Is there an Issur of Tzar Baalei Chaim on humans?
There is a prohibition of harming a human being, including inflicting pain (even without lasting damage). That's straightforward.
The question is as follows, if someone/something is already (unfortunately) suffering:
When do we say that pain and suffering alone, assuming no danger to life or limb, override a Halachic prohibition?
or in Yeshivish,
When are we matir something because of Tzaar Baalei Chayim?
The Chavos Yair (Rabbi Bachrach, lived in Germany in the late 1600s), responsum 191, explains that the answer may be different for humans and animals.
As a quick, imperfect illustration, and I'm not saying this is definitively the halacha, but it gets across the point:
The reasons for that are an interesting philosophical question. E.g. Fluffy was never commanded by God to keep Shabbos; Shmerel was. Assuming Shmerel is an adult, he can understand what's going on and why; etc.
The Chavos Yair never said it's allowable to inflict pain on a person. It's that a person in (I repeat, non-life-or-limb-threatening) pain may not get all the Halachic exceptions that an animal in pain may get. The Chavos Yair's question involved a Cohen who, to avoid contamination near a corpse, would have to be out in the bitter cold for a prolonged period. He allows it (in a case of rabbinically-prohibited contamination), but warns that the rules are more nuanced than an analogous case for animals.
The Chavos Yair in Kuf Tzadik Aleph says no, because humans have intelligence and they can choose to process it as they please (it is what you make of it). Hence there is no such thing as Tzaar Ballie Chaim for Humans.
I'm not answering halachickly but logically. If there is an avaira for hurting a squirell then certainly a human who is created B'tzelem Elo-kim should be much worse. I would think hurting a human is tzaar baalei chayim-we are baalei chayim.
Like i said this is not the halacha!!!!!
Please correct me if i am wrong.