by the possibility concerning faster clock speeds for smaller animals, the possibility of lesser unity in non-human animals (which one might value at >1x for the same reason one might value a dually-conscious split-brain patient at ~2x), and the possibility for greater intensity of experience in simpler animals.
Interestingly, some variant of each of these would also seem to apply when comparing the moral weight of adult human vs. infant/toddler humans; while human infants probably don’t have a higher clock speed than adults in the same sense that small animals might have a higher clock speed than humans, there is the widely-known point about young children regardless seeming to have a much higher subjective speed than adults.
Interestingly, some variant of each of these would also seem to apply when comparing the moral weight of adult human vs. infant/toddler humans; while human infants probably don’t have a higher clock speed than adults in the same sense that small animals might have a higher clock speed than humans, there is the widely-known point about young children regardless seeming to have a much higher subjective speed than adults.
And if we are willing to ascribe moral weight to fruit flies, there must also be some corresponding non-zero moral weight to early-term human fetuses.