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An alternative explanation is defense from predation. Black bear cubs use trees for defense, whereas brown bears and polar bears, which regularly inhabit treeless environments, rely onaggression to protect their cubs. Like brown bears and polar bears (and unlike other myrmecophagous mammals, which are noted for their passivity), sloth bears are easilyprovoked to aggression. Sloth bears also have relatively large canine teeth, which appear to be more functional for fighting than for foraging. Like brown bears and polar bears, slothbears may have evolved in an environment with few trees. They are especially attracted to food rich grasslands; although few types of grassland persist today on the Indiansubcontinent, this type of habitat was once widespread there. Grasslands support high densities of tigers, which fight and sometimes kill sloth bears; sloth bears also coexist withand have been killed by tree-climbing leopards, and are often confronted and chased by rhinoceroses and elephants, which can topple trees. Collectively these factors probablyselected against tree climbing as a defensive strategy for sloth bear cubs. Because sloth bears are smaller than brown and polar bears and are under greater threat from dangerousanimals, they may have adopted the extra precaution of carrying their cubs. Although cub carrying may also be adaptive for myrmecophagous foraging, the behavior of sloth bear cubs,which on their mother's back at the first sign of danger, suggests that predation was a key stimulus.