In my post last week about animal navigation – part 1, I wrote that animal navigation is a complex area of scientific research. While more work is needed, the present findings on marine animals’ ocean navigation are very interesting. Let’s take the example of the harbour seal. So far, the only creatures known to be able to navigate with the help of individual stars, were us, humans. The study of two harbour seals conducted in a custom-made swimming planetarium taught them to recognise Sirius, the brightest star in the night sky, that was projected onto the water. After some time both seals would follow Sirius while swimming to their destination. The scientists involved in the study concluded that harbour seals are able to develop a star compass system like that used by Polynesian sailors. They wrote: ‘We suggest that marine mammals might learn to identify lodestars in the pattern of the night sky and to use these lodestars as distant landmarks …to steer by in the open sea.’
For some time now scientists have worked on a theory that marine turtles circle the whole North Atlantic Ocean using the Earth’s magnetic field as a guide. After years at sea, marine turtles congregate around the edge of many beaches in various places in the world. Female turtles come on to the beach, and after digging deep into the sand, they deposit several eggs. Then they depart back to the sea. When the eggs hatch, the youngsters instantly trudge down the beach sloping towards the sea. They have to overcome many challenges as there are predators waiting for them on the beach and in the shallows of the sea. Once in the water, they swim as quickly as possible away from the land. After several years at sea, the turtles return to the same spot where they were born. The question that puzzled scientists was, how do they know where this spot was? We are now told that it was established; the turtles rely on Earth’s magnetic field to find their way back to the place of their birth. The coastline of each beach has its own magnetic field and the turtles remember this and use it as an internal compass when returning years later. “It is pretty fascinating how these creatures can find their way through a vast expanse of nothing”, wrote J. Roger Brothers, one of the scientists involved in the study of the marine turtles’ navigation.
The studies were lengthy and laborious, and other researchers found out that when the turtle hatchlings run towards the sea, it is because this side is brighter. Once they are in the sea, they swim into the waves, which are corresponding with orbital movement under the water. The turtles are directed by the magnetic compass, and together with the findings of the magnetic fields of the coastlines, it gives credence to the evidence that the adult marine turtles navigate using both coordinated magnetic maps.
The observation that Monarch butterflies migrate in late October from their breeding grounds in northeastern Canada and the USA was first made 500 years ago. Since then, the question has been where do they go? It took centuries before it was discovered that the destination was the remote side of a mountain in Central Mexico, to hibernate in oyamel fir trees. The woods there provide wintering sides for the millions of butterflies, the biggest colony of Monarch, in the world. It is one of the greatest spectacles of nature and we have seen it in countless documentaries. Tourists travel there in their thousands, but the question of how do the butterflies find the same sides still remained unanswered, until now. After years of getting nowhere, the scientists had the idea of attaching small round paper tags with a unique number to the underwings of a few butterflies. It also included a request for sending back a report. With the help of many volunteers, 300,000 butterflies were tagged. Slowly it emerged that Monarchs were heading to Texas and then to Mexico.
After more years of research, it was established quite by chance that the butterflies spend their winter in the cool air of the mountain, at a height of 3000 metres. The millions of butterflies clung to the trees, firs, cedars, and pines, so thickly that the trees bent under their weight. The ground under the trees was carpeted with millions of dead butterflies. When spring arrives the butterflies wake from their hibernation and mate. They then move like a huge cloud northwards, many of the males dying on the way. Females have to find milkweed plants, on which to lay the eggs, and then they die. The caterpillars hatch, then turn into pupae and then into a new generation of Monarch butterflies. The puzzle of how they navigate very long distances with such precision was finally solved quite recently. The butterflies are using a time-compensated sun compass. The Monarch butterflies, similar to desert ants and honey bees, respond to the position of the sun. The mechanism is situated in butterflies antennae. It helps them to establish the changing height of the sun in the sky. Scientists think that this requires involvement from a second clock in the brain, but this is not yet fully understood. It is thought that in the final few kilometres of their journey the butterflies use as a guide the imposing landmark of the mountain, and even the smell of the dead butterflies littering the ground under the trees. The migration of Monarch butterflies is, without a doubt, one of the most spectacular wonders of nature. When once asked why are we here, Goethe replied; ‘Zum Erstaunen bin ich da’, I am here to wonder.