Battle of the sexes

It's tough being a guy in a Darwinian world. Sure, you can sit back and let the female bear many of the costs of childbearing: producing costly eggs filled with precious nutrients, or even carrying them to term within their own bodies. Sure, there's that, but males face a wholly different challenge: how can they know if the hatched babies they're helping to care for are their own?
Cuckoldry is rife in the animal kingdom (and not just in the animal kingdom); is there any better strategy for gene propagation at minimal cost than letting some other poor schmuck look after your kids while you swan off to mate with other females? It is predicted by theory, then, that males would have developed strategies to counter this and, indeed, many of these have come to light. For instance, the first thing a male lion will do on taking over a new pride is to kill off all the cubs, as it is really not in his best interest to raise somebody else's genetic progeny, or to allow his new harem to do so at the expense of raising infants with his own genes. It has even been speculated that we look like our parents so our fathers can be sure of their, um, involvement. A recent study, however, has documented behaviour which really takes the cake: A species of fish in Indonesia is actually more likely to cannibalise children, which are at least in theory his own, if he is unsure of his claim. The likelihood of this happening is, predictably, predicted by the number of males present at spawning.
Nature is nothing if not unrelentingly, horrifyingly efficient.