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Exception to herbivorous theory

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Posted by: pilirin at Thu Nov 30 17:27:03 2006 [ Report Abuse ] [ Email Message ]

Leeway Corucia Research Center (LCRC)

I wish to question a theory proposed by Keith A. Metzger and Anthony Herrell on herbivorous lizard evolution with an example based on a largely herbivorous monitor.

Here is the abstract form the paper:

Correlations between lizard cranial shape and diet : a quantitative, phylogenetically informed analysis

METZGER Keith A. ; HERREL Anthony

Biological Journal of the Linnean Society (2005)

Résumé / Abstract

Although the relationship between dietary and phenotypic specialization has been well documented for many vertebrate groups, it has been stated that few such general trends can be established for lizards. This is often thought to be due to the lack of dietary specialization in many lizards. For example, many species that are reported to be insectivorous may also consume a variety of plant materials, and the reverse is often true as well. In this study, we investigate whether a correlation exists between general cranial form and dietary niche in lizards. Additionally, we test previously proposed hypotheses suggesting that herbivorous lizards should be larger bodied than lizards with other diets. Our data indicate that lizards specializing in food items imposing different mechanical demands on the feeding system show clear patterns of morphological specialization in their cranial morphology. True herbivores (diet of fibrous and tough foliage) are clearly distinguished from omnivorous and carnivorous lizards by having taller skulls and shorter snouts, likely related to the need for high bite forces. This allows herbivores to mechanically reduce relatively less digestible foliage. Carnivores have relatively longer snouts and retroarticular processes, which may result in more efficient capture and processing of elusive prey. When analysed in an explicit phylogenetic context, only snout length and skull mass remained significantly different between dietary groups. The small number of differences in the phylogenetic analyses is likely the result of shared evolutionary history and the relative paucity of independent origins of herbivory and omnivory in our sample. Analyses of the relationship between diet and body size show that on average herbivores have a larger body size than carnivores, with omnivores intermediate between the two other dietary groups.

Here is a pronounced exception.

The Gray's Monitor (Varanus olivaceus) or Butaan has a slender neck, body, short height, long narrow pointed head. Indeed, it looks identical to it's other carnivorous tree monitor relatives. It does not possess the shortened snout with associated strong jaw apparatus as indicated in the above theory. The Butaan consists as an adult, only on a diet of oily fruit from a native Luzon palm tree. This fruit is so tough that pliers are needed to extract it. But the Butaan removes it with ease. It can be argued too if one were to say that this is a case of inital evolution towards a non -carnivorous diet, this would not be so as this species has an enlarged Caecum as well as an enlarged intestine which would have taken time to evolve.

Sincerely,
Brian
LCRC

I have also seen such fruit seed’s in the stomach of wild imports rudicolis, also in recent years I was informed that eastern morphs of niloticus witch undergo teeth transformation in accordance to diet change when adult, often eat more plant matter then the molluscs themselves while feeding. These are based on observation made by trappers and park rangers alike.

I tested this by feeding many snails in tubs full of algae and water plants, and nil’s just grab the snails and gulp everything that comes with them.

I believe all monitors species eat the same ration of plant matter, some as niloticus may take it directly while most take it indirectly via insects, mammals and birds witch feeds on greens. I don’t see much of a difference between a 20lbs nil stuffing it self with mouthful of algae and a prasinus feeding on numerous grass hoppers who themselves fed on plant matter all day. To me its equivalent and buttanes or any other varanids have the same requirements, they simply adapt to environment and availability.
rgds

Several problems with this. You say, all, when taking about them eating pandana fruit. Unfortunately that is not true.

Then there is one huge problem. And that is context. What do greys consume for different tasks. Reptiles, and particularly monitors comsume different prey for different reasons. This is not known, or even thought about.

As a person who breeds monitors, we see they have a completely difference perference when gravid, both in size and prey type. This in not taken into any consideration at ALL. The work so far, has no context. Its simply getting information, in very restrictive cicumstances. Its really very little about the greys monitors. Its more about hoping to find a method to study greys monitors in a certain situation and at certain times.

Also, something totally neglected(ignored) is food perference in captivity. Greys recognize all normal monitor food as prey. They simply eat all normal food items. Which means they understand all that as prey. They do not ignore mice over pandana fruit, or grapes. Or fish, or crabs, or bird eggs, etc. They understand all these things readily. Are you going to tell me they will not eat, crabs, snails, birds, nestlings, eggs, if they come across them? of course they will. If they are restricted to fruit, because they cannot find other prey types, then that may be a very good explination why they seem to be rare. Again, context. because a species exsists, does not mean its healthy or doing what its designed to do.

You must understand, A greys is a greys, in nature, on different islands, different populations, and in captivity. They have the same brains, the same genetics and the same behaviors. And the same perferences. Those that think different are sadly mistaken.

The problem is obvious to me. There is almost no information on this species. Yet, people want to take incomplete information and use it for a base of other studies. Not a good practice. The current information is very weak. Nothing against the fine folks working on them. Just ask them, how much do you see, compared to how much you do not see. Sadly, if they are truthful, their answer will reflect an almost total ignorance of these monitors.

With that in mind, it makes this a study problem. There is no good/real/effective way to study them in nature. To say they depend on one certain fruit, is to explain why they may be on the road to exstinction. At least on a local level. Cheers

Frank is essentially correct in stating that V. olivaceus is really an omnivore, not a herbivore. According to the experts on trophic ecology, an animal must have a diet consisting >75% of a single type in order to be classed as a specialist. V. olivaceus' diet in the wild consists only of 66% fruit. The remainder is animal prey. Those like Frank and I who live in the western desert are familiar with a lizard which is a true specialist, the desert horned lizard, whose diet is 77% ants. And true specialists cannot persist in captivity unless fed their natural diet. V. olivaceus has been kept for many years at Dallas-Fort Worth and Los Angeles Zoo without panadanus or other of its natural fruit diet.
Frank, we do know a lot more about V. olivaceus ecology than most monitors, thanks to the great study by the late Walter Auffenberg.
Mr. Leeway makes two errors in his comment on Metzger's work.
1) V. olivaceus does not eat just pandanus fruit. It also take some 15+ other kinds, while at the same time rejecting many, many more kinds available in its tropical forest habitat.
2) Contrary to popular belief, grey's monitor does not "extract" fruit. It swallows all fruits whole allowing the digestive tract to separate pulp from seeds. That is why it has evolved a larger colon (caecum). The seeds are excreted whole and undamaged.

Yes,
Varanus olivaceus does pass seeds. This was never intended to be otherwise.

The research done on Luzon over many weeks with not only direct observation, but also motion sensor cameras, seems to bear out that this species, in an adult state at least, is entirely fruitivorous. Snails may be taken on occasion or more so by the young. However, the fact that this species has evolved the longer intestine and Caecum even if Omnivorous, should show some minute change at least in evolution towards a shorter and taller skull if the theory proposed is correct.

Sincerely,
Brian
LCRC

At the risk of talking too much, 1)This paper defines herbivory as "diet of fibrous and tough foliage", which doesn't apply to Varanus olivaceus 2) evolution of cranial structure in Varanus olivaceus is probably driven by the need to crush shells rather than consume fruit. All of the characteristics mentioned can be explained by selection for durophagy rather than frugivory. Most likely scenario for me is a large molluscivorous lizard that has recently started to exploit fruit resources whose main consumers were now-extinct megafauna. 3) Other than the alimentary tract there isn't much evidence of structual adaptations towards frugivory at all, and the presence of a caecum suggests leaf eating rather than fruit eating. I'm not aware of any evidence that leaf eating is a strategy regularly employed by any monitor lizards and I'd be very surprised because of the apparent lack of necessary gut bacteria and the fact that they don't chew like folivores. 4) All known populations of olivaceus/mabitang complex eat different fruits, but Pandanus is the only genus that is eaten everywhere so it's fair to say that the animals depend on it. 5) I'd disagree with everything said on this thread about the diet of the species and whether or not it is a "specialist". Its diet consists almost entirely of fruit and snails and they eat a lot more fruit than snail by both weight and volume for at least ten months of the year. I'll repost my interpretation of Auffenberg's data to justify this. Again.
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Mampam Conservation

This is pasted from another forum dated 26 May 2006@

What we know about the diet of this lizard in the wild comes from Walter Auffenberg's study in southern Luzon that finished in 1983 and our work based on Polillo that started in 1999. Walter's data is basically 116 animals that were dissected, our is 977 fresh fecal samples. Feces don't give exactly the same results as stomach contents, the differences are very slight and the method has a lot of other advantages but some people don't think it is good enough, so we will just concentrate on the dissections. I presume that the people who claim that the frugivorous part of the diet is incidental have read Auffenberg's book. He talks about diet largely in terms of numbers of prey item and very little about the relative weights of different food types. So using three of the tables he provides I reconstructed the diet by month according to the relative weights of animal and fruit items. The graph is at http://www.mampam.com/gfl/waltdiet.htm
It isn't perfect for many reasons and the original material is lost, but it shows that the animals are eating a lot more fruit than animals. In fact this looks like the diet of an obligate frugivore that is only taking a minimal amount of animal prey for protein and calcium. There isn't much data for October - January (only 17 animals in total) and they are not eating a lot. Maybe the fact that there is only 1 fruit in the December sample makes people think that the fruit is supplementary. My experience on Polillo makes me suspect it's due to small sample size because lizards don't move about much during the coldest part of the year.

Graph expanded

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Mampam Conservation

First Daniel, you know I like and respect you and your work. Please for all to understand, its not about Daniel. He is doing the best he knows how.

But, there is a serious problem as I see it. And yes, I know I see things much differently. So Please take it for what it is, just something to think about.

When you do a study on a species, like your doing. You gather information and make all sort of assumptions and graphs and diagrams and such. You read them, and others read them and think this and that. You think its real and meaningful. The problem is, what does this information have to do with that species?

For instance your post is to correct the original poster because they were making weak assumptions based on previous poor assumptions. Theory based on theory and false assumptions, does not make good information. This is what your post is trying to correct.

Lets go on, Of course I have heard you say this and that about the importance of this species to that prey species, and visa versa. Which is always true even it they did not eat them. All species are important to other species, in a ecosystem.

The problem is, you have no base to work from. As with most other varanid studies I have read or seen. You and others work from what has been written, and use the animal to verify that work. Or so it seems. The problem is basic, its not about what was written, good bad, or indifferent. Its always about the species or individuals being studied, whether they agree with previous studies or understandings or not. Is not important, not to the animals or the REAL understanding of the animals.

The Key is to form a base to work from. Not from what was written about a species, but what HAS to occur for this animal to exsist.
1. This animal(Greys or any species) Must hatch from an egg. It must as far as we know, IS THAT TRUE?
A. have you seen eggs, or hatchlings, nests or associated evidence of this? how much?

2. These hatchlings must grow up to reach reproductive ability. Is this true?
A. what are the hatchlings feeding on to grow up? Have you seen this? how much?
B. are hatchlings abundant? they should be.

3. Once mature, females must copulate to produce offspring. Have you seen this? How much?

A. nests, nesting behavior. HYST?(have you seen this)
B. pairing to copulate, HYST?
C. or simply a fat female suddenly becoming skinny. HYST?
Its a biological fact, they must copulate. In captivity we KNOW, they are not a wam bam thank you mam, type of breeder. The males attend females for an extented period. Have you seen this? You know, a pair. or even males following females, mam do they do this commonly. This is not about the difinition of social, its simply a fact, varanid males attend females for 10 to over thirty days.

For the purpose of simplicity, I will leave it at a simple, 1, 2, 3. now understand, each of this catagories has many many subcatagories. These are what define the species. Not one simple part of a diet, fruit. This is why I am a bit negative about this type of study. You are stuck on fruit and not looking at the animal. What would happen if you took the fruit away? of course you think the animal will not exsist, but this animal understands a wide range of prey types. I would bet it would simply feed on something else. As that is what monitors do. (of course you will argue that)

I also understand from our previous conversations, that your study site is limited, and I am sure your right, but that has little to do with Greys monitors, it mainly has to do with you and may restrict your study.

This brings up lots of questions. For instance, in auffenburgs study, I remember reading he found Greys sheltering in rock crevices. Have you found where they shelter? The reason I ask is, wait, you know I am a reptile detective. I go in the field and figure out what a certain species is doing. I do very well at this. When doing this, such important things are where they perfer to shelter, is a dead give away as to what else they are doing. Its a place to start to look for more evidence. Why, monitors and a good percentage of all reptiles, shelter in holes, crevices, etc. not in the open like some other lizards, Chams, etc. It appears from what I asked you in the past, yours go up to the top of the trees. Are there holes up there? or are they sheltering on a branch? The reason I ask this is, Residents shelter in holes, transients shelter anywhere. The key here is, you need to be studying the residents, not the transients. Of course, this may not occur on your site, you may have nothing but transients. First you have to understand, there are transients and residents.

Which brings up many questions. Since you started, have you tracked single individuals throughout their lifes. What percentage of RE-observations do you see? or are you simply following different individual transient animals. Sir, this is very important when it comes to diet. Transients have a survival diet, and residents use a progressive diet. This should be clearly understood. Which again takes me full circle. What are your study animals doing? What have you seen to indicate they are doing anything more then exsisting. Remember, exsisting at minimum levels is what reptiles are expert at. But at some time or another they must change and progress in order to add to the eco system(recruit)

I hope I am somewhat clear with this post. Two things, you must find evidence of what the animals are doing. This will indicate, what the diet is supporting. In simple terms, I do not argue with your findings, your study animals are eating fruit, but what the heck are they doing with it. Its what they are doing on that diet thats important to your findings, ARE THEY EXSISTING or ARE THEY PROGRESSING. What is the fuel used for?

The question of eating friut is too darn simple, of course they are, how does that add or support their life, thats the question. Do you have evidence of this? Your friend Frank Retes

Hello Frank
I just saw this and am pressed for time. So I'll reply bit by bit. The questions we are concentrating on are basically 1) why do butaan stop exisiting in degraded and fragmented forests? and 2) can any features of plant distribution and forest structure can be directly attributed to the behaviour of butaan?
Obviously there are a lot of other questions but I concentrate on these because I thnk they are important and achievable.

You said:
1"Lets go on, Of course I have heard you say this and that about the importance of this species to that prey species, and visa versa. Which is always true even it they did not eat them. All species are important to other species, in a ecosystem."

The lizards are important to the prey species primarily because they move seeds against gravity. That is a very specific relationship. I don't know how they might be important to the prey species if they didn't eat them. The idea that all species in an ecosystem are important to each other isn't convincing because they are all in competition with each other rather than in a web of mutually beneficial relationships. Most predators confer no obvious benefits to their prey.

2"The problem is, you have no base to work from. As with most other varanid studies I have read or seen. You and others work from what has been written, and use the animal to verify that work. Or so it seems. The problem is basic, its not about what was written, good bad, or indifferent. Its always about the species or individuals being studied, whether they agree with previous studies or understandings or not. Is not important, not to the animals or the REAL understanding of the animals."

All that was written about the Butaan was Walter's book. His methodologes were basically catching butaan, dissecting most, observing a few in enclosures and releasing a few with radio transmitters. Other than a few that went to the USA there were no survivors. So we couldn't work from what he had written and had to devise a set of minimally intrusive methodologies that yield very different types of data. A lot of that contradicts Walter's findings, but because all the specimens from his study are lost it's very difficult to make any direct comparisons.

more later, all the best
Daniel
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Mampam Conservation

Daniel,
I love your work but I disagree with this statement "Most predators confer no obvious benefits to their prey". Predators are a major selective pressure on their prey and due to this, they are definately a benefit to their evolution. This relationship is like the "arms race" that drives change, or advancement of a population. The selective pressure of the predator forces the total prey species' fitness to increase or it will eventually become extinct. After which will follow an unfortuate chain of events to the ecosystem unless that niche can be filled by some other species.
Best,
Jason
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"Relax, Don't Worry, Have a Homebrew!" Charlie Papazian

Hello Jason
It's a bit of a circular argument because predation just selects for characteristics that avoid predation. So you could argue that cheetahs benefit antelope by selecting for speed, but if there were no cheetahs they wouldn't need to be fast!! In fact predation isn't a suitable description for the relationship I wa talking about, I was trying to use it in the context of monitor lizards: that the prey of monitor lizards don't benefit from being eaten, with the exception of the fruits eaten by frugivorous species..
Daniel
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Mampam Conservation

Ok,
Daniel, I agree. I will accept that I took you out of context a little, but that statement just poked out at me.
Best,
Jason
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"Relax, Don't Worry, Have a Homebrew!" Charlie Papazian

You said:
"1. This animal(Greys or any species) Must hatch from an egg. It must as far as we know, IS THAT TRUE?
A. have you seen eggs, or hatchlings, nests or associated evidence of this? how much?

2. These hatchlings must grow up to reach reproductive ability. Is this true?
A. what are the hatchlings feeding on to grow up? Have you seen this? how much?
B. are hatchlings abundant? they should be. "

Obviously they must hatch from eggs and grow to maturity. But we have still never seen eggs or nesting, and don't expect to, unless we find a felled tree that contains a nest. We have seen two hatchlings. So in total based on over 200 years of observations in those forests (i.e. the six people who are 50 who work with me and have always lived there) we have seen one hatchling and no nests. That suggests to me that all the action is going on high above our heads. I consider it a limitation of working with an extremely shy animal in some of the world's densest and tallest forests, rather than a failing on my part. If we wanted to look at the reproductive biology of the species we would need to start interfering a lot more. The questions I am asking assume that reproduction occurs wherever the requisite animals exist. Walter thought that hatchlings fed only on animal prey. One of the ones we saw was eating a frog, the oher was feedng on Grewia fruits. The animals I know about are those from 300g upwards. They are eating fruit and snails and the 1-3kg animals form the bulk (70%) of the known population which is around 60 animals in total. Given the low numbers of adults I wouldn't expect hatchlings to be particularly abundant although they may outnumber the larger age classes
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Mampam Conservation