Sunday, December 4, 2011

Home for the holidays! How birds use their sense of smell to find their way home

Happy holidays, everyone! It's a time of eating lots of delicious food, spending time with friends and family, and celebrating long-held traditions. For many, it's also a time of finding their way back home, whether it's in the town where they grew up, or in the company of loved ones (or both). This also means that for many, it's a time of airports and cars and lots of frustrating travel. For us humans, navigating home involves making reservations, getting on a plane in one city and landing in another. Or it means climbing into the car, punching in an address in the GPS, and hitting the gas. But what does getting home mean for other animals? They don't have a GPS with a vaguely snarky voice to tell them which way to turn, nor do they have massive(ly disorganized) transportation hubs in major cities that quickly shuttle them back and forth to destinations. So what happens when you take an animal, put it somewhere where it's never been, and let it try and find its way home?

That's actually a pretty big question when it comes to animal navigation. Different animals have very different ways to navigate--for example, some use the position of the sun to orient themselves. Others can see polarized light, and use that to navigate home. Yet others use strategies more familiar to us, they look for familiar visual cues and landmarks, and use that to orient themselves. You've also probably heard of animals using magnetic fields (known as magnetoception) to discern their location on the globe. Most famous for this form of navigation are the birds, who have small particles of magnetite in their upper beak that allow them to sense the magnetic fields over the earth, and orient themselves within those fields. This form of navigation comes in handy quite a bit, especially on overcast days when birds cannot use a sun compass effectively, or when they're in an unfamiliar location and cannot identify any familiar landmarks. Magnetoception certainly plays a very important role in bird migration but, surprisingly, may not be the go-to strategy for a bird finding its way back to a familiar "home base" from a completely novel location. In this situation, studies have shown that it may be a bird's sense of smell that it primarily relies on to find its way back home.

Homing pigeons are most commonly used as model species to determine the homing ability of birds. However, determining exactly how they navigate is a bit tricky. They have an innate ability to use magnetoception (meaning they're born with the ability), but they have to learn to use the magnetic compass in relation with familiar visual landmarks to get a sense of a "magnetic map" in order to find their way home. When they're about three months old, they then use to learn a solar compass to aid in navigation, by using the position of the sun in relation to the magnetic and visual cues. Most studies use young birds that haven't learned the sun compass yet, or birds that haven't been allowed to learn a visual map (by keeping them in captivity), to determine innate homing capabilities. When you bring these birds to a novel location and let them go, they are usually able to fly home, even if you prevent them from using the magnetic compass by severing the nerve that relays the magnetic information to the brain. However, if you take these same birds and make them anosmic (unable to smell), they tend to get lost and have a hard time finding their way home, if they do at all. But what happens when you take adult birds that have been allowed to learn a visual map and put them in the same conditions?

A study by Gagliardo et al in 2009 did just that. They allowed birds to become familiar with the area around their home loft, and then split them up into three groups. One group had the opthalamic branch of the trigeminal nerve--which relays magnetic information to the brain--severed, rendering their magnetic compass ineffective. Another group had their olfactory nerves severed, rendering them anosmic. And another group was left intact as a control. They then took these birds to novel locations either 50-60 km, or 75-100km away from their home roost, let them go, and observed their homing abilities. Here are their results:

This is a figure from the study representing the homing success of the birds at locations 50-60km away from the home loft. The three circles at the top of the figure represent the three groups, with C being the control, V1 being the magnetically-impaired group, and ON being the olfactory (smell)-impaired group. The "H" at the top of the circles represents the direction of the home loft from the release site, and the arrow from the center represents the average vector of the direction the birds were flying in when they vanished from sight. Each triangle represents the direction of the individual birds of each group as they vanished from sight. In both the control and magnetically-impaired groups, the birds were generally flying in the direction of the home loft, but in the olfactory-impaired group, they were generally flying away from home.

The bottom-left section shows the homing speed (in km/hr) of the pigeons that found their way home the same day of the release. To the right shows the number of pigeons (represented again by each triangle representing an individual pigeon) that found their way home the next day, more than one day later, and ones that never returned home, or were "lost." This shows a pretty clear demonstration that when pigeons were deprived of their sense of smell, they were more likely to become completely disoriented and lost after being released from a novel location. The data from the group released from 75-100 km from home shows a similar pattern, though all the pigeons in each group showed greater difficulty in homing regardless of their sensory capabilities.

Whether or not the magnetic compass or olfaction is the primary navigational tool for homing in birds is still pretty heavily debated. It's been suggested that the various methods to deprive birds of smell may also get in the way of magentoception, because the nerves and sensory organs involved in both senses are in very close proximity to each other in the birds' anatomy. However, this and other studies provide quite a bit of evidence that olfaction is very important in a bird's ability to find its way home. This study in particular lends a lot of weight to the idea that olfaction is necessary for navigation considering it used adult pigeons that were experienced in navigation, unlike many studies of the past.

So, if you have the misfortune of getting stranded in an airport or stuck in a traffic jam while making your way home this holiday season, you can at least take some comfort that you don't have to rely on your sense of smell to get home!

Reference: Gagliardo, A.; Paolo, I.; Savani, M.; Wild, M. (2009). Navigational abilities of adult and experienced homing pigeons deprived of olfactory or trigeminally mediated magnetic information. The Journal of Experimental Biology 212, 3119-3124.


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