In Depth Questions Regarding Tortoise Digestion and Fruit

[TechnoCheese]

While debating someone on the quality of their redfoot's diet, I came across this recent study on the effects of starch and fiber on pyramiding in redfooted tortoises. https://www.sciencedirect.com/science/article/abs/pii/S1095643321002397
As I expected, it found that there was no correlation, but that's not what interests me. This study found that tortoises fed a high starch diet had a lower bone density, lower mineral content, and increased plastron width, as well as plastron width growth rates. Additionally, tortoises fed a high fiber diet had a higher digestibility coefficient for protein.

Very useful source for anyone trying to explain the importance of a high fibered diet and avoiding grocery store foods for tortoises.

Speaking of grocery store foods, one of the main aspects of tortoise diet I see people reference that I still don't understand is phosphorus ratio. What specifically is phosphorus used for in a tortoise, and how is it processed/absorbed? (I.E in the liver, in enterocytes, denatured by acids in the stomach, etc.) It would really help me out if someone could give me a run down. Is phosphorus just one of the minerals you would find in reptile vitamin supplements?

Quick correction, the high starch diet lead to an increase in plastron AND carapace growth rate. I believe they may have just been using width as a measure of growth, and did not find that the width was larger at the same shell length as the high fiber diet. I can't seem to find anywhere they clarify. Need to see if I can log in through my school and get the full study for free.

 

[TechnoCheese]

Here is the full version of the Redfoot study. It's a very interesting read. It's also normally behind a paywall, so this is probably legally (ethically?) questionable.

As expected, while the high starch diet produced steeper levels of pyramiding sooner than the high fiber diet, redfooted tortoises of the same size, regardless of diet, had the same degree of pyramiding on average. Because individuals on the high starch diet experienced faster growth, the pyramiding increased at that same rate. Growth rate had an effect on how fast the pyramiding developed, remaining constant with growth, but did not have an effect on the final result. Individuals on the high fiber diet, when compared to similarly sized individuals on the high starch diet, had the same degree of pyramiding, even while growth rates were different. I do realize I've just said the same thing four times.

The diets also had equal calcium/phosphorus ratios, and the only difference between the diets was the addition of corn meal (in the high starch diet) or sugar cane fibers (in the high fiber diet). I wonder if the decrease in bone density and body mineral content were caused by an increased growth rate with the same amount of Ca/P, as in less calcium/phosphorus consumed per gram gained, and not specifically by the abundance of starch or lack of fiber.

The 2003 study, Influence of environmental humidity and dietary protein on pyramidal growth of carapaces in African spurred tortoise, found that protein had a minor, positive (additive) effect on pyramidal growth at the highest levels of dietary protein measured. To this day, Zoo Med still has "No pyramiding" written on the side of their grassland tortoise pellets, and I assume that was, at the very least in part, influenced by this study. However, I suspect that the minor effect measured was due to the size of the tortoise, not the protein itself. There is a correlation, but not necessarily causation, between protein and pyramidal growth when measured against time, but when measured instead against size, likely in shell width, I would expect there to be no statistically significant correlation between protein and pyramiding in humid conditions. Unfortunately, that study is also behind a paywall that I'm not willing to subscribe to, and the site does not accept my university to view it for free, so I cannot see the methods used and results to know for sure how it was measured.

I know that the consensus that pyramiding is not caused by diet is common on this forum, but outside it, most people still believe that it is. I find it helpful to look into why people believed that it was to better argue against it, which I've been doing a lot recently.

 

[TechnoCheese]

@jaizei was kind enough to provide me with the full version of the study. I've attached it to this post.

As I expected, the pyramiding was measured at the end of the 5 month trial, in essence measuring it against time, and did not take tortoise size into consideration, as greater levels of protein cause increased rate of growth. In addition, with a p-value of 0.06 and a significance level of 0.05, protein level only barely made the cut off to be considered significant. Environmental humidity also fluctuated greatly per trial, with a low of 45-47.5 and a high of 99 measured in the groups kept most humid, group E and D. All of the tortoises experienced some level of pyramiding, made visible by the lack of any group containing tortoises with a negative or 0 H-value, meaning that the humidity was not kept high enough to completely prevent it. This also means, unfortunately, that group E and D, the two high humidity groups with differing diets, cannot be used to compare pyramiding with increased protein in humid environments, as the average final weight of group E was greater by a whopping 38.5% of the weight of group D.

 

[TechnoCheese]

Also, the redfoot study sources another study, which found that levels of calcium, from slight deficiency to 3x necessary amount, had no effect on pyramiding. I think I'm going to compile all of this stuff into it's own post later, since this thread was not meant to be about pyramiding. I know we already have lots of resources on pyramiding here, but to have a collection of empirical studies that support the idea that diet does not cause pyramiding, all in one place, would be very valuable. Less work for me when I'm arguing on the tortoise subreddit, anyway.

 

[Maggie3fan]

You lost me at “ to preface” haha

you can't read anyhow

 

[TechnoCheese]

I found the answer to question one! Completely by chance, actually. It is, indeed, length of small and large intestine that varies between fruit eating and grass eating species.

From the red-footed tortoise wikipedia page: "Forest-dwelling tortoises in the Chelonoidis, Indotestudo, Manouria, and Kinixys genera are omnivores with upper and lower intestines about the same length, while herbivorous genera such as Gopherus and Testudo have longer large intestines to digest fibrous grasses. Most omnivorous tortoises have no other specialized digestive structures, reflecting their generalized, flexible diet."

Here is the journal it references to support this.

Flexibility of digestive responses in two generalist herbivores, the tortoises Geochelone carbonaria and Geochelone denticulata - Oecologia

To test the prediction that digestive responses digestibility, intake and passage time-of generalist herbivores vary with different diets, feeding trials were conducted in Venezuela with two sympatric tortoise species, Geochelone carbonaria and G. denticulata. Three single-species diets (two...

link.springer.com

Unfortunately, this site does not accept my university, and I am not willing to pay $40, so I don't have access to the full text to verify this. Actually, I have friends that go to UT, so I'll see if one of them can grab it for me. It seems like an incredibly interesting read, and from the summary, seems to imply that intake and mass gain did not change when the tortoises were fed different diets, and instead the tortoise changed the way it digested the food to account for the change.

 

[TechnoCheese]

My UT friend came through. Here's the full study. I have yet to read through it completely.
It does not seem to mention other tortoise species when comparing intestine lengths, but does reference "Green turtles."

From the "Gut morphology" section:
"A flexible digestive strategy requires a gut morphology that permits differential processing of various types of digesta and does not fix the rate of passage. Both tortoise species studied here have a relatively simple gut morphology. The cecum is only an eccentric dilation of the proximal colon, and the small intestine is roughly equal in length to the large intestine (Guard 1980; Bjorndal, unpublished). The expanse of the small intestine is sufficient to deal with a diet that is digested primarily or exclusively by the herbivore's endogenous enzymes, and the large intestine is not so expansive as to cause unnecessary delay in passage of such a diet. However, the large intestine is capable of retaining digesta long enough to allow for extensive microbial activity. Reverse or anti-peristalsis may aid tortoises in adjusting passage rates to different types of food (Hukuhara et al. 1975). Varying the ratio of reverse to positive peristaltic contractions could adjust the passage rate of digesta in the large intestine. Geochelone carbonaria and G. dentieulata have small and large intestines of nearly equal length to maintain a flexible digestive strategy. One would expect that herbivores specializing in diets that are either digested largely by endogenous enzymes or largely by a hindgut microbial fermentation should have a higher proportion of either small intestine or large intestine, respectively. Evidence for this prediction is found in the green turtle, Chelonia mydas, an herbivore that specializes on a folivorous diet and depends on a symbiotic microflora in its hindgut (Bjorndal 1982). Green turtles have a large intestine about three times as long as the small intestine (Bjorndal 1979, 1985). "

 

[Chubbs the tegu]

you can't read anyhow

you can't read anyhow

Oh how ive missed u