From a PNAS paper by Yamada et al. That is, the macaques were willing to take greater risks for a reward when their pre-existing "wealth" is greater, and the possible lost utility is therefore relatively smaller. The wealth in this case was water - either in the form of a drink of water, or their internal store of water as measured by blood osmolality (the macaque's water bank account). More applications of the St. Petersburg lottery here.
Yamada H, Tymula A, Louie K, Glimcher P. Thirst-dependent risk preferences in monkeys identify a primitive form of wealth. PNAS. Published online before print September 9, 2013, doi: 10.1073/pnas.1308718110.