Misc: an estimated ---------- of animals on earth have six legs?

Questions

A ---------- has the largest penis of any other animal in relation to its size.?

A barnacle has the largest ---------- of any other animal in relation to its size.?

A ---------- 's backbone is just as straight as a horse's.?

A camel's backbone is just as straight as a ---------- 's.?

A ---------- male reindeer is called a bull.?