Translation:I am a mammal because I am human.
Just rejected was my "I am a mammal since I am human." In English some uses of "since" are not equivalent to "because" (as in "I've lived here since I was a teenager"), but other instances--like the one in this exercise--are.
No, in the sense of logical inference, 'since' is what I would naturally use in this case. If "Είμαι θηλαστικό επειδή είμαι άνθρωπος" is natural Greek, and "I am a mammal since I am a human" is natural English, it follows that 'since' should be accepted for this exercise.
Grammatical terms seldom have good one-to-one translations, but often overlap in usage.
I will cook because you are busy is not exactly the same I will cook since you are busy. The same exact (slight) difference exists in greek between αφού & επειδή. Είμαι θηλαστικό επειδή είμαι άνθρωπος is not the same with είμαι θηλαστικό αφού είμαι άνθρωπος in the same way that "because I am human" and "since I am human" are not the same.
In English they are close enough in meaning for "since" to be accepted here, it seems to me. As this article on Merriam-Webster's website points out, "Since as a causal conjunction is almost unremarkable except to a few stick-in-the-muds, and is sometimes preferable when you want the cause to be less directly linked to the effect." https://www.merriam-webster.com/words-at-play/since-as-because-usage
No offense at the "stick-in-the-mud" remark. Σιγούρα, δεν αναφέρεται σε εσάς. ;-)