A new study in the journal Nature has shown that pregnant males give birth to a larger number of young from attractive females than from unattractive ones. As males tend to be a prized commodity, you see competition and courting behavior among females for mates. The chosen female then engages in a dance with the male during which she deposits the eggs in his brood pouch to be fertilized and carried. The pouch seals itself shut until the eggs hatch, and after hatching the male is ready to be impregnated again in under an hour!
However, the study found that the male pipefish doesn't treat all eggs equally. Males allocate more care (oxygen, nutrients) to eggs from larger (= more attractive)females. In general, males prefer to mate with larger females and these larger females transfer more eggs (relatively more young survive). Additionally, secondary, smaller females just never seem to measure up to the ex-girlfriend. Males that have a large female as a first mate have less egg survival the second time around with a smaller girl.
...pretty much its the don't put all your eggs in one basket routine. Unless that basket was mothered by the large, skinny, beautiful girl. After all, there are only so many resources to go around.
But wait! There's more!
The study also showed that the male pipefish absorbs some of the developing young - mostly those from the unattractive mate. Yeah, dissolving basically equates to eating in this context. Lovely. Its that whole resource allocation thing again - all about costs and benefits.
Computer models show that it is beneficial for males to mate with smaller females and raise at least some of their brood. Mainly because males are picky and large females are in short supply. While the male is pregnant with the less desirable eggs he can look for a more desirable mate. Way to multitask.
In other species, you typically see such judgements before mating (think frog calls, bird plumage, etc.) or after mating (voluntary aborting, flushing sperm, etc.). In the case of pipefish, you see both, a strategy that is relatively rare.
Here's the link: http://www.nature.com/news/2010/100317/full/news.2010.127.html
(images from daveharasti.com and divegallery.com)