I'll call in my help some words from Valeria Simonova, a senryu scholar:

A senryu is not the transmission of an important aesthetic experience in which man merges with nature (as in haiku), but a shrewd penetration into the world of human nature, weak and imperfect, but also tender and sometimes moving. A poet of senryu extracts from his quiver irony, humor, sarcasm, parody, and with these arrows he hits the reader.
According to a famous saying, "if a haiku is the finger that indicates the moon, a senryu is an elbow in the ribs".

A haiku about animals is simply a haiku, but if you talk about an animal to make a metaphor about the human condition, then it can be a senryu.
For example, you can write about ants running amok outside and inside the anthill, and let it be understood that they are like commuters at rush hour.

Sorry for the long and boring comment, hope I could clarify you something ...

(Marco)