sit in direct sunlight for long periods without fatally overheating, as long as they were moist. Bufonids are relatively tolerant of drying, reaching their critical activity point when dehydrated to 41% of their initial hydrated body mass (Hillman 1980). Their warty skin is moderately resistant to desiccation, and the thin skin of the lower abdomen, referred to as the ‘pelvic patch’, enables them to absorb moisture from the ground (Green and Campbell 1984). Although toads can venture relatively far from water compared to other amphibians, they must rehydrate daily in some sort of standing water (Campbell 1970). Davis (2000) occasionally found toads sitting in water, and observed that suitable moist microsites were readily available within cut-over areas on Vancouver Island. Newly metamorphosed individuals however, have a higher surface area to volume ratio, and are more vulnerable to desiccation than adults (Livo 1998).
Western toads hibernate from three to six months each year, depending on the location. Jones and Goettl (1998) radio tracked toads to their hibernacula in Colorado over a three-year period. Hibernation began in early October, but toads associated with the hibernacula by late August. They found that toads used a variety of sites, including the underside of a bank above a spring seep, willow clumps (sometimes in association with a seep), the base of an Engelman spruce, Picea engelmannii, and a burrow associated with a spring. By far the majority of toads used burrows of golden-mantled ground squirrels, Spermophilus lateralis, even some that were not abandoned. Burrows that were deep enough to prevent freezing and moist enough to prevent desiccation were used. Toads hibernate up to 1.3 m under ground (Mennel and Slough 1999). Campbell (1970) states that B. b. boreas must hibernate in contact with water all winter. Jones and Goettl (1998) found that hibernacula were reused across years, and often occupied by more than one toad. Toads maintained an average body temperature of 5 to 6°C between November and April. Jones and Goettl (1998) noted that a group of toadlets was found under a rock after winter, meaning that they had experienced freezing, or near freezing, temperatures. The fate of toadlets that do not reach adequate hibernacula sites before the onset of freezing, because of their late emergence at high elevations and latitudes, is unknown (Jones and Goettl 1998). In northern regions, toads may be dependent upon areas of high snow accumulation, that lack permafrost, to survive through winter (Cook 1977, Slough 1999).
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