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For the last 14 years, we have been studying the sociobiology, demography, and population dynamics of blacktailed prairie dogs (Cynomys ludovicianus) in Wind Cave National Park, South Dakota. Our study colony covers 6.6 hectares (16 acres) and has not expanded during the period of research; in late spring of each year the colony contains a mean ± SD of 133 ± 29 adults and yearlings and 81 ± 33 juveniles. We have discovered four surprising aspects of the demography and populations dynamics of prairie dogs. (1) Mortality during the first year is approximately 50% for both sexes. Those males that survive the first year can live as long as 5 years, and females that survive the first year can live as long as 7 years. (2) Litter size ranges from 1 to 6, the mean ± SD is 3.05 ± 1.08, and the mode is 3. (3) Although individuals of both sexes usually defer first breeding until the second year, 9% of females and 3% of males first produce offspring as yearlings. (4) Infanticide is the major source of juvenile mortality, accounting for the partial or total demise of 51% of all litters born. In the most common type of infanticide, lactating females kill the unweaned offspring of their sisters and daughters.