SPORT: The clash of opposing linemen at every down in American Football?

Questions

The leech has 32 brains - 31 more than a ----------?

The cells which make up the antlers of a ----------?

Pigeons and hummingbirds have tiny magnetic particles in their heads that respond to the Earth's magnetic fields and are used for ----------?

In Russia, dogs have been trained to sniff out ore deposits that contain ----------?

The word "struthious" refers to something that resembles or is related to ----------?