Months before Bruno Iksil became famous as the "London whale," the trader who contributed to a loss of more than $2bn at JP Morgan Chase, he earned a different nickname: the "caveman," for pursuing trades that rivals sometimes thought were overly aggressive but often led to huge profits.

Late last year, Iksil, a London-based trader in JP Morgan's chief investment office, turned heads among fellow debt-market traders with a wager against a group of junk-bond-rated companies, traders say.