If a species is hungry and CAN eat, it MUST eat. With that in mind, page 14 of the Rule Book where it goes into more detail about each card, it says
Up to 2 Population may be ignored when determining if this species is Hungry. Those Population don't need to be fed during the Feeding Phase and they won't be lost due to Hunger at the End of Feeding.
Based on that, I'd say they can go unfed if you want, but remember that food equals points too. The food is available now, but it may not be next round when your population bumps up thanks to the Fertile trait. Of course, that all hinges on whether the other players still need to feed as well.