Why Are Hyaenas Smart?

Behavioural data indicate that there has been considerable convergence between primates and hyaenas with respect to their social cognitive abilities.

From here (hat tip to Razib Khan).

I have long attributed the exceptional intelligence of hyaenas, one of the most intelligent and successful African megafauna, to the fact that unlike most other mammals, the reproductive organs of hyaenas require both the male and female to be simultaneous aroused to reproduce. This has the effect of giving female hyaenas much more control of their reproductive decisions than comparable felines and canines. 

Hyaenas are the poster children for Planned Parenthood of the animal world.