Understanding Reptile Vision: Parietal Eyes – January 2014

By Jennifer Greene

In this third installment of understanding reptile vision, we’ll be discussing the parietal eye that is present in many species of lizards, and its impact on how your lizard perceives the world.  The parietal eye is often referred to as a “third eye”, and in some species of lizards you can even see the scale or spot on the top of the head where the “eye” is present.  The parietal eye in lizards is tied to their pineal gland; a gland that produces, among other hormones, melatonin – the hormone that helps you sleep at night.  The pineal gland is a fascinating organ, and one that we don’t know nearly as much about as we’d like to.  Even in humans, the functions of the pineal gland are still somewhat of a mystery, so it’s understandable that in reptiles, we struggle to understand fully what impact it has on their day to day lives.

If you have never seen a parietal eye, or are unsure of what exactly one is if you wanted to look for it, in our pet lizards it is usually a small, circular scale in the center of the top of the head.  It can be grey in color, or just a slightly different shade than the rest of the lizard.  The third eye is most pronounced in the prehistoric tuatara lizard – their third eyes have similar parts as their two main eyes, including a lens similar to a cornea.  The third eye is quite primitive, “ much more like the retina of an octopus rather than that of a vertebrate” (Schwab and O’Connor, 2005).  This eye cannot see in quite the same way as the main eyes, instead likely only detecting shapes and shadows rather than full pictures.  They are also highly sensitive to light – producing markedly different hormones based on time of day, with one study showing a system of neurons reversing their reactions based entirely on the daily photoperiod of the lizard. (Engbretson and Lent, 1976)

Older studies done on lizards to examine the purpose of their parietal eye experimented with removing the eye as well as simply covering it up.  In humans, the pineal gland is what helps control our circadian rhythm, and in lizards the combination of the third eye and pineal gland serve a similar function.  Experiments that removed the third eye from common North American fence lizards found that lizards missing their parietal eyes were more active for a longer period of time than their counterparts with intact eyes.  On the surface, this doesn’t seem like such a bad thing – why wouldn’t a reptile want to be out and active for the most amount of time?  Turns out, that’s only a good thing if you’re a mammal.  We are active as much as possible to get as much food and energy as possible.  For us, just sitting costs energy, while for reptiles, the less they move, the less energy they expend.

So, when a lizard is active for a longer period of time, but is not necessarily consuming more food, being that active becomes a hindrance, not a help.  Parietal eyes helped limit the amount of time that fence lizards were spending out basking or moving around.  In addition to the differences in activity between lizards that had parietal eyes and those that didn’t, lizards with their third eyes removed were harder to startle and scare away, with speculation in one study that “the pronounced heliothermism perhaps works antagonistically to the normal retreat reaction.”  (Stebbins, pg 35)

Not only do the parietal eyes help set a lizard’s internal clock, control hormone production, and help them determine needed activity levels – they also use their parietal eyes to navigate.  A study done with Italian wall lizards found that they used the sun to navigate through a “Morris water-maze” (click link for the wikipedia article), and by tricking their biological clock to be 6 hours faster or slower, the lizards were no longer able to reach their goal at the end of the maze.  Painting over or removing the parietal eye entirely caused the lizards to no longer be able to navigate the maze at all.  (Carnacina, 2009)

All of this just scratches the surface of what the parietal eye and, through extension, the pineal gland, are responsible for and control in a lizard’s life.  This sensitivity to light is one reason for the common recommendation to provide basking lizards with bright, white lights to bask under – you are helping your lizard to keep its biological clock ticking at the right speed.  Any diurnal lizard is particularly sensitive to light, and understanding how heavily they rely on external sources to help guide their lives will help you as a keeper provide them with a rich captive life.   This is, of course, most relevant to true lizards, such as iguanas, skinks, lacertas, bearded dragons, and other similar reptiles.  There are few, if any, lizards that are nocturnal, and in fact searching online for “nocturnal lizard species” will instead bring up the gecko family.  Geckos do not appear to have the same parietal eye as diurnal lizard species, but as we saw in the last article, that does not at all mean that their sight is less attuned to light!

For your diurnal lizards at home, please be sure to provide them with a regular day/night cycle, including a nice bright, white basking light for them to heat up under.  As studies have shown, diurnal lizards rely heavily on lighting as well as heat to determine activity levels as well as hormone levels.  Good lighting not only encourages natural behaviors, but enables your pets to thrive all the way down to a cellular level.  Lighting is vital to proper husbandry, and an integral part of any set up for diurnal lizards.  Asking for help from any experienced lizard keeper in a sick, non-feeding, constantly sleeping lizard will immediately earn you questions about your lighting set up – and is it any wonder?

References:

Augusto Foa, Francesca Basaglia, Giulia Beltrami, Margherita Carnacina, Elisa Moretto, and Cristiano Bertolucci (June, 2009) “Orientation of lizards in a Morris water-maze: roles of the sun compass and the parietal eye”, Journal of Experimental Biology, 212 Retrived from:http://jeb.biologists.org/content/212/18/2918.short

Gustav A. Engbretson, Charles M. Lent (February 1976) “Parietal eye of the lizard: Neuronal photoresponses and feedback from the pineal gland”, Proc. Nat. Acad. Sci USA Vol 73, No 2, pp 654-657 Retrieved From: http://www.pnas.org/content/73/2/654.full.pdf

I.R. Schwab and G.R.O’Connor (March 2005) “The lonely eye”, British Journal of Opthalmology, V. 89(3), 256 Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1772576/

Robert C. Stebbins and Richard M. Eakin (February 1958), “The Role of the “Third Eye” in Reptilian Behavior, American Museum Novitates, Number 1870 Retrieved fromhttp://digitallibrary.amnh.org/dspace/bitstream/handle/2246/4659//v2/dspace/ingest/pdfSource/nov/N1870.pdf?sequence=1

The Language of Love: Breeding Behavior in Reptiles and Amphibians – January 2013

By Erin Lane

Introduction

For most of the country, it’s the middle of winter.  Cold storms are moving through, the thermostat is turned up on the furnace, and going out in anything less than a parka is out of the question.  Your herps, however, are likely blissfully unaware—either because they are basking in their own temperature controlled enclosure, or because they are sleeping it out until the Spring.  This doesn’t mean though that they aren’t getting prepared for breeding season.  As your temps start to warm up here in the next month, your pets will be undergoing physiological changes that will prepare them for breeding season.

Whether you have snakes or frogs, monitors or turtles, most of their wild counterparts have a well defined period of time when the conditions are just right for procreating.  In the tropics, this may be the start of the rainy season.  For creatures living in the temperate zones (which is most of the U.S.), this means the beginning of warmer days and milder weather.

This is also the time that most of our pet reptiles and amphibians begin to gear up as well.  The means of communicating the readiness to breed is different from species to species, but there are some common types of communication when advertising love.  As we get ready for springtime here at the Reptile Times, we will be discussing the many ways in which our ectothermic (aka ‘cold blooded’) animals attract and choose mates.

Green Tree Frogs like this one are found throughout the East Coast of the US

Part I: Auditory Announcements

Although we initially rely pretty heavily on appearance when attracting or finding a mate, many species would find all of this primping and grooming a waste of time.  For most anurans (frogs and toads), the voice is often the most important tool to attract a mate.  But why?  What is the benefit of vocal communication, and how can that help to find a suitable partner?

Well, think about the areas in the world that have the highest densities of anurans.  These are wet places with dense vegetation, such as rain forests.  Even if you have good eyesight, it won’t help you to find other small, cryptically colored frogs hiding out under leaves and in small pools of water.  With so much thick foliage, it won’t do much good to see well.

Chorus Frogs, like this one, get their name from the distinctive calls they are known for making

Picky listeners

Sound, on the other hand, carries far, even when visual barriers are present.  Having a strong voice means that you can be heard over a greater distance, thereby increasing the number of listeners (and potential mates).  As for most animals, it is the males that do the majority of calling.  Females tend to be choosy since they typically have a very limited number of reproductive opportunities in a given season.  Making eggs is taxing, and, depending on the species, they may only have one opportunity to have their eggs fertilized.  In that case, finding a mate becomes a choice that could be the difference between their genes getting successfully passed on to the next generation, or not at all.  If the chosen male doesn’t pass on good strong genes to your offspring, they may not live long enough to have offspring of their own.

Males, on the other hand, typically have a very different take.  Where females are all about quality, males are all about quantity.  Unlike females that produce large, energetically costly eggs, males make millions of small, energy efficient sperm.  In most cases, the number of females they can breed with is dependant primarily on how many they can convince.  So when it comes to attracting a mate, males are usually the ones with the tougher job.

Size matters

You have likely heard the saying “appearances can be deceiving,” but it’s hard in the animal world to disguise a voice.  Vocalizations, or calls, are usually closely tied with the size of the animal making them.  For example, a small frog can’t make a deep call, just like a snare drum will never sound as big and deep as a base drum (think of an American bull frog versus a chorus frog).  It’s all about the size of the instrument, and it’s hard, if not impossible, to alter a call enough to trick a discerning ear.

But why is being bigger important?  It isn’t always, but being bigger usually means that the male is older, has more access to food, or both.  If a male lived long enough to become big and can make a nice booming call, it likely has good genes that have allowed it to find food and escape predators.

Standing out in the crowd

Often times, males stay put and call while the females seek out the one they wish to breed with.  In some species, the males form a lek, which is essentially a gathering place where they can show off for females.  The females come to the lekking spot, and can suss out which male would be best to breed with.  In some cases, the mere act of being able to participate in the lek is the only qualification a male needs to attract a mate.  Males that live long enough to make it there every season are usually those that are higher quality, and therefore suitable for a choosy female.  In other species, females will find vocalizing males from a distance, led on by the sound of their call.  Only males that have strong, attractive calls will be sought after, meaning that males with less attractive ones will be left singing to themselves.

An important point to make here is that every species will have its own particular call structure and/or frequency.  Ideally, females will only be attracted to the call of their own species.  If they weren’t, it would be a waste of their time, and they wouldn’t be likely to reproduce.  They are tuned in, so to speak, to the vocalizations of their own kind, and discern between calls that are within certain acoustic parameters.  Some of them within those parameters just happen to be more or less attractive.

This is an important element in vocal communication because of possible interference from other species.  In your mind, go to that lush rain forest with all of those frogs.  Think of how many species there are, and how that might sound.  Pretty confusing, for an animal that is relying on sound to find a mate.  Animals need to be able to ‘filter out’ the sounds of other species, and focus in on those of their own.  Not an easy task for us, perhaps, but one that comes naturally to even the most inconspicuous little frog.

This little Chorus Frog was caught in a surprise spring snowfall – he hopped away to sing another day!

Curtain call

As the country begins to thaw out in the next few months, we will start to see our reptiles and amphibians perk up, and get into breeding mode.  This is the time of year that is often most rewarding to serious herp breeders and casual hobbyists alike.  Your pets begin to wake up, eat more, and, often times, start looking for a mate.  The way they do so depends on the species, and it can make for truly interesting behavior.

Although we rely heavily on sight when finding a potential mate, many animals use other senses to discern a good partner.  For frogs and toads, it’s all about the voice.  A call that sounds out over the rest is sometimes the most enticing attraction for a female, making it a male’s most important asset.  However, sound is not the only way that animals communicate and find mates.  Next month we will be discussing how some species use chemical cues to find and attract each other.  In the mean time, keep an ear out for local frogs and toads as they get ready to sing their hearts out in search of springtime partners.