what makes a monitor lizard a specialist?

[mbayless]

When I look at African monitor lizards, I see 'specialists' = those animals that have a dietary speciality - and supported by their two different teeth morphologies. African monitors go through two steps of tooth development: 1) their baby (Insectivorous teeth) and 2) their adult crushing mulluscivorous teeth...do other monitor lizards do this?

Although the teeth of V. dumerilli resemble the V. salvator, V. dumerilii appear to be molluscivorous feeders, while V. salvator eats anything = so does the tooth morphology seen in African apply with Asian Pacific monitors? Doesn't seem so. V. prasinus stomach contents suggest they feed a mjaority on walking sticks and katydids - but does that mean they are specialists or generalists?

Comments? Ideas?
markb

[treevaranus]

Good topic Mark,

I think that monitors of the sub genus Polydaedelus are prime candidates for such a discussion, as it is quite evident that the structure of their teeth vary greatly with other extant subgenera of Varanus.
While I am not sure of the dentistry on V. griseus, upon looking at the , flatter/blunter teeth of V. albigularis, V. exanthematicus, etc. it is very easily for one to assert that such dentition would suggest durophagy. Many people will say that they evolved these teethto allow for a more specific diet...

But what if it is the opposite of that? Perhaps the teeth evolved to allow for a greater diversity in diet. Maybe Polydaedelus evolved teeth like that to be able to incorporate shelled mollusks into their existing diet. It is evident that these monitors are still quite capable of killing and subduing mammalian or other prey with their teeth.. What if a rodent/snake/ or terrestrial bird diet was an ancestral trait, and members of Polydaedelus have slowly changed the shape and structure of the teeth which allowed for durophagy, thus creating an even more diversity in potential prey items than their ancestors. This idea would suggest that they have evolved those teeth to become more opportunistic.

There could be a very clear cut answer to this topic, as I am not all that knowledgeable of Polydaedelus, or the paleontology of Varanids, or the evolution of jaw and tooth structure in Varanids.... I will try to read up a bit on ancestral Varanids, and see if Molnar's work mentions anything about dentistry, and how it has changed through time.

As for analyzsis of stomach contents in preserved museum specimens, is there not a possibility that the findings may be flawed?? Things that could afffect the findings may include the time of year of the animal's capture, as some food items may be more abundant at one time of the year than others.. Wet season- more insect prey available... Perhaps all of the specimens were collected during the wet season, as it is easier to find and trap individuals then than the dry season(may or may not be true, just an example); would it be fair to claim that that is the only thing they eat year round?? There could very well be a different prey selection/abundance at a different time of year... I am not sure if Greene's V. prasinus paper addressed when the specimens were collected..

But it definately is an interesting topic, and needs to be studied in more depth.. Very little is known about varanid ecology. Diet being just one of many unknown/misunderstood subjects..

Cheers Mark,

Bob
treemonitors.com (http://www.treemonitors.com)
http://treemonitors.com/images/d26.jpg
Varanus beccarii female

[mbayless]

Hi Bob,
I can't believe I am talking with the same fellow who 6 months did not know the difference between a plant and animal cell - Good Grief Bob = you speak Biology like a pro!! Your classes have definetely worn off on you....

It has never occured to me that this durophagus teeth morphology might be a new trait, going backward towards specialization = I always looked at it that any animal with polymorph teeth, like African monitors are, was agenerality to speciality move, not the reverse! I will have to go back and look at the fossil African Varanid materials and see = and recheck Molnars materials.....

Nice skull/head you share with us here....
Thanks Bob,
markb

[kap10cavy]

Ok guys, I aint going to use all the big words but i believe most monitors are specialist. Traits that have been passed onto their species through the eons. I believe it goes further than teeth, look at the rudi. Long narrow snout, great for digging bugs from their hiding places.
If you want to talk teeth, I like the crocs teeth, perfect for grabbing a bird.

Scott

[V.hb]

Aren't crocs known for grabbing small primates? I always love reading marks posts.. Like a live action monitor movie!!

[mbayless]

Yeah crocs grab monkeys, and anthing else they can catch.

V. rudicollis is an amazing species - its stomach content notes, which are few, but damn interesting include crabs, shellfish, a spider, a frog, and beetles...they're shellfish feeders by what the stomach content studies say; but the snout says insect; and the body which swims like a U-Boat says fish/frogs...their habitat is 20-100 feet in the tree canopy - in groups no less - so like Orangutans, do they only come down from the trees to feed?? I believe V. rudicollis to have semi-social proclivities = group living. One of my favorites.

markb

[kap10cavy]

I always assumed crocs were bird eaters from the long, sharp and hollow teeth, great for piercing feathers.
I could be wrong.

Scott

[SHvar]

After all depending on the age, size, and location, time of year, etc food sources always vary. Albigs love birds (King and Greens book quote from African song bird watchers), they love snakes, and other lizards, but if there are tons of large snails, lots of available eggs, small tortoise, and tons of locust available they will eat what is most plentiful, and I believe adapt to eat those items over many many generations, its advantageous after all.
Griseus love snakes, lizards, and small animals, as well insects so it stands to reason what is most available will make up their diet.
According to stomach contents taken from African varanids (from the study Mark sent me) the nile has the broadest range of food sources, well almost eveything goes to water to drink or to eat, so this stands to reason why niles in the wild eat so much more variety. The albig stays far from water, this stands to reason that snakes, birds, scorpions, and insects will make up alot of their diet.
I like reading discussions like this.

[mbayless]

Diet studies are not conclusive, only suggestive - but if you have enough stomach studies, I bet patterns begin to show. In S. Africa, near Aliwal, tortoises make up 40% of their diet - in Zimbabwe, its invertebrates, and in Equatorial Tanzania, its beetles and snakes... so it is not only a suggestive study, it is regional as well, as you say about the Nile monitor Shvar.

Only 1 study has been done on the Forest Nile (Varanus ornatus), and it supports an idea I have long suspected, and makes sense: male and female monitors have characteristically different diets which along with time sharing reduces their stress levels for competition in their over-lapping ranges....cool huh. That way she can have pickles and ice-cream, and he can have hot dog and beer without fuss! haha.

I have always been a proponant of Natural diets and monitors, and as these field studies show, and are suggestive of, we are only beginning to understand these animals...I love it! If true, it would seem quite important to insure mineral levels were higher for females than males (for specialists) = which fits nicely with a 24/7 photo period feeding them mice on an hourly basis, and then they drop dead from hepatic necrosis, renal failure etc. haistens their death exponentially....you can't feed an animal of a reptilian physiology and treat it as though it is a mammalian physiology and expect it to live for too long, now could you...its physiological unfeasible....
markb

[treevaranus]

I do not disagree in that there are specialist monitors out there, I brought up the Polydaedelus example for a reason. Like I said, it is known that members of this group are quite capable of catching and consuming non-shelled invertebrates, and that even with these 'specialized' teeth, they are able to incorporate great diversity of prey into their diet.

When I see an animal who has a specialized diet, I see an anteater, as the way that its mouth/jaw/tongue has evolved in no way allows for a diversity in the diet. Or look at the prime example of adaptive radiation- Darwin's finches. The finches evolved separate beak morphologies to allow for a specialized diet. The long, narrow beaked insect feeders would have a very difficult time cracking open seeds and fruits as the short, thick-beaked seed eating finches could... I see those as specialists.

With Polydaedelus, i see the modification of their teeth not limiting the diet that they can and do eat, but allowing for a greater diversity in teeth. While varanids do not have heterodonty like we do,( different types of teeth to allow for different functions-allowing for a greater diversity in consumable food items), I think that the teeth of V. albigularis, V. exanthematicus, etc... are similar in a way, as that type of tooth structure allows for variety in diet, not just shelled inverts.

As for V. olivaceus being a specialist, there is very little known about their wild ecology. Looking at the dentition of V. olivaceus, I would think that they would be consuming other animals, being they have rather sharp teeth, not what you'd expect on a fruit eater. Not to mention the obscene quantity of fruit/plant matter an animal with such a high metabolic rate would need to consume daily in order to provide it with enough energy to power its metabolism.. Like I said, I know virtually nothing about V. olivaceus, but I would guess that they are generalists, who have simply added plant material into their existing diet.

Just my thoughts on dietary specialization in monitors..

As for my favorite varanid complex, the prasinus group, I think that they are tree dwelling specialists. Many of the adaptations will allow for/benefit life in the canopy would have a harmful effect if they spent any significant portion of their lives on the ground/forest floor. Coloration in several of the species would not help in crypsis on the forest floor. But most of all(in my opinion), their body structure is ultimately designed for a tree dwelling lifestyle..

Have any of you ever seen a tree monitor run on the ground?? They are very awkward and clumsy. Their body plan is slender and stretched out. If you look at any other varanid which spends much time on the ground, their body plans are much more compact and squat.. An awkward moving, and uneasy monitor on the forest floor would easily be picked off. WHile they are very fast and agile when climbing, they are very clumsy and uncoordinated when running across the ground.. Therefore, I feel that these lizards will very rarely come down to the forest floor, perhaps to move from tree to tree, but all in all, I would have to think that they are tree specialists...

Now the difference between this and my interpretation of the diet topic, is that the teeth of Soterosaurs do not limit individuals to an exclusively shelled mollusk diet, whereas many of the 'tree dwelling' adaptations in the V. prasinus super species limit the animals to using the trees only, as their "arboreal" adaptations would effect the survivability of the animal if it were in fact a generalist, and used all different situations regularly..

I could see that in an area where there is great competition for food among similar animals, the need for specialization, ie. Darwin's Finches-evolved to fill separate ecological niches/separate diets. Do we really see this same type of trophic competition among Varanids that would require such a radiation? Perhaps if these animals were forced to live within a limited area, with a limited amount of food resources, we would see specialization- so that pressures on food supplies would be decreased by having two species(or more) of completely different diets- competion no longer exists- again, Darwin's Finches are a prime example of this..

Even with Indonesian varanids, where there may be three separate species dwelling on a single island, has there been that much competition to allow for the need to specialize in diets?? In this case, I would think a specialization in ecological niche/choice of habitat would be favored over diet. For example, the V. prasinus group moved 'up'. Where the doreanus/inducs group remained on the ground..

This is a great topic. It is a shame that all of the discussions could not be this interesting and though-provoking... Mark, any word on the evolution of varanid dentistry? Is durophagy a plesiomorphic/ancestral condition?

Cheers folks, sorry for such a long post...

bob

treemonitors.com (http://www.treemonitors.com)

[mbayless]

Hi Bob,
Its funny - the only photo I have from James Menzies, is a V> prasinus on the ground, among the rocks/boulders of the Fly River Shore-line, on the ground - and his accompanying letter says are often seen there....as you move inland, you encounter the nefarious and graceful Artrela V. salvadorii.

I am not sure is the Polydaedalus - African monitors teeth morphology is pleisiomorphic condition - I have to go look at the 45 year old papers on Reptilian tooth morphology - The wieshampel theory of tooth development or the ...... damn, I cant remember the other theory for it...and work backwards....

Cheers Bob,
markb

[mbayless]

Come on Folks = Lets hear from you....what do people think about this? Bob, Scott and Shvar have said what they think - anyone else??

OR are people only interested in posts prefetted/blaspheme 'words' are bantered about....?

markb

[treevaranus]

Hey Mark,

One thing that might shed some lite on the ancestral traits of Varanids, is that although it was once believed that Varanids arose in Gondwana, it is now believed that they arose in Leurasia, where there was a radiation which ultimately branched off to form the african species, whereas the other lineage went on to form the asian and indo australian species.. So there is a possibility that durophagy was aquired after this radiation.

I have done some reading of some of Molnar's work, and it appears that durophagous teeth evolved multiple times within ancestral lineages leading up to monitors.. There were several platynotans that exhibit diversity in teeth structure and shape, however the majority seem to have sharp, pointed teeth.

If you look at the closest living relatives of monitors- other extant members of Varanoidea(Heloderms and Lanthanotus), they all have sharp, pointed teeth. This leads me to believe that sharp pointed teeth are a primitive/ancestral(pleisiomorphic) condition of varanids, and the durophagic teeth seen in Polydaedelus most likely evolved later on.

I tend to think that mollusk-crushing teeth had evolved later on in the Varanid lineage, after the radiation between african and asian groups... as that would be more parsimonious it seems...

Here's an abstract of some of the work that Lynn M. Clos has done on Varanid ancestry, and discusses teeth a fair bit... check it out:
Fossil Monitor Lizards Article/Abstract (http://www.fossilnews.com/1995/varanus.html)

What does everyone else have to say about this topic?? ARe some monitors dietary specialists???

Cheers everyone,

Bob
treemonitors.com (http://www.treemonitors.com)

[treevaranus]

oops, forgot to comment on V. rusingensis.. That article says that it had somewhat durophagous teeth, although not as evident as modern day Polydaedelus..

While it is supposedly the earliest known varanid fossil, it does not mean that it IS the earliest/ancestral varanid, as V. rusigensis is from Kenya, which was once part of Gondwana...

The most recent findings suggest that Varanids came from Leurasia...

Interesting stuff nonetheless... I would still believe that sharp pointed teeth are the plesiomorphic condition when comparing them to other sister taxa which share the most recent common ancestor- members of Varanoidea

[kap10cavy]

I find this subject interesting. I might even find it facinating if ya'll would use words a poor country boy could understand.
I am tired of reading these post with an encyclopedia in my lap. hahahaha

Scott

[mbayless]

Talking to Bob - one needs a dictionary = hahahah - I can't believe how fast he has gotten into the zoology stuff...

Yes, I have read Molnar's damn good chapter in Varanoids of the World a few times, and seen the teeth morphology of those... and I too believe Africa monitors [polydaedalus] are an older group - old enough to have baby teeth and adult teeth - for adaptation to hard bodied prey = all of the 'known' African monitors have such teeth - but what the 2 unknown species I know of, and am writing about now? We know nothing of their teeth.

Yes, the oldest know African varanid fossil is Varanus rusingensis, and Lynne Clos has shared with me alot of notes, and photos of the fossil before it was sent back to Kenya Museum - she and her museum had it for a long time. But there are fossil Varanus dating 7 million years old from Chad; 1 (V. albiuglaris?) ~2 million years old from South Africa; and a few fossil varanids from Uganda and Namibia (Rage, 2001). So there are alot more fossils than just a few years ago. Personally I see V. rusingensis as a fossil Nile monitor, found on Rusinga Island, where living V. nilitocus live today (Bayless, 2002) - not because they both lived there, but the skull morphologies of both are strikingly similar. As Lynn says about V. rusingensis it is not entirely durophagus teeth morphology - was this specimen changing from baby teeth to adult? Were these its adult teeth? It is hard to say 14 million years later....but there are least 1, probably 2 new/unknown African monitors out there - and I think we will be in for some surprises when I get all the information I am waiting for: I have a photo of them, habitat notes, in one instance -why they are rare ((incident in 1727 escalatd this), and there range - I am waiting for a specimen.

Yes, I too think monitors made a N -> S migration - perhaps via the cold, moved south as hominids did (and in The Day after tomorrow for same reasons) = in Africa, the large carnivore mammals were already there, so there were fewer niche to fill, as compared to S.E. Asia and Australia where there is more niches to fill and bloomed, and where Reptile is still king in this Age of Mammals era. In Africa there is V. griseus, V. niloticus, V. ornatus, V. exanthematicus, V. albigularis, V. yemenensis, and 1-2 surprises = that is not many for such a vast Continent when compared to 22 in Australia - and how many throughout Indo-Malay-Thailand and Philippines?

Thanks for keeping this thread going - and good ideas....
Cheers,
markb

[Sam Sweet]

This is an interesting thread, but it begs the question as to whether various monitor species can actually be referred to as specialists. I am inclined to say that they are not, although you can make a strong argument that monitors as a group are highly specialized lizards.

Varanid lizards (the whole family) differ from all other lizards in having structural and physiological features that permit them to use aerobic metabolic pathways while active -- in other words, monitors are more similar to mammals than to other lizards in that they do not tire out quickly even during long periods of high locomotor activity. This is certainly a specialized trait. Monitors are also specialized compared to other lizards in having acute long-distance (color) vision, sensitive hearing, and very well-developed senses of smell and tongue-based vomeronasal organ "taste". Monitors additionally seem to be able to integrate all these senses well, and are pretty smart, for want of a better term.

These are all features that make monitors effective as widely-foraging predators, and they are very good at this. However, it is a fairly good general rule that predators cannot afford to be particularly choosy about what they eat, or overly specialized in the ways they go about searching for prey. (As an aside, many snakes violate this generality by being highly specialized for finding and consuming only a few kinds of prey.) While monitors may capitalize on seasonally abundant prey (for example, turtle eggs), all of the published studies of monitor diets show that individuals eat a wide range of prey types (basically as items are encountered). In other words, monitors that encounter a lot of freshwater crabs eat a lot of freshwater crabs, and likewise for other lizards, large insects, and so on. The Philippine monitors V. olivaceus and V. mabitang are unusual in feeding in part on fruits (they also eat snails), but there is actually very little in their structure that signals this dietary difference.

Much of the discussion here has focused on dentition, when in fact there is not a huge amount of variation among species in tooth form. Sure, old individuals of several species develop blunt rear teeth and tend to eat a lot of snails, and a few, like croc monitors, have rather long and narrow teeth. I need to point out to MarkB that croc monitors do not have long teeth to catch monkeys (since there are no monkeys within 400 miles of New Guinea). However, it is hard to make such minor variation in tooth structure evidence for "specialization" between species. All of these species in fact eat lots of different prey types *when they can find them*.

This last point is the key. While you cannot make a strong argument that any monitor species is a dietary specialist, you can certainly show that many species are habitat specialists (and since this restricts the sorts of prey they encounter, you do therefore tend to see crabs inside of mangrove monitors, bearded dragons inside of Gould's monitors, and canopy insects inside of green tree monitors). It's the cart that follows the horse, folks.

Ecological differences between species of monitors lie mostly in body size and habitat use, with differences in diet being the consequence of those primary distinctions. Even so, there is not that much difference in body form evident among tree- or rock-dwelling species, semiaquatic species, or those that make their living on dry ground. Limb and tail proportions and general stoutness vary, but this is not very dramatic across the group. This is one reason that systematists continue to regard all monitors as members of a single genus, Varanus.

[mbayless]

Hi Sam,
Nice to see you here - did I say V. salvadorii eat monkies!! Ooops!!! I cant imagine me making an error like that, but worse things have been known to occur....Yes, many varanids are ecological specific - whether by human intervention or by design, but I believe the durophagus tooth morphology is a diet-specific adaptation. The diet of African monitors does change somewhat, more so in V. albigularis and V. niloticus from insectivorous hatchlings to vertebrate and especially invertebrate (Achatina fulica) giant snail feeding, which both do actively feed upon on a seasonal basis, and during the wet, feed abunadantly upon these - you need crusher teeth to feed on snails, as Dracaena and some Mosasaurs also did/do on a regular basis.
I think this is a specialist adaptation trait...are there any other varanids whose teeth change with 'puberty'/maturity? Some Bengal monitors has a somewhat similar durophagus tooth morphology, as does V. olivaceus which does, as you say Sam, feed on fruit, and decapods.
Have a Good Memorial Day all,
markb

[SHvar]

One of the more informative and detailed posts around on such a specific subject.
Sam great seeing you, and great response, how are you and the lizards (hope your all great).
I wish more posts went this way on forums.

[kap10cavy]

Thanks Sam, I can understand your post without an encyclopedia.
So it pretty much comes down to the will eat just about anything they can catch and consume.
My lizards are basicly goats with scales. hahaha
Ok, enough funny stuff, "I" do believe certain critters have adapted through the ages to consume different prey.
I have stuck mussels in all my cages, the argus(ok, he's crazy) used it for a toy. The albigs did the big chomp and ate them.
The savs sniffed them and rolled them around trying to get a firm grip and eventually open a few and ate the contents.

Scott

[mbayless]

HI Sam,
I read my earlier posts - and I did say "Crocs eat monkies" = I meant 'crocodiles' on that post, not croc monitors. Sorry for the confusion there...

Isn't it fun how they 'play' with food when they are not entirely sure what it is - like a 4 year old, or a 50 year old whose wife is a bad cook! haha....
markb

[kap10cavy]

Ok, now to a new question but close to the same subject.
What about jaw pressure? I can stick different foods in my cages and get different actions. If the fod item is too big to swallow, my argus will hold it and shred it. My savs will grab it and shake it, sometimes doing a death roll to tear off chunks. My albigs just crush it until it fits down the ol' gullet.
Does anyone have anything on jaw pressure with different monitors?

Scott

[mbayless]

Hi Scott,
There is a paper on two on power bite and so on written by Kathleen Smith - and a paper closer to your question:

Alice Sinclair, 1987. Estimates of forces exerted by the jaw muscles of some reptiles. Journal of Zoology London 213:107-115.

I cant find the paper right now, but when I do, I will relate what it says about our animals.

markb

[mbayless]

And here is a paper title suggestive of durophagus teeth and 'specialization' in Reptiles:

Durophagus feeding adaptations in a Amphisbaenid. Journal of Herpetology 18(2):186-191.

cheers,
markb

[infernalis]

I found this long retired thread in our archives, after reading it, I thought it very worthy of bringing it back out.

R.I.P. Mark Bayless.

A beautiful eulogy can be read here in the first issue of Biawak.

http://varanidae.org/Vol1_No1.pdf