Animal migrations and the navigation skills they require have long been a source of fascination, and they continue to be an active research field in biology. Despite progress over the past century, major questions about how animals sense direction remain unsolved mysteries. Facts such as how a godwit can fly non-stop for more than a week across the Pacific between Alaska and New Zealand without missing the target warrant explanation.
Biologist James Gould and popular science writer Carol Gould's latest book provides some answers to the puzzles. It is all about clocks, compasses and maps, but animals' concepts of these things are not necessarily the same as humans'. Their daily and seasonal rhythms are provided by the revolution, spinning and tilt of our planet, while the stars (including the Sun), the atmospheric polarisation of light, odours and the Earth's magnetic field give directional information. A general rule seems to apply: if there is information, animals have evolved a way of using it. The Goulds explain the basic principles of time-keeping and direction required by an animal to find its way and review the classic model systems used by researchers to study navigation. These include the honeybee, desert ants, the homing pigeon, sea turtles and, of course, migratory birds. The honeybee dance language used to tell other workers the direction and distance to a food source is given plenty of space, which reflects James Gould's research.
Animal compasses include the solar azimuth, the polarised pattern of skylight, the rotation centre of stars, odours and the Earth's magnetic field. The sensory abilities of animals are fascinating, but I find the ingenious experiments and tricks researchers have invented to study them equally intriguing. If you wish to show that adult birds have some kind of map, catch a sparrow in western North America, fly it to the East Coast, attach a radio transmitter and track it after release using a small aeroplane. Adults will fly towards their Mexican winter range (towards the south-west), while juveniles continue on their normal course towards the south, unaware of the displacement.
Alternatively, take the same sparrows on an ice-breaker ride across the magnetic and geographic North Poles, and fly them with helicopters to field sites for an orientation-cage experiment to study the sensitivity of their magnetic compasses. Behavioural experiments suggest that a bird's magnetic compass measures the inclination (or dip) angle of the magnetic field lines relative to the horizontal. At locations where the dip angle deviates only 2 degs from vertical, birds appear capable of using the directional information. How? The search for the magnetic sense in vertebrates is the search for the holy grail in the field. Scientists have found concentrations of magnetite in the beaks of pigeons and other animals, but recent studies have challenged the interpretation that these are magnetic sensors. Instead, the magnetic sense seems to be located in the eyes of animals, where a light-sensitive pigment (cryptochrome) works as the physiological gateway for magnetic information into the brain. But exactly how this sense works remains a mystery.
Research on animal navigation sits at the interface of physics, biology and many different cultures, and has seen many heated debates, past and present. Nature's Compass is an excellent introduction to the field and hopefully will serve as inspiration for new research. Although it contains a few minor mistakes, I found it enjoyable and would recommend it to anyone interested in the subject.
Nature's Compass: The Mystery of Animal Navigation
By James L. Gould and Carol Grant Gould. Princeton University Press. 320pp, £19.95. ISBN 9780691140452. Published 6 June 2012