Unlike bats, cucumber slices can be easily transported to a safe eating place, rarely fight back, and are ideally suitable for precision nibbling.

The skin of cucurbits is usually fairly high in cucurbitacins (usually much higher than the flesh).

that's not unusual; most rodents do it, and also ants and plecostomuses (although in their cases it may be less out of preference and more due to mouth limitations)

true! But that's also because cucurbitacins, given the size of a rat I'd imagine it wouldn't have to eat much cucumber skin to get pretty sick.