GG on beating the game! Glad you liked it :)
My original idea was that each animal's value was equal to the length of its name, but I had to scrap that since the distribution of name lengths is so uneven... there's plenty of 5s (PANDA, TIGER, SHARK, etc) but anything above 8 got really iffy, and as far as I know there's no english animal names with 1 or 2 letters (swedish has LO and ÅL (lynx and eel, respectively) but I didn't want to force people to learn a second language on top of the maths). I scrapped the name-length numbering in the last half-hour of development and just numbered animals sequentially, which is why there's no good system to see their numbers. It's probably less of an issue if you play the game back-to-back without long breaks.
The maths system has an implicit rule that every operation fully finishes before the next one starts, so a * b + c * d = ((a*b) + c) * d. I think one of the earlier levels cover this with an oddly-placed multiplication operator. This was made just to make the implementation of the maths system easier (it just steps through each operator and performs it using an ongoing total value and the next cage in line, with an invisible plus before the first cage. And when I say "invisible plus", I literally mean that there's an invisible plus operator there, that's how I implemented it).