For the surviving males, the odyssey has just begun. The females will arrive within days, urged on by celestial forces demanding they reach the males and the sea in time. 

 

Like the crabs, creatures around the globe are moving to the music of the spheres. They register the sun, the moon, the earth’s magnetic field and move at their bidding, even the navigational feats of the most delicate of creatures beggar the imagination and confound our understanding. 

 

And in tiny slices of endangered Mexican forest, hundreds of millions of the world's most astounding navigators are stirring. These monarch butterflies have waited patiently through the winter, barely moving, for the sun to tell them it's time. They stream to water  to slake a winter-long thirst, water that will bring spring's sexual awakening. Males pluck females from the trees like reluctant flowers. And they mate.