Costing seems very imbalanced. Basically the only sensible buying strategy is: Buy green snakes only until you can afford mice, then just buy mice forever as they offer the most pies per second relative to their cost in pies:
Green snake: 100 pies per pie per second
Corn snake: 110 pies per pie per second
Mouse: 90 pies per pie per second
Magpie: 135 pies per pie per second
Bear: 133 pies per pie per second
Serpent: 118 pies per pie per second
Light wolf: 116 pies per pie per second
Dark wolf: 103 pies per pie per second
Is there some sort of goal or benefit from buying the less cost effective animals?
Can't tell if feeding and happiness actually do anything, do I just wait until they hit 100% and see?