Anaxyrus hemiophrys (Cope, 1886)
© 2011 Richard Sage (1 of 1)
Bufo hemiophrys Cope, 1886
Michael A. Ewert1
1. Historical versus Current Distribution. Canadian toads (Bufo hemiophrys) have their widest distribution across the prairies and aspen parklands of Canada from south of Great Slave Lake in the Northwest Territories, across eastern Alberta, central Saskatchewan, and southern Manitoba (Hamilton et al., 1998). In the United States, their historical range includes an isolated site in northeastern Montana and extends across northern North Dakota, then southeastward into northeastern South Dakota, and also into the Red River Valley and adjacent areas of western Minnesota. Reports during the 1970s extend their known distribution into southwestern North Dakota (e.g., Seabloom et al., 1978). This distribution is unique among amphibians in lying completely within regions of North America covered by the late Wisconsinan ice sheets (Flint, 1947; Underhill, 1961). Further, the range of Canadian toads tends to mirror the former extent of glacial Lake Agassiz (Underhill, 1961). Historically, this glaciation may have allowed Canadian toads (or their precursor) to range south and west of their current range. This notion is supported by the relict population of closely related Wyoming toads (B. baxteri, formerly designated as a subspecies, B. h. baxteri) in southeastern Wyoming (Henrich, 1968). Canadian toads range in altitude from 300–2,130 m (Stebbins, 1985).
2. Historical versus Current Abundance. In the recent past, Canadian toads have been considered "very abundant" in northeastern North Dakota (Fishbeck and Underhill, 1960). In northwestern Minnesota, at the 640-acre Waubun Prairie Research Area, Mahnomen County, the population size exceeded 5,100 individuals in 1961; numbers fluctuated at lower levels in other years (Tester and Breckenridge, 1964b). In any given year, juveniles (second-year animals) outnumber adults with juvenile/adult ratios varying from 1.5–14.1 (Kelleher and Tester, 1969). Two amphibian calling surveys were conducted in northern and eastern North Dakota during 1995 (Johnson and Batie, 1996; Bowers et al., 1998). Both surveys sampled according to a predetermined grid of focal areas and found that the apparent distribution of Canadian toads was much smaller than the historical one given by Wheeler and Wheeler (1966). The more extensive of the two surveys (Johnson and Batie, 1996) found Canadian toads at only 4 northeastern focal areas (11 call listening posts) among about 30 sampled focal areas within the range known in 1966. However, such surveys may underestimate species with special habitat needs and spotty distributions. The historical status of Canadian toads in Alberta is well documented (Hamilton et al., 1998) and provides a perspective on the near present. At very least, a range contraction appears to have occurred in south-central Alberta since the mid 1980s (Roberts, 1992). Declines pertain especially to populations along the lower Medicine River and adjacent portions of the Red Deer River in Alberta. Sufficient concern for this apparent decline has resulted in yellow-listing ("may be at risk") in Alberta. Saskatchewan is without a comprehensive survey for Canadian toads, however the province also lacks obvious signs of population declines (Didiuk, 1997). In the United States, only Montana, which is peripheral in the species range, protects Canadian toads and lists them as Endangered (review in Hamilton et al., 1998).
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. In northwestern Minnesota, adults generally move from overwintering sites to breeding sites soon after emergence during late April to May (Tester and Breckenridge, 1964b). Atypically warm early spring weather fostered emergence as early as 28 March (1958) at Delta Marsh (southern Manitoba); however, return to freezing weather for an additional month stalled breeding and may have killed the toads that emerged early (Tamsitt, 1962). Chorusing normally commences during May. In flooded ditches in Minnesota, breeding congresses tend to include < 20 individuals. At Waubun Prairie, however, breeding congresses composed mostly of males varied from 43–112 individuals and totaled 207 individuals on 21 May 1966 (Tester et al., 1970; M.A.E., personal observations). Not all males in a population appear to breed in any given year. Females approach the chorus singly, lay eggs, then leave (Tester and Breckenridge (1964b). Chorusing activities tend to decline during June; but at Delta Marsh, Manitoba, calling has continued sporadically into July and early August and even to 12 September 1958 (Tamsitt, 1962), which seems non-adaptive. The call is a series of low-pitched, soft trills of short duration, suggesting slightly the call of American toads (Bufo americanus), but much softer (Breckenridge, 1944; Conant and Collins, 1998).
ii. Breeding habitat. Breeding habitat includes the shallows of lakes, in ponds, and other bodies of water (Wright and Wright, 1949; Roberts and Lewin, 1979; Oldfield and Moriarty, 1994). In one instance, 14 sexually mature adults occurred in a lake in mid May in Day County, South Dakota. Sampling at this same lake in early July yielded seven newly metamorphosed animals (Underhill, 1958). Toads also use flooded roadside ditches, especially in the flat agricultural country that currently occupies much of the species' range in Minnesota (Breckenridge, 1944; Brown and Ewert, 1971; M.A.E., personal observations). Breeding habitat may include flowing water, such as the Red Deer River of Alberta (Roberts and Lewin, 1979). Water associated with breeding habitat may be in permanent, semi-permanent (Tamsitt, 1962; Tester et al., 1970), or temporary basins (roadside ditches). At Delta Marsh, Manitoba, Canadian toads favored interdunal ponds near Lake Manitoba as opposed to more distant marshes (Tamsitt, 1962). Canadian toads can be syntopic with Great Plains toads (B. cognatus) in temporary basins in the Red River Valley but rarely hybridize with them (Brown and Ewert, 1971). The presence of fishes has not been noted and seems unlikely at places where Canadian toads breed in northwestern Minnesota (M.A.E., personal observation).
i. Egg deposition sites. Chorusing and egg deposition sites are often in 10–50 cm deep water in areas with a mix of open water and vascular plant debris, new blades of grass or sedges, and/or sparse cattail. Canadian toads avoid calling and egg deposition in dense beds of cattail or rushes (M.A.E., personal observations, Mahnomen and Norman counties, Minnesota). In Alberta, some oviposition sites lack any vegetation and may render the eggs and early tadpoles at risk from water movement (Roberts and Lewin, 1979).
ii. Clutch and egg size. Egg counts from three individuals in northeastern Alberta ranged from 3,354–5,842 eggs (Roberts and Lewin, 1979). These eggs are joined in long strings (Breckenridge, 1944; Roberts and Lewin, 1979). The early embryo is about 1 mm in diameter and positioned about 1 mm from the next embryo in the string (Breckenridge, 1944).
i. Length of larval stage. The exact duration for individual freely ranging tadpoles is unknown; metamorphosing toadlets appear at Waubun Prairie from late June to mid July. Thus, if eggs are laid from 15–20 May and hatch within 4–5 d, the tadpole stage lasts approximately 6–8 wk (see Breckenridge and Tester, 1961).
ii. Larval requirements. Canadian toad tadpoles spend daylight hours in warm areas of wetlands that are thermally stratified. Further, in response to these thermal preferences, several bufonids will aggregate, including closely related American toad and Wyoming toad tadpoles (Beiswenger, 1978).
a. Food. Tadpoles of Canadian toads, as with all North American bufonids, are generalized suspension feeders, ingesting a range of organic and inorganic material associated with mud, plant, and other surfaces.
b. Cover. The need for cover, such as to seek shielding from UV-B radiation, remains undocumented and seems unlikely.
iii. Larval polymorphisms. No larval polymorphisms occur.
iv. Features of metamorphosis. Canadian toads metamorphose between 9.0–15 mm SVL (Wright and Wright, 1949; Breckenridge and Tester, 1961).
v. Post-metamorphic migrations. At Waubun Prairie, Minnesota, large numbers of juveniles remain near their natal wetland to feed along open mudflats (Breckenridge and Tester, 1961). At Delta Marsh, Manitoba, newly metamorphosed toads move to the edges of sand dune ridges and gradually move out onto them (Tamsitt, 1962).
D. Juvenile Habitat. Juveniles (defined as animals < 45 mm; Breckenridge and Tester, 1961) between 18–36 mm were found near the remaining pools of a drying stream in northeastern North Dakota (Wright and Wright, 1949). Young of the year often occur in high numbers along the margins of spawning areas in Alberta (Roberts and Lewin, 1979). At Delta Marsh, Manitoba, second-year juveniles were common on the dune ridges and by mid summer frequented the open beaches of Lake Manitoba at night and regularly moved across beaches, between interior marshes and the lake side—75–120 m (Tamsitt, 1962). Age-specific growth rates are described in Breckenridge and Tester (1961); first-year growth rates are detailed in Roberts and Lewin (1979).
E. Adult Habitat. Canadian toads occur in mesic (but not arid) prairies and savannas, usually near streams, lakes, pothole wetlands, irrigation ditches, and flooded fields (Wright and Wright, 1949; Roberts and Lewin, 1979; Stebbins, 1985). They appear to have evolved to associate with wet prairie or pothole country in places with enough water in their hydrocycle to not render them saline (M.A.E., personal observations). They favor the aspen parkland country (Hamilton et al., 1998), which has groups of trees interspersed with grassland. In northeastern Alberta, Canadian toads are more abundant in areas of grass or willow bog than in areas of aspen or spruce (Roberts and Lewin, 1979). In a pilot study of choice for semi-natural cover in Minnesota, adult male Canadian, American, and Great Plains toads were released within a fenced area (19 x 6 m) that ranged from mostly white spruce woods at one end to short grass at the other. Whereas Great Plains toads clearly favored the short grass, Canadian toads were more moderate in their preference for grassy areas, and American toads favored tall grass and woods (M.A.E. and J.R. Tester, unpublished data). During droughts, Canadian toads will enter gutters and sewers in small towns, such as Walhalla, North Dakota (Wright and Wright, 1949). Canadian toads are good swimmers (Stebbins, 1985), more aquatic than either American toads or Woodhouse's toads (Bufo woodhousii; Underhill, 1961), and the most aquatic of Minnesota's toads (Oldfield and Moriarty, 1994). Canadian toads are active around the clock during the breeding season (Stebbins, 1985; Fischer et al., 1999; M.A.E., personal observations). On cool nights, Canadian toads tend to burrow into sandy or loamy soils, but can be active on warm nights (Tamsitt, 1962; Stebbins, 1985).
F. Home Range Size. In a traditional sense, home ranges probably do not exist. Canadian toads have local focal areas for breeding, summer feeding, and overwintering. Individual toads often remain in one place for several days and then suddenly move some distance to another. The extent of these daily movements varies from almost no movement to > 225 m (Breckenridge and Tester, 1961, table 2). The day-to-day movements of Canadian and American toads are similar in distance but differ from the longer movements of Great Plains toads (Ewert, 1969).
G. Territories. As with other species of North American Bufo, Canadian toads move among focal areas but do not defend territories (e.g., Breckenridge and Tester, 1961; Ewert, 1969).
H. Aestivation/Avoiding Dessication. During the numerous observations of Breckenridge and Tester (1961), they "… found only one animal that exhibited anything suggesting aestivation." This animal remained dormant, buried in grass and surface litter for periods of 5, 7, and 8 d, from late June to early August. Upper lethal temperatures are about 40 ˚C (Schmid, 1965a, fig. 2). In a laboratory comparison of deep body temperatures with adjacent environmental surface temperatures in American, Canadian, and Great Plains toads, deep body temperatures of American and Canadian toads were cooler than the substrate, especially at high temperatures. Deep body temperatures within Great Plains toads remained similar to substrate surface temperatures (Tester et al., 1965). These differences seem unlikely to reflect simple evaporative cooling and instead could represent behavioral thermoregulation (Schmid, 1965b).
I. Seasonal Migrations. The annual period of activity for Canadian toads at Waubun Prairie, Minnesota, begins in May with their emergence from hibernation and their congregating along the pond margins. These events are dependent on weather conditions (Breckenridge and Tester, 1961). Findings of post-breeding adults most frequently occur along the ponds margins, with no differences between sexes. The cessation of activity in autumn is gradual, with animals burrowing to hibernate over the course of several weeks. The distances of "long movements" of toads are given in Breckenridge and Tester (1961, table 3). These movements are not correlated with rainfall events.
J. Torpor (Hibernation). Retreating to overwintering sites at Waubun Prairie takes place over a period of several weeks, with many of the larger toads burrowing in late August to early September (Breckenridge and Tester, 1961). Here, Canadian toads overwinter communally in small earthen (mima) mounds (Breckenridge and Tester, 1961; Tester and Breckenridge, 1964a; Ross et al., 1968; Oldfield and Moriarty, 1994; Fischer et al., 1999). The mounds, apparently derived through the burrowing activities of the toads and several mammals, are ~0.5 m high, 3–12 m in diameter, and located about 25 m from wetlands (Breckenridge and Tester, 1961; Tester and Breckenridge, 1964a; Ross et al., 1968). Individual toads repeatedly overwinter in the same mima mounds and show 87–95% homing fidelity to these areas (Kelleher and Tester, 1969).
Within the mima-type mounds, Canadian toads move vertically in the soil horizon in response to temperatures. By mid October, marked toads in one study had burrowed to a depth of 46–66 cm and tended to stay there until January–February. Then, when soil temperatures dropped dramatically, toads burrowed deeper to a maximum depth of 117 cm (Breckenridge and Tester, 1961; Tester and Breckenridge, 1964a). Emergence appears to be stimulated by thawing of the soil. Spring soils thaw from both above and below the frozen layer, and toads begin to emerge once the thaw is complete. Breckenridge and Tester (1961; Tester and Breckenridge, 1964a) note that emergence occurs over the course of several weeks. The earliest toads to emerge from hibernation are adult males, with females and juveniles following (see also Kelleher and Tester, 1969). The final stage in emergence is often triggered by rains.
Canadian toads occur, at least in small populations, in many places in the United States where mima mounds are absent or not evident. One of us (M.A.E.) suspects that, as with Great Plains toads (Ewert, 1969), Canadian toads select roadside berms and other spots with good drainage as places for overwintering. In the Northwest Territories, Canada, Canadian toads overwinter communally (apparently > 500 individuals) but in a sandy hillside with a southern exposure, somewhat reformed by a road cut (Kuyt, 1991). The sandy nature of this location contrasts sharply with the structure of the mima-type mounds. The mounds have a thick layer of black silt loam overlying dense yellow clay (Ross et al., 1968). A comparison of soil characteristics with depths attained by burrowing toads (Tester and Breckenridge, 1964b) suggests that toads normally enter the dense clay layer.
K. Interspecific Associations/Exclusions. In North Dakota, Canadian toads, American toads, and Woodhouse's toads have complementary distributions (Fishbeck and Underhill, 1960). Canadian toads will hybridize with Great Plains toads in northwestern Minnesota (Brown and Ewert, 1971). Canadian and American toads have high survival in reciprocal hybridization crosses in the laboratory (Blair, 1972b). Natural hybridization with American toads occurs in eastern South Dakota (Henrich, 1968) and has been studied extensively in southeastern Manitoba (Cook, 1983; Green, 1983; Stebbins, 1985; Green and Pustowka, 1997). Based on morphology and allozyme data, the steepest gradient in the transition from B. hemiophrys (west) to B. americanus (east) is about 20 km across. This zone appears to have drifted nearly 10 km westward (toward Winnipeg) between 1968–'69 and 1977–'79. Although there is some evidence for hybridization on either side of the transition, the width and character of the overall zone seems to be stable; the introgression is not expanding. Green (1983) and Green and Pustowka (1997) conclude that the zone has natural causes dating back to the retreat of glacial Lake Agassiz during the early Holocene epoch. Hybridization with American toads seldomly occurs, if at all, across a sharp forest-prairie transitional zone in northwestern Minnesota, directly west of Itasca State Park (J.R. Tester, unpublished data; M.A.E., personal observations).
L. Age/Size at Reproductive Maturity. Males grow to 56–68 mm and females tend to reach 56–80 mm; hence, females tend to be larger (Wright and Wright, 1949; Breckenridge and Tester, 1961; but see Underhill, 1961). Individuals over 45 mm SUL enter breeding choruses and have thus attained adult size (Tester and Breckenridge, 1964b). This length is achieved during the second summer, and the first opportunity to breed follows winter when the toads are nearly 3 yr old.
M. Longevity. Populations that emerged from overwintering sites (mima-type mounds) at Waubun Prairie fluctuated between ~1,000–2,500 toads during the 5-yr period, 1962–'66 (Kelleher and Tester, 1969). Post-metamorphic mortality tends to be high, such that 1–2-yr-old juveniles constitute 93% of a spring emergence—only 7% are adults (Tester and Breckenridge, 1964b). Fewer than 1% of yearlings survive to be 6 yr old (Kelleher and Tester, 1969).
N. Feeding Behavior. The bulk of the diet of 16 adults from southern Alberta consisted mainly of ants and secondarily of ground-dwelling beetles, but included flies and insects from six additional orders and some small spiders. Small ground beetles dominated the diet of 19 juveniles from the same region. Small flies, mites, and springtails followed in abundance, and ants were not a large dietary component of these juvenile toads (Moore and Strickland, 1954). Moore and Strickland (1955) concluded that the diet of adult Canadian toads did not vary appreciably from that of adults and subadults of western toads (B. boreas), which they had studied personally.
O. Predators. Documented predators of Canadian toads are plains garter snakes (Thamnophis radix; Breckenridge and Tester, 1961; Tester and Breckenridge, 1964b), badgers (Taxidea taxus; field notes of M.K. Nelson, cited in Breckenridge and Tester, 1961), and red-tailed hawks (Buteo jamacensis; Tester and Breckenridge, 1964b). Additional predators likely include raccoons (Procyon lotor; Tester and Breckenridge, 1964b) and other mammals, birds, and snakes.
P. Anti-Predator Mechanisms. Canadian toads avoid some kinds of predators by being mostly nocturnal during warm weather. They are also cryptically colored and produce toxic secretions from their parotoid glands. When they are disturbed near water, Canadian toads often move toward the water for escape, where they may swim over 30 m and either float at the surface or dive to the bottom (Breckenridge and Tester, 1961; Underhill, 1961). However, fleeing toads have sought other alternatives on the beach near Lake Manitoba and moved toward upland vegetation rather than toward the open lake (Tamsitt, 1962).
Q. Diseases. Field-collected but laboratory-maintained Canadian toads died from mycotic dermatitis caused by the fungus Basidiobolus ranarum (Taylor et al., 1999). Two malformed Canadian toads, one from Minnesota and one from North Dakota, have been reported (NARCAM, 1997).
R. Parasites. In a sample of 40 adult Canadian toads from Alberta, Canada, there was one species of trematode (Gorderina simplex) and three species of nematodes (Cosmocercoides variabilis, Oswalocruzia pipiens, and Rhabdias americanus). The nematode R. americanus had the highest prevalence (73% of the toads sampled), the nematode C. variabilis had the highest intensity (average 26 individuals/toad; Bursey and Goldberg, 1998).
4. Conservation. Despite evidence for species declines in Alberta (Hamilton et al., 1998), Canadian toads retain an overall large geographic distribution. This large distribution clearly distinguishes Canadian toads from their closest relative, critically endangered Wyoming toads. Even though the most proximate threats to Wyoming toads are not clear (see B. baxteri account, this volume), Canadian toads seem unlikely to become endangered globally. Montana, which represents the northwestern extreme of the U.S. distribution, protects Canadian toads and lists them as Endangered (reviewed in Hamilton et al., 1998).
Canadian toad populations in the southern portion of their range (i.e., in the United States) experience a periodic (10–11 yr) natural stress from severe droughts. Population contractions and expansions due to the availability of surface water are no doubt a natural occurrence, but droughts likely have a more profound effect on amphibian populations in today's fragmented landscapes than they have had in the past (Lannoo, 1998a).
In the United States, Canadian toads range mainly within the prairie pothole breeding grounds of waterfowl and other wetland birds. This area remains rich in wetlands, as federal and state governments and NGOs have protected habitat for waterfowl. Within the historical range of Canadian toads in northern and eastern North Dakota, there are at least ten separately named National Wildlife Refuges. Minnesota has at least three named National Wildlife Refuges along the eastern edge of Canadian toad range, and South Dakota includes at least two National Wildlife Refuges. It is beyond the present scope to assess how much management within these refuges is optimally compatible with the needs of toads. However, a cursory review of internet information suggests that some wetland-upland combinations (e.g., Type III wetlands with adjacent prairie grass uplands and permeable soils) are maintained. These refuges should buffer Canadian toads against extirpation during extreme droughts.
Information on whether Canadian toads or other amphibians and reptiles actually occur on the refuges is less readily available than the internet postings of resident bird and mammal checklists. Sand Lake National Wildlife Refuge (in South Dakota) lists Canadian toads as present, but Audubon National Wildlife Refuge (North Dakota) does not include them among their four listed amphibian species. It is unclear whether amphibian and reptile surveys have occurred at many of the refuges, although any environmental impact assessment would require an evaluation. In general, keeping with state non-game programs, lists of all vertebrate wildlife should be created and maintained. While amphibians and reptiles did not generate the funding to create these refuges or to maintain them, the problem of declining amphibian populations has generated wide public interest, including visiting refuges, and non-game funding for many species is increasing.
Canadian toads occur along the northern edge of agricultural lands with extensive pesticide applications for row crops. Overlap between toads and pesticide applications seems likely in the Red River Valley of Minnesota and North Dakota. Some pesticides formerly assumed to be harmless are now known to act as endocrine disrupters on amphibians (e.g., atrazine acts as a testosterone suppressor, which in very low concentrations leads to feminized male amphibians; Hayes et al., 2002a). Actual effects of pesticides on Canadian toads remain unknown (Sparling et al., 2000), and this gap in the toxicological literature must be filled.
Acknowledgments. M.A.E. offers warm thanks to John R. Tester for providing guidance, encouragement, and financial support during the author's studies on toads in northwestern Minnesota during the 1960s.
1Michael A. Ewert
2Michael J. Lannoo
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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