In The Call of the Wild, what traits or skills did Buck have that could ensure his success in the wild?

Buck was healthy, strong, and fit.  Although he had lived an almost pampered life on Judge Miller's ranch, "hunting and kindred outdoor delights had kept down the fat and hardened his muscles".  Buck also came from a strong lineage; his father was "a huge St. Bernard", and his mother a Scotch shepherd dog (Ch.I).

Buck was also extremely adaptable, having an uncanny "capacity to adjust himself to changing conditions" (Ch.II).  Buck was intelligent, and able to learn quickly from both observation and experience.  Introduced precipitously to the Darwinian world of survival of the fittest, Buck was able to immediately discern what the most expedient course of action was in a given situation, and was able to perform accordingly.  Although his spirit remained strong throughout, Buck, when confronted by "a man with a club" (Ch.I), "was too wise to rebel" when he knew it was futile (Ch.II).

Buck had strong natural instincts; "the dominant primordial beast was strong in (him)" (Ch.III).  Once he was forced to learn to survive in the new order, "instincts long dead became alive again".  Buck's "sight and scent became remarkably keen...his hearing developed...acuteness".  The need to survive awakened "the old life within him, and the old tricks which they had stamped into the heredity of the breed...came to him without effort or discovery" (Ch.II).