Dinner for Two: Dumpster-diving bears. Photo: Jon Beckmann/Wildlife Conservation Society

It has long been known that garbage attracts bears and causes them to ditch their natural diet; who wants to scrounge for berries when there’s half a Big Mac in someone’s uncovered garbage can? But Jon P. Beckman of the Wildlife Conservation Society and Carl W. Lacey of the Nevada Department of Wildlife went further. They followed 22 female black bears—12 in an urban environment and 10 in wildland habitats—from 1997 to 2006 in the northern Sierra Nevada Mountains, including around Carson City and Lake Tahoe, to assess the impact of people and their garbage over the bears’ lifetimes, they report in the journal Human-Wildlife Conflicts.

They find that bears in urban areas weigh more (about 30 percent more than bears in wild areas, because their natural diet is heavily supplemented by garbage) and get pregnant at a younger age (on average, when they are between 4 and 5 years old, rather than 7 to 8 years for bears in wild areas; some urban bears around Lake Tahoe even reproduced at the age of 3).

Despite these advantages, female bears in urban areas do not leave more offspring over their lifetime because their lifetime is cut short. Due to collisions with vehicles, all 12 urban bears in the study were dead by age 10, while only 4 wildland bears died young. As the scientists conclude, “bears in urban areas have experienced elevated levels of mortality that exceed reproductive rates, even though urban bears are more fecund than wildland bears.”