Ants have something like a single fertile female to several males. The same applies to bees, and larger animals - elephants too. Is the sex ratio universally skewed in favour of the female? What animal species exhibit this ?
It is advantageous to have a skew towards more males. Males can be expendable as they are only required briefly for impregnation and can then focus on providing resources for the mother and young and protection. This breaks down when this is no longer the case, for example in humans we are closer to a 50/50 divide.
This article notes that in aphids, there are twice as many females as males, the same is true for daphnia. I don't think the reason behind this is known. In reptiles, the female to male ratio can depend on temperature of incubation thus this can lead to strong fluctuations.
An interesting principle is Fisher's principle: