You need two things to find your way to a place: a map and a compass (Or one of those GPS navigation systems, such as a RoadPilot that combines the two). The first shows your current location in relation to your destination, and the second tells you the direction you are moving in. Biologists believe that animals have these instruments hard-wired in their brains and sensory organs.
Navigation in homing pigeons was studied for over half a century. Since the sense of sight is useful only if you are in a familiar place, experimenters wondered if pigeons used the sun to navigate. Shifting the birds' body clock by artificially lighting their lofts at night and darkening them during the day neutralises this system. When released on sunny days, the unfortunate birds were disoriented, and couldn't find their way home. But on overcast days, they had no problems at all because in the absence of the sun, they may have been relying on another navigation system.
Perhaps, they were using the earth's magnetic field. Like many birds and animals, pigeons have magnetite (or iron oxide) particles on the upper part of their beaks (we have a single crystal between our eyes and behind the nose, and minute crystals in the brain), which help them sense the earth's magnetic field, like a compass. To deactivate this sense, experimenters attached magnets to the birds' heads, and anaesthetised their upper beaks. Such magnet-neutralised birds got lost on overcast days (because they couldn't navigate using the magnetic field or the sun), while on sunny days, they found their way to the loft. If deprived of one sense, the pigeons appear to be compensating with another.
An American geologist, Jon Hagstrum, came up with yet another theory to explain how birds are able to tell where they are. The earth's crust is thought to make a constant low frequency infrasound. Like the magnetic field, every part of the planet has its own sonic signature, and Hagstrum says that some birds may be able to hear these infrasounds and recognise where they are. Some others suggest smell may play a part while others disagree. So, the jury is still out. Whether animals use sight, smell, sun, stars, the earth's magnetic field, infrasound or a combinations of these remains a magical mystery.
I should mention that homing pigeons are selectively bred for their remarkable ability to return home from hundreds of km away. In other words, it is a genetic trait; some have it and others don't. Likewise, not all animals that are displaced find their way home, some get lost and wander aimlessly.
Over the years, Rom and I have argued over who has the better sense of direction just like so many others. But one exceptional field researcher, J. Vijaya, had a remarkable sense of direction. Apparently, she would wander through an unfamiliar forest without notching a tree or snapping a branch, and yet unerringly find her way back. We don't know how she did it!
Back home, a couple of years ago, this homing instinct backfired on me when our war on the Croc Bank ants began in earnest. I remembered ma-in-law's dictum as well as our previous experience at the Croc Bank.
Grudgingly, I caught two toads from the garden, and left them under the sink. They wouldn't have any of it; they ran back into the garden at the first opportunity. Feeling a little like the shame-faced princess who had to kiss the toad, I gave them a nice cosy little cardboard box. They didn't want that either.
So, I swore that the next toad which came into the house would get red carpet treatment, stinky scats notwithstanding. I'll even kiss it, as long as it takes care of the ants from hell!
(The author can be reached at firstname.lastname@example.org)