Reptile & Amphibian Forums

There is a fairly basic principle in animal behavior that could be called "can/can't, do/don't". If animals physically can't do something (such as pigs and flying), it's a safe bet that they don't. However, knowing that an animal can perform some feat does not tell you if it does, in fact – in that case you need to look. Monitors' eyes are a very straightforward case of the can'ts.

A bit of background about the types and roles of light-receiving (photoreceptor) cells first. These cells come in two basic types, rods and cones. Rods simply record light w/o distinguishing colors. They are often hooked up to nerves in a "summing" fashion, whereby amounts of light that are insufficient to stimulate a single rod are added up among all the rods in a local group, to generate a single nerve impulse. This permits vision in very dim light, but the sharpness (resolution) is low because several to many rods go into making a single 'pixel' in the image.

The other class of visual cells, cones, includes 2-4 subtypes. Each type of cone responds best to a particular band in the spectrum of visible light, and combinations of these cone types allow animals to perceive a range of colors. Unlike rods, cones are usually hooked up to neurons in a 1:1 ratio, making for sharp images (many small 'pixels'), but at a cost of being relatively insensitive in dim light.

Most animals have mixed retinas, containing both rods and cones, and the proportions generally align with whether the species is most active by day or at night. Humans have cone-dominated retinas, with the proportion of cones being greatest near the focal point, grading into increasing numbers of rods towards the periphery. Our night vision is poor and colorless for these reasons. You see best at night by not looking directly at an object – there are more (and more sensitive) rods in play when you use peripheral vision. Colors are of course there at night as well, but you cannot "see" them because there is too little light to stimulate the cones in your retina. A fully nocturnal animal has a rod-rich retina, and often a reflective surface behind it (called the tapetum lucidum) that bounces back at least some of the light that missed rods on the way in. This is why the eyes of some animals reflect a flashlight brightly, others less so. Nocturnal animals also tend to have large eyes (think owls), simply because a bigger eye can have a wider pupil and a larger retina that can be packed with rod photoreceptors.

OK, monitors, then. All monitor species that have been studied have 100% cone retinas. No one has ever reported monitor eyes with rod receptors. Further, almost all of the cones are in a 1:1 relationship to neurons. What this means is that monitors have color vision (2-3 distinct cone types) with maximum resolution (= tiny 'pixels'). Other optical properties of their eyes are very similar to those of birds, such as hawks, that have far better visual acuity than do humans. Monitors have no tapetum lucidum. Their eyes are not large in proportion to their body size, as for example among geckos.

Short answer, monitors are night-blind. Their vision on a night with a full moon is roughly the same as yours would be in a forest on a moonless night – no detail visible, no depth perception, and only the most marginal ability to detect an object moving (and that object would have to be large and of a color very different from the background).

So, it's a matter of can't do. Pigs don't fly, either.

Monitors "can't" be nocturnal? They can't see in the dark?
They have other sense for finding food. I have witnessed one of my lizards go into a burrow (it's dark in there) and come out with worms and crickets.
Can't wait to hear about tounges and nostrils.

Scott
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Do not meddle in the affairs of dragons, for you are crunchy and taste good with ketchup.

So, monitors can’t see in the dark.

This doesn’t mean that it doesn’t do anything at night.

We can’t see in the dark either… that’s why we have headlights on our cars.

Let’s pretend that a monitor uses its tongue as headlights when it goes out at night.

Case closed.

??

Michael

Umm, no. Think about it. Try to find your way around in Home Depot with your eyes shut, using only your tongue.

You like the headlights, eh?

If I could send signals directly to my brain from my tongue, I wouldn't need my eyes.

Mike

Eyes part I was analog, eyes part II is digital, takes a minute to write up. After that, tongues, OK? Be patient.

And by "headlights" I mean that since it can't see well at night a monitor's tongue assists the eyes, since a tongue probably can't sense movement.

Sorry, case isn't closed.

Tongue and headlights, not an ideal analogy.

The most fabulous sense of smell in the world will not stop you from bumping into things. Headlights prevent you from bumping into things, sonar prevents you from bumping into things (bats are way cool at this), lateral lines and associated sensors prevent you from bumping into things (watch a blind cave fish navigate its way around a tank) but I can't think of any animal that navigates by smell in the dark.

A monitor bumping around its enclosure at night doesn't count. It's safe, it knows the exact dimensions of its enclosure and where everything is. A monitor in the wild has to worry about nocturnal hunters (mammals, birds) which would kill it before it could smell them coming. It also has to worry about bumping into things. Can you imagine the cacophony of smells a rainforest floor would produce, how would any animal be able to discern tree stump, rock or cliff edge by smell alone?

I have seen how they get around, they search using alot of tongue flicks, and as they do in the day prodding and digging with their nose tip, the only difference is they dont see very far in comparison. Their night vision isnt very good, obvious but they are not helpless during the night, they navigate more by scent than during the day. Ive also watched them in burrows in the dark in their cages, yes they dig them so therefore know where to go, but Ive also seen my ackie chase crickets, and speedy lobster roaches through his burrow in the dark and catch them with his eyes shut, yet it didnt slow him down as much as you would think. So theres more to scent navigation and feeling than you think. Ive watched rodents being chased at night in the dark by my monitors and others, they needed to be closer to the rodent but still it never stopped them in from a catching one even maneuvering around, through, and over objects as well jumping in the air, they rarely ran into something hard enough to slow themselves down down, catching a mouse at full speed in the dark is no fun for a person either, yet they can do it. I dont think the simple minded, slow brained animal that was described above accurately fits a monitor. Ive watched timors, niles, waters, albigs, bosc, ackies, dumerils, argus, and others all chase mice in the dark and catch them.

Really quick question -- if it's really "dark", how did you see them do all these things?

I can see that the word "dark" gets defined differently around here too.

bah, I was about to say the same thing. Good thing I checked your response first.

Every human on this planet has a different range of Nightvision...Some even have night vision equivalent to a cat...Special forces testing includes Night vision determination. I, personally, have extremely good night vision including detail determination at distances...

I'm of the opinion that any family of species would be much the same. Individuals would have different degrees of vision in varying conditions. Why? Because they may need it to navigate unfamiliar territory in escaping other predators. How do you think monitors navigate if their burrows are ripped open by, say, Hyenas... lions or the like? They aren't like raptors. Raptors remain completely still in the dark. Thats why they're hooded by falconers. To keep them in the dark and calm and from moving much. Few Hawks, eagles and Falcons will fly at night unlike owls.

Monitors have a combination of senses that would help them navigate in the dark. It isn't all about one sense seperately but how ALL of them operate together. And you'll also find most animals have an awareness of how close they are to something even if they can't see it..Humans can have the same thing if they pay attention to the subconscious stimuli they're recieving all the time through all of their senses. Monitors have even more acute senses of smell and taste than most...and bidrectional sense of smell at that.. Any animal that has lived in an environment long enough learns to distinguish harmless scents like those emitted from trees or grass.. flowers or rock from those that might be a predator or even prey. Eye sight is going to be a poor second to the sense of smell a monitor has. Again.. Take Komodo's for instance (My favorite species by far save for V. macraei) they will travel between the islands merely on the chance that a scent trail in the air will lead them to food eventually. They aren't seeing the prey.. they're smelling it... Sight seems to be used more up close to pinpoint in daytime.. In the dark, their sense of smell can do the same thing. They emply whatever tools they must to gain that advantage. I'm with FR on this one though I'm by no means as experienced as he is....While their eyes in 'scientific study" may not be built for nocturnal sight, the darker hours aren't spent in complete stillness nor is the time spent in the burrow spent in just sleep. They have other senses to rely on if they must. In most animals sight is secondary to scent anyway. except human of course. Our sense of smell is almost dead compared to what a monitor is registering.
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Lucien

1.1 Columbian Redtail Boa (BCI)(Sutekh and Isis)
3.5.3 Leopard geckos (2.0 Blizzards (Caine and Goliath), 0.1 Tangerine Albino (Tequila Sunrise ...Tiki for short), 1.0 Rainwater Albino (Mycah), 0.4 Poss. Het. Albino (Annika, Lace, Rain and Aris) and 2.1 dbl. het blizzard x tang albino (Malice, Malfeas, and Mystic))
0.1 Savannah Monitor (Kiros)
13 rats
2 Dogs (Loki and Storm)
3 cats (Ashe, Sahara and Hercules)
6 Fish (4 Red Danios, 1 Cardinal Fish, and 1 Tiger Barb)
8 Ramshorn snails
"And a Partridge in a Pear Tree!"

1. This discussion is mainly about whether or not monitors are active enough at night to lose their 'diurnal' classification, not whether or not they move around while in their burrows, hollow logs etc. No one is saying monitors lie still in their burrows or in hollow logs, I think we can all agree that they do.

2. A diurnal animal that scoots off into the night when its burrow is torn open by a predator does not become nocturnal. It still a diurnal animal scooting off into the night because its burrow was torn open by a predator.

3. No matter how spectacularly acute we think our night vision is compared to our neighbour, no matter what special army forces we may belong to, we are still humans with human eyes and accompanying human eye limitations and will never be able to see as well at night as a normally sighted cat. There's a reason cats have such huge eyes for the size of their head, vertical pupils which can open incredibly widely, a preponderance of rod cells and an extremely reflective tapetum lucidum. We might think we can see like cats, but that's because we have no idea what a cat sees.

4. Monitors (including Komodos) make those long journeys over water by day. The fact that they can't see the upcoming island and are going by smell alone doesn't mean they are travelling blind, they just can't see their destination. When I leave for work each morning I can't see my destination, either, but I can see the journey. There's a big difference. As I said above, I have no doubt monitors use their sense of smell to find unseen things (a potential mate that has passed by earlier, a scentmarked spot etc etc), but that's very different from using their sense of smell to 'see' in the dark.

5. No one has explained to me yet how any animal can navigate in the dark by sense of smell alone. Sure, monitors can tell a stick from a rock from food by smell alone, but those smells don't say how wide, long or tall those objects are - important parameters if you want to avoid bumping into things in total darkness. They can slowly move their way around, smelling as they go, but remember that in the wild there may be predators chasing them or prey running away - times when slowly smelling one's way around things just doesn't cut it as a survival mechanism.

I know monitors are occasionally seen out at night, I'll even accept that some species are regularly crepuscular, but for most species if you get 500 day sightings for every single night sighting, does that make the species nocturnal or is still a diurnal species given to occasional wandering at night? Are monitors really equipped to do the normal things in total darkness, such as hunt, flee predators, etc, when they can't see where they are going? Let's forget all accounts of our captives bumping and stirring in the night (I know mine do on occasion, while I've yet to see a wild lace monitor out at night) and talk about wild monitors out there in the real world, where predators far better equipped for night activity are wandering around in the darkness as well. Taking one's monitor out in the garden for a stroll at night and watching it bump around doesn't necessarily tell us that wild ones make a regular habit of doing so.

"No one is saying monitors lie still in their burrows or in hollow logs, I think we can all agree that they do."

I meant to say that I think we can all agree that they do move around.

lol

I remember reading an article about some monitors that preyed on migrating birds. It seems these birds always came to a certain place every year, late at night. The monitors would climb the trees and wait for supper to land. So for me it's all about need. The monitors needed food, and knew when and where it would be. Did they see the birds? I don't know.
Somehow they knew where the birds were in the trees, did they smell them? Probaly. Did they hear them? Probaly. I am no expert or do I have a PHD in anything other than redneckology.
All I know is these animals will use what ever senses they need to survive.

Scott

PS. I don't remember where I read it, maybe Mark has seen it.
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Do not meddle in the affairs of dragons, for you are crunchy and taste good with ketchup.

Would it be safe to assume that these birds migrate through that area once or twice a year? If the monitors climbed those trees at night during those two periods, but were mainly active by day for the rest of the year, would you call them nocturnal?

I'd still label them as diurnal, for in general they are usually active during the day. Truly nocturnal animals are mainly active at night and have adaptations to cope with the darkness, such as light adapted eyes, or sonar etc.

If I remember correctly, the birds fly through twice a year, migrating and migrating back. They always stop in the same area and rest in the same set of trees.
What I am saying is they are neither nocturnal nor diurnal. They will move when necassary and when the need arises.
If they can find an abundance of food at certain times of the night, they will move at night. If food is available during the day, they will move during the day.
I see at as simple as that, It's about need.

Scott
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Do not meddle in the affairs of dragons, for you are crunchy and taste good with ketchup.

"What I am saying is they are neither nocturnal nor diurnal. They will move when necassary and when the need arises"

I understand what you are saying, but the fact remains that night time activity is a relatively rare occurrence, whereas daytime activity is a normal, everyday occurrence, which makes them diurnal. The exceptional foray at night doesn't prevent them from being generally diurnal animals. A twice yearly feeding on birds in trees at night is still only twice out of the year. The rest of the time they are most likely active by day.

I'll give you another example. There are a lot of lace monitors in the region in which I live. Over the time I have lived here, I have experienced perhaps one or two thousand sightings. I've yet to see a single one at night. This isn't to say they never come out at night, for a good friend has seen one on the road side one night (it appeared to have been dirty and he thought it may even have been a nesting female). However, this doesn't stop me from thinking of them as diurnal animals. They are diurnal animals which occasionally may come out at night for some reason or other. As a rule, they tend not to.

I just got back from Fiji with Peter - he told me you did the layout for the conference - nice work, thanks!!

Thanks, Jeff. What Peter doesn't know is that he'll be returning the favour by checking up on my animals when I go away. LOL.

np

I really do love your version of the world. You read the manual(books) and define how the car will run. While thats almost true, its not always the case. Thats why they test drive cars, even to this day, even with very advanced computers and design programs.

The "point is" They, varanids, "are" active at night and in dark places, like holes tunnels and hollows. I did find them crossing the road at night, and so have others. Its published in several books that both, V.tristis, and V.glebopalma, are dusk and night active. They do come out at night here in our outdoor cages and hunt, in the dark, all on their own accord. Many many of us, see them active in the dark in our cages. This sir, is reality. It really does not have to fit, your opinion of their design parameters.

I bring this up for a very important reason, while I did say monitors are predominately diurnal(their behaviors) They also include, nocturnal behaviors. The educated question is, why and how often.(not shoot the messenger) My responce to this is, they incorperate nocturnal behaviors, when pressured to do so. These pressures are of both of positive and negative types.

Examples of positive types would be to exploit a food source. Examples of a negative type would be to avoid unwanted external stresses. Like extreme heat, or predators, or the stress of predators/unwanted other monitors, pressense(common in captivity)

This is why the subject was brought up, it happens, happened, and I hope will happen. Captive montiors will choose alternate behaviors to avoid unwanted stresses or take advantage of resources. Are these behaviors normal, yes they are, even if they are uncommon in nature or precieved as uncommon.

I guess this reminds me of a little fish you should be familar with, a mudskipper(on the banks of the rivers in your study area) They are fish and live in water, we all know fish live in water, don't we? But mudskippers live on land too. So, they live in water and on land. Now, all hobbyist fish keepers know fish live in water. So what are they going to do? keep mudskippers in water like other fish, or keep them on land? The answer is easy, keep them with both water and land, unfortunately, the average fish keeper, does not want to do both, so what will they do? This applies to monitors, they, the monitors, live in both an inside world, holes hollows, etc,(the water) and they live outside(the land) but most keepers(maybe you) label or judge them by your preception of what you think and not what they think. You judge captives for only a oneside view, inside.

Most field folks, like you and Daniel, and of course me, view monitors kept in cages as "different" then the ones we see in nature. The natural ones, are strong, fast, aggressive, WILD, and would never put up with sitting on someones lap and falling asleep, would they? But what would the wild ones do, if they are inside a log? Or even picked up. Well Sam, you should know this, they play dead. They do not run, stand on their hind legs and fight(if cornered), they play dead, just like the ones sleeping on folks laps. Is there an epiphany here or are you too stubborn?

So in my view, I do not think monitors believe that cages are "outside", they know they are still "inside" They behave in cages as if they were inside logs, holes and such. Its no wonder that the most tame of monitors, often(not always) go wild when placed outside.

I also do not believe monitors think a litebulb is the sun, not in any form or fashion, they treat a lite bulb as a heat source, period.

Now please understand Sam, I am at an advantage. You mentioned, that you would love to have your captive monitors in indoor/outdoor cages. My advantage Sam is, I have had that situation for over a decade with many many types of monitors. Which I believe is valid reason to view them different then you.

Its about preception, you and I are only going by what we preceive. Well, I do not preceive a cage as most do, maybe even you. I think, monitors treat a cage as inside something, not like outside.

You and others, think when a monitor is under a lite, its outside basking, and when it retreats to a plastic pipe or some such hiding area in your cages, its now going inside. Then you think of the litebulb as the sun.

As a person who has outdoor cages, indoor cages and indoor/outdoor cages and with monitors in all of them, I do not think of such things. It preceive monitors know what the sun is, and a litebulb is not it. Again, They do not behave in a cage, like they do outdoors. They do behave in a cage, like they do inside or underground, when outdoors. I hope you enjoy, FR