Basic animal physiology for common animals is pretty well nailed down, I presume you could look that up. We probably also know how big their internal organs are, on average, just as an example.

Also note, for free vs no reward are not the same. Being cited as an author of something, anything, is important for advancement and recognition in your field. Students seldom get much financial compensation for the research necessary to graduate, for instance.

That said, I’m guessing. I am not in biology or any of its offshoots.