Alex was skewering a white rabbit last night. (love a chick who eats raw meat with her hands... growl, but I digress). So we can assume that with all of the experiments run in the past, and that scene, that rabbits can procreate (and *blank* like rabbits) and we've seen little boar..kiddies? So the animals on the island can get pregnant and carry to term. Now I know what you're saying there House 'bunnies and boars ain't no humans' but that can probably eliminate the gas as the reason for the pregnancy problem, since I have seen absolutely NO animals wearing gas masks.
So what else is it? I figured it was the magnetic properties of tthe island at first, disrupting flow of ambiotic fluid, or abnormal growth rates, but wouldn't that affect the animals as well?
Lastly, i figured Jacob had a hand in this, but then why would Ben even attempt to thwart the grand Poobah's plans or actions..

Time loop... what.. arrghh, it's killing me.