Homing and Survival in the Manitoba Toad, Bufo Hemiophrys, in Minnesota

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Homing to overwintering sites, spring emergence, and structure and density of a population of the Manitoba toad, Bufo hemiophrys, were studied in northwestern Minnesota from 1960 to 1967. Data were obtained on 15,837 individuals as they emerged from their wintering sites. Toads generally returned to the same Mima—type mound each fall, although individuals from various mounds were mixed in the breeding choruses and at the pond margins during summer. The high percentages of toads which homed to the wintering site correctly, ranging from 88 to 95%, indicate remarkable orientation capabilities. Toads committing an error when homing are much more likely to select a mound near their "home" mound, rather than a more distant one. Error rates were 10.2% for toads released as year—old juveniles and 2.9% for adults. Emergence occurs during a 5—6 week period, generally starting during the last few days of April. In general adult males emerged before adult females with juveniles emerging last. Deep frost in the soil delays the period of emergence. Population structure and density were determined mainly from toads wintering in four mounds. Yearly trends in juvenile numbers have been unanimously reflected by all four mounds. The 1966 population size, estimated by the Lincoln index, was 510 males, 1,276 females and 2,415 juveniles. Adult sex ratios varied from 33 to 38% males. The best estimate of the age structure of the total population at the time of breeding varied from 1.47 to 14.06 juveniles per adult. Survival of all toads from year to year ranged from 23.9% in 1963 to 44.4% in 1965. Juveniles had lower survival rates than adults, with the greatest mortality occurring in 1963 as the result of light snow and subsequent deep frost.