"For whom" here is a reference to the animals. It might be more common to use "for which" here.
"For whom" here can be understood as meaning "where it is the case that" or "regarding whom", "in relation to whom/which" or just "where".
You also asked about redundancy. In this sentence...
In the first, we have animals that display stereotyped, predictable behaviors towards their dead, and for whom much of what we understand about them comes from experimental studies
the terms "that" and "for whom" are both relatives that restrict the scope of "animals".
However, in practice, we would interpret the sentence in exactly the same way if the term "for whom" were missing, so it is effectively redundant.
In the unlikely event that we were distinguishing between this group of animals and another that displayed these stereotyped behaviours but where our knowledge wasn't derived from experiments, then "for whom" would serve a purpose here.
The wording makes it improbable that the "for whom" only applies to a subset of the animals showing stereotyped behaviours; the comma makes it almost impossible.
It is, though, the presence of the word "and" which means that technically "for whom" is a restrictive relative (because it is parallel with the preceding "that", which is restrictive) rather than a non-restrictive one. So instead of omitting "for whom", omitting "and" is also an option (this changes this second relative to a non-restrictive one, but as discussed, that would not affect the intended meaning) - or we could omit "and for whom" altogether, and change the comma to a semi-colon.
Another option is to change "for whom" to "about whom" (or "about which") and then omit "about them".