Male platypuses have venomous ankle spurs. Why are they probably not mainly prey-hunting tools?

Answer: Seasonal male weapon

Seasonal male weapon ✓ — Correct. The venom apparatus is tied to adult males and peaks around the breeding season, which points to competition between males more than everyday prey capture. A shrimp-hunting tool should be useful to females and juveniles too, because they also eat. The odd lesson is that a mammal's venom can be more about mating conflict than about killing dinner.

Fish-stunning tool — Fish-stunning sounds plausible because venom often means predation, but platypuses mainly eat aquatic invertebrates and locate them with the bill. The spur sits on the hind ankle, not at the mouth or bill where prey is collected. If it were the main fishing weapon, its seasonal male-only pattern would be hard to explain.

Year-round predator shield — A defensive role is possible in a broad sense, but 'year-round shield' misses the strongest pattern. The venom system is functional in adult males and peaks around breeding season, which points to male-male competition as the main story. If defense were the whole explanation, the seasonal male bias would be much less telling.