Why is it that some animals are able to detect the scent of human pheromones even when HUMANS cannot?

Dolphins, for example, will go crazy and start showing a visible erection around a woman diver that is experiencing her ovum cycle, whereas her husband or boyfriend will be completely unaware that it is her fertile time of the month, dogs too, although even dogs don't get as frisky as the dolphins do, want proof, lol, check this out a href="http://youtu.be/OwQObOHbTAk" rel="nofollow"http://youtu.be/OwQObOHbTAk/a