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!

Images courtesy of The Journal of Experimental Biology.

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.

Lizards!

Lizards.

I grew up in the North--first New Jersey and then I went to college in Oberlin, Ohio.  So when I recently moved down to Florida to start a new job, one of the most exciting things I found about my new location was the new fauna.  Particularly all the little lizards running around!  The most common lizards to Florida belong to the Polychrotidae family, and are generally known as Anoles.  However, "lizards" are a broad and diverse group, with many different species with very different morphologies and life styles.  Here are a few fun facts about lizards:

• Lizards hear through the conduction of sound through small bones in their lower jaw vibrating a tympanic membrane (sometimes visible on the outer skin surface)
• Some lizards have prehensile (grasping) tails that aid them in climbing.
• Other lizards have more elongated tails that they can use for defense as whips.
• Some only have two limbs, while others are limbless (not to be confused with snakes).
• Some have webbed limbs and even specialized skin flaps that act as parachutes or gliders (I highly recommend that you go and watch that link, it's a really cool video from Animal Planet that I couldn't embed here.)
• Some have specialized fringes on their toes that allow them to run on water!  (See video)

However, being a nerd I find one of the most intriguing things about all lizards is that--unlike us--they are ectothermic, or basically cannot internally regulate their own body temperature.  This is what is generally referred to by the term "cold-blooded," although that term is considered a bit archaic by the scientific community due to the fact that cold-blooded animals do not have cold blood.  In fact, most of the time lizards probably have blood that is warmer than the blood of "warm-blooded," or endothermic animals (i.e. mammals and birds).

In order to understand the implications of being ectothermic, we should first consider the importance of body temperature in general, regardless of how it is regulated within an organism.  At it's most reduced, life is the result of many physical and chemical interactions occurring within the cells and tissues of the body.  The chemical interactions in particular (such as the transcription of DNA into RNA, and the translation of RNA into proteins; the breakdown of proteins and fats by enzymes; and the break down of ATP into ADP for energy... the list goes on), will function most efficiently within a certain temperature range.  If that temperature is too low, for example, then all those processes happen much slower.

What is it like to not be able to physiologically regulate your own body temperature?  Well for one, it vastly limits the type of environment you can live in.  While there are a few lizards that live in higher latitudes and altitudes, most lizards live in very warm climates.  This is because in order to keep their body temperature within an ideal range (somewhere around 40° C), they have to bask in sunlight and on warm substrates to keep warm, and then get to shade or be able to bury themselves in colder dirt or sand when the surrounding environment gets too hot.  In this way, you could consider lizards as behaviorally regulating their own body heat, but they still lack the physiological mechanisms by which endothermic animals regulate their own body temperature.

As you may guess, the environment ends up playing a very large role in determining the lizard's activities throughout the course of the day.  But as with any trait, it wouldn't have lasted so many generations if there wasn't a significant benefit.  Endothermic animals, in order to maintain their high internal body temperature, must have a very high metabolism to produce enough heat.  This high metabolism needs to be powered by energy gained from food, so a significant amount of the animal's time and energy must be allocated into foraging just to maintain their metabolic rate.  Lizards and other ectotherms don't have that problem; none of their energy intake needs to be put towards generating heat, so they can spend less time foraging and thus can allocate their energy towards rapid growth, social behavior, and reproduction.

The social behavior is what personally intrigued me by the Anoles of Florida.  If you watch these little guys for more than a few minutes, you will probably see them bob their heads up and down and extend a red projection from their neck, known as a dewlap.  This general behavior is seen in male Anoles, and is thought to serve to both establish territoriality and dominance over other males, as well as to attract females.  Moreover, the specific factors such as number of body push-ups, head bobs, and the degree of extension of the dewlap can convey specific signals within the same species of Anole.  It is also thought that the color of the dewlap varies between species, and that Anoles are able to visually detect the differences in color.  Some lizards are even capable of detecting ultraviolet wavelengths, thus adding another dimension to the visual spectrum that we cannot see ourselves.  Accordingly, the dewlaps of these lizards are able to reflect UV light.

In addition to being able to communicate with each other visually on a wider spectrum of visible wavelengths than we can, lizards (and many other animals) are also able to communicate using chemical signals called pheromones.  While the jury is still out on whether or not humans use pheromones to communicate, it is pretty well known that lizards can emit and detect such chemical signals.  In lizards, there seems to be some differences in the methods by which the two sexes employ the chemical signals.  Males seem to emit pheromones to communicate with other males, whereas females use their pheromones in a more male-directed fashion.  Lizards are able to detect these chemical signals using their olfactory system (sense of smell,) their gustatory system (sense of taste) and another sensory system a little foreign to us, the vomeronasal system, which specifically detects pheromone chemicals.  In lizards, the vomeronasal ducts open to the mouth, and they use their tongues to pick up the chemicals and then flick them over the olfactory and vomeronasal ducts.  This is the underlying reason for the tongue flicking behavior that you probably associate mostly with snakes.

There is quite a bit more interesting information about lizards, particularly in the shapes of their skulls and how sound is conducted through their lower jaws.  But in the interest of not writing a whole book on the matter, I'll cut this post off here.  But now you know a little more about the crazy lives of lizards!

Reference and disclaimer

Most of the information I find for the posts I write on this blog comes from a combination of knowledge that I gained through classes and through internet sources.  I generally link to these internet sources in the text of my posts, but sometimes I also get information from books (those archaic sources.)  Most of the information from today's post came from Lizards: Windows to the evolution of diversity by Eric R. Pianka and Laurie J. Vitt (University of California press, 2003).  The rest came from my own notes or internet sources where linked.

Thanks for reading!