By Paul Donovan
There are some questions you get asked which, on face value, you give an answer to but upon reflection later, make you wonder if you gave the correct answer. Take this one. I am often asked “Will the snake you have just removed from my yard find its way back into my yard?” My instinct is to say “no, it shouldn’t”. But is that the correct answer? Bearing in mind many reptiles and amphibians have homing instincts, breeding grounds, denning sites, or just preferred places of habitation.
A Sense of Direction & a Case of Magnetism
For an animal to return to the place from which it was removed, it must have the ability to navigate back there. In order to do this, it must be able to determine direction. Physically removing a snake from someone’s yard and then relocating it several miles away, it would not have that sense of direction because it was being physically moved. It would, therefore, not be aware of the surroundings from point A to point B. However, if it were moving by itself, then it would have a sense of direction, because as it moved it would be building up a visual/chemical/olfactory picture of its surroundings. It may also be orienting itself using magnetic cues. So, theoretically, it is perfectly feasible that it could return.
Migratory birds navigate an annual course over thousands of miles each year. They have something in common with homing herps. That one feature is that they have a built-in “compass” of some description which is affected by the earth’s magnetic field. Crocodilians have been found to have this, and can find their way back home when relocated several miles away. We should also not forget how they can navigate back to precise areas where annual migration of wildebeest takes place. The crocodiles do not remain in this area year-round waiting for the migration, but roam wide and far.
Even the humble tortoise has been known to find its way back home, albeit slowly, after being moved. However, other than migratory birds, the group which has the most sophisticated of navigation systems are the sea turtles. For a long time we have marvelled at the ability of a female turtle to travel thousands of miles across the oceans, back to the same beach it was born, to lay its own eggs there. It has to be one of nature’s most amazing spectacles.
When that little sea turtle hatches, it has the innate knowledge (some people call this instinct) to make its way straight to the ocean. As it digs its way to the surface, it immediately orientates itself using celestial navigation. Using light from the moon and stars as it reflects off the ocean’s surface, this is the cue the little turtle uses to move towards the water. As it takes its first steps into the ocean, it aligns itself perpendicular to the waves which carries it out into the vastness of the expanse beyond. While in the ocean, research has found that turtles do not just swim around haphazardly, but navigate using magnetic cues.
By taking DNA samples, and placing tracking devices on these turtles, researchers have found that sexually mature female sea turtles can navigate back to their nesting sites, while covering distances in excess of 1200 miles. Although some visual cues may play a part in navigation in some parts of the ocean, in the vast waters of the open ocean, the most important clues are the earth’s magnetic fields and celestial navigation.
Not Random
We are all familiar with the denning habit of rattlesnakes and garter snakes. Leading up to winter, hundreds of individuals make their way back to the same denning site, year after year. As spring approaches, these individuals vacate the denning site to embark on another year of feeding and breeding. But how do they find their way back?
You could be forgiven in thinking that as they exit these denning sites, they do so in a random manner. But this is not so. They actually disperse using predictable pathways which they orientate using visual/chemical/olfactory and magnetic cues. But this begs another question “how do they know where these denning sites are in the first place?” “How does a newborn rattlesnake or garter snake know the location of a denning site, if it were not born in it to gain some sort of cue where to return to?” They do not all meet up by a boulder on a particular day, and make their way there in convoy. Though they do arrive within a short time frame of one another. From a simple question the answer could be very complex.
Landmarks
Just as we use landmarks to find our way around, so do as it appears, most reptiles and amphibians. These landmarks are used to find their way around their home range/territory, locate preferred basking rocks, and secure shelter at night. This can be for short term or long term gain. What this indicates, is that an individual not only has the capacity to learn about its surroundings, but navigate around it using specific features it has come to identify. Depending on the species, it also seems likely that an individual will use elevated positions to navigate towards preferred habitat choices. This is particularly the case with lizards.
Navigating using landmarks may explain how denning reptiles find their denning sites year after year, but have difficulty in locating them if certain features are removed or obstacles built in their way. As many amphibians and reptiles have restrictive home ranges, it becomes easy for them to find their way around it.
Third eye
You may have read about some reptiles, particularly lizards (and some amphibians), having what is called a third “eye”. The correct name is the parietal organ, and is situated on top of the head between the eyes. It is far more evident in some lizard species, than others. For example in monitor lizards and the Green Iguana it is clearly visible as a small raised “lump” in the centre of the head. It is often opaque in colour.
It is not an “eye” as such (they are not freaks of nature), but a functioning light receptor which contains a cornea, lens and retina, which is connected to the pineal complex in the brain. It is believed that this structure “sees” polarised light, and therefore functions as a polarised light receptor. Although a primitive structure, the parietal organ plays an important role in navigation, as it allows the individual to set its internal compass based on the angle of polarised light.
As light radiating from the sun hits the earth’s atmosphere, some of it is deflected into a plane perpendicular to its original plane of entry. This deflection of light is called polarisation. The scattered proportion of this light, polarised light, travels in a straight line following a path called the e-vector. Because the e-vector remains perpendicular to the sun’s entry plane, and not to the earth surface, its orientation changes as the earth rotates (if there are any physics teachers out there and I’ve got that wrong, please let me know; I failed A-level physics convincingly). This changing orientation allows the individual to determine direction.
In many experiments, if the parietal organ is covered over, the individual is unable to orientate properly, leading us to believe that its function is to detect polarised light and set a directional compass allowing for orientation. From the limited amount of research carried out so far, it would appear that in some species of lizards, greater reliance is placed on using the parietal organ for navigation, than their visual, or even olfactory system. Until more research is undertaken, we do not know how widespread this phenomenon is.
Reflection
Leading on from polarisation, we now have a phenomenon whereby those reptiles and amphibians which can detect polarized light, are able to navigate using the inverse relationship between reflection and polarisation. In other words, how light reacts on its contact with the earth’s surface. On dry surfaces, reflectance is high and polarization is low, while on water and wet surfaces, reflectance is low and polarisation is high.
The effects of the polarised light on wet and dry surfaces, allow semi-aquatic reptiles and amphibians to differentiate between the texture of the surface, and navigate towards the habitat of preferred choice. For example, a turtle moved some distance from its pond will be able to navigate back using polarisation, even though it may not be able to physically see the pond. Of course, all is subject to the level of polarised light. On a bright clear day, polarised light levels will be high, whereas on a cloudy day they will be low, because the clouds prevent the polarised light from filtering through.
Navigation using reflectance/polarisation, would be a bit like us seeing a mirage in the middle of a desert and walking towards it. We see the shimmering light of reflectance on the sand’s surface, even though there is no water there. The turtle, frog or salamander is seeing the same effect on the water’s surface, but of polarised light.
Chemical Cues
Considering that many pond dwelling reptiles and amphibians live in water sources with a less than favourable odour on the nostrils, it is not inconceivable that odour may also be used as a form of navigation. Quite a significant amount of research has been undertaken with navigation in toads and salamanders, which has proven to be quite interesting.
For much of the year, many toads and salamanders are terrestrial, living in the damp undergrowth. However, during the breeding season, they will migrate back to regular breeding ponds. On a clear night, the toads and salamanders can navigate back to these breeding ponds with some degree of accuracy over distances of several hundred feet. If the night is cloudy, they can still navigate back, but with less accuracy.
When removed from a pond, displaced toads and salamanders can also navigate back over distances of several hundred feet. As I have said, on a clear night navigation is precise, but on a cloudy evening, not quite so. If the toads were rendered blind, they could still navigate on a clear night, but only to the same level as if it were cloudy. In other words, not very accurately. It is possible there exists some sort of auxiliary light receptors somewhere on the body which allows for celestial navigation, even on a cloudy night.
Orientation takes on an entirely different approach, though, if the same toad’s sense of smell is removed. On a clear night they can still orientate themselves, but in a rather, less precise manner. This points to the toads placing a greater reliance on chemical cues to navigate back to breeding ponds, than celestial navigation. However, it seems plausible that the toad locks on to chemical cues which sets an internal compass. Once the compass is locked onto a direction, the toad then uses celestial navigation to find its way back to the pond.
The use of chemical cues for navigation is an interesting field of research, as it could open the door to understanding how other reptiles find their way around. There is a theory that rattlesnakes, and garter snakes may use chemical cues to find their way back to their denning sites each year. However, denning activity is a sophisticated behaviour and involves travelling over great distances, similar to those experienced by the sea turtles, so it seems probable that all elements of navigation are involved.
Tail-end
While the ability to navigate has been widely studied in birds for many years, in the reptile and amphibian world it is less well understood. From what we understand so far, is that light and magnetism play a big part in the way some members of these two groups get about. The use of landmarks can explain how hibernating snakes find their way back to denning sites each year. And the most astonishing of all, how a female sea turtle will remember where it was born, and 50-odd year’s later return to exactly the same beach to lay her eggs. It also shows that reptiles and amphibians have the capacity to learn, and build up a sensory picture of their surroundings which they can use on a day to day basis, or over the long term. And who said we humans were the more advanced species, considering many people find it difficult to navigate out of their front garden without a GPS!
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Awesome, informative piece!