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Macrocephaly in turtles

Discussion in 'Water turtles' started by JTExotics, Jan 22, 2019.

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  1. JTExotics

    JTExotics Member

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    Macrocephaly is seen in lots of turtles in the southeastern U.S. (To easily find pictures of this, go to Instagram and go to these guys’ pages: groverbrown, alabamaturtles, luke_pearson2, and cheloniagodwin) However, I’ve seen it in a few Asian species as well.

    Macrocephalic Revees turtles (although other species may be present in the same photos)

    7CDAC739-14E3-40F2-8F0F-DB49B7D3DEBA-328-00000022F4B029E6.jpg

    IMG_1408.jpg

    Macrocephalic Malayan Flat-Shelled Turtle:

    EA5ADF27-7370-4105-80E9-80FF11173FEC-328-0000002284E3C983.jpg

    (Photo from http://turtlesurvival.nonprofitsoapbox.com/component/taxonomy/term/summary/109/7#.XEegF7pOmEf )


    I think it can happen in more species than you will typically see, it seems to mainly depend on the diet of the animal, although genetics have some influence as well. I’ve even seen it in a red eared slider that are only snails it’s entire life:

    IMG_1409.jpg

    (Not my photo)
  2. KarenSoCal

    KarenSoCal Well-Known Member

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    Eating snails = too much of something or too little of something? How would it affect the size of the head?

    Do these turtles live normal lifespans?

    I know nothing about turtles, but this is fascinating!
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  3. cdmay

    cdmay Well-Known Member 10 Year Member!

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    Yeah, an interesting subject.
    Loggerhead musk turtles in Florida can show a pronounced difference in adult head size depending on the river system and diet. Adult male loggerheads will show the most extreme variation but why is that? Why is it that males show this and not so much the females?
    Conversely, FEMALE Reeves turtles in China display the oversized heads and larger body size than their male counterpart. Why?
    In the genus Graptemys usually the females are the ones with oversized heads. But this has been shown to be because of diet. The males remain insectivorous as do the juveniles, the females graduate to eating snails and clams, i.e. durophagous. Um, I think I spelled that right? Anyway, the diet makes the head get bigger, or remain the same proportions as a juvenile.
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  4. enchilada

    enchilada Well-Known Member 5 Year Member

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    Eggs need calcium—— >clams and snail shells are calcium ——> female turtles eats more shellfish than males —-> MACROCEPHALY
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  5. cdmay

    cdmay Well-Known Member 10 Year Member!

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    Sorry about the triple posting above. My IPad wasn’t showing that my post had attached.
    Anyway, another factor is genetics. Some species are genetically predisposed to produce individuals with enlarged heads if the right environmental conditions are present.

    For example loggerhead musk turtles in the Chipola River — the bottom of which is fairly carpeted with small clams and snails, will develop enormous heads as they get older.
    But captive hatched and raised loggerheads from that same river system will grow to adulthood with only moderately sized heads.
  6. JTExotics

    JTExotics Member

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    I’ve also heard of macrocephaly in some populations of alligator snapping turtles, further exacerbating their already giant head size. However, I’ve never heard of or seen it in common snappers. That probably has to do with diet. Unsurprisingly, I’ve never seen or heard of it in any tortoises. I wouldn’t think it occurs in tortoises, unless some individuals continue to eat hard shelled nuts and fruit throughout their life. I know the extinct meiolaniids had very large heads, although I don’t know if that was for defense or their diet.
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  7. Michael231

    Michael231 Member

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    This is a great thread!

    Everyone is bringing up very interesting points about this subject!

    For the past ~3 years I have worked on a population of odoratus that display extremely megacephalic heads in New Braunfels, TX. However, if you move over to the Guadalupe River located just a few miles Northeast of the specific site, to my knowledge, there are no megacephalic odoratus there. Interestingly, sometime during the mid-late 20th century, Asian immigrants to the area brought over Ramshorn snails to the river we work on (separate from the Guadalupe). Since their release, the snails have taken over the area, and we have proven the odoratus have a particular penchant for eating them. At this site, the odoratus get so megacephalic that in some extreme cases the keratin of their rampotheca begins to fall off and bleed.

    I am particularly interested in what @cdmay points out, in that it is almost always the larger sex that is durophagous. Furthermore, the larger sex in aquatic turtles is generally the female, who needs to produce eggs (i.e. a larger volume would allow for more eggs to be produced and stored). Could it be that these “broad-headed” turtles are mainly developing in the female sex to favor calcium consumption from snail and mollusk shells, as to make the egg shelling process less physically demanding (on the females nutrient reserves)? While this statement seems compelling to me, I should add that at our New Braunfels site we do see males with very broad heads. So this may not (or not always) be the case. I’m just throwing the idea out there. However, Graptemys are sexually dimorphic in size as well, whereas odoratus are not, or very weakly. Maybe a competitive property is at play here, whereas it is lacking in odoratus.

    As for the pattern of megacephaly over different families, perhaps we are seeing evolution in real time here. All of the species mentioned above are known to be avid mollusk eaters. Kinosternon minor has a large range over much of the southeast, and like @cdmay mentions, in the Chipola, it eats mollusks and develops large heads. Another study on minor determined that the species eats invasive Corbicula snails that were introduced into its waterway. However, not all minor get megacephalic in the larger sex. I have seen pictures of large individuals that look quite normal (from multiple springs around central FL). It seems like the trend for megacephaly follows either the species inhabiting an already mollusk inhabited waterway (i.e. Graptemys, Malayemys, Sternotherus) or inhabiting an environment where mollusks have been introduced (i.e. Sternotherus). In the latter the range of individuals displaying megacephaly is generally broader, where some larger individuals don’t display the trait, possibly as a relict of turtles slow generation turnover (assuming genetics plays some role here), whereas in the former, megacephaly is generally sexually biased towards the larger sex, and always pronounced.

    This is a very interesting topic, and I’m curious what you guys think about the above...

    Lastly, I like the Meiolaniid reference, and based on the size of some of their skulls I’d say they were definitely megacephalic, but they also had horns on the back of their heads, so I’m leaning towards saying these animals just had large noggins with relictual horns used in combat.
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  8. Michael231

    Michael231 Member

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    Al Redmond cites a population somewhere in Georgia that had Macrochelys with giant heads. He says he thought this was because the turtles had a diet high in mollusks.
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  9. JTExotics

    JTExotics Member

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    That makes sense. They probably looked ridiculous!
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  10. JTExotics

    JTExotics Member

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    Yeah, I was thinking that too, but I think their diet must have played a role in those large heads
  11. Michael231

    Michael231 Member

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    They probably did..

    I’m imaging a cross between a tortoise with an overgrown beak smashed on a snappers face!
  12. Michael231

    Michael231 Member

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    What do you think that they ate? I’m not sure anyone’s really looked into that....

    Any thoughts?
  13. JTExotics

    JTExotics Member

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    I go back and forth. I don’t see them being predators, I mean, unless they are somehow fast and maneuverable . Maybe they used that massive head to crush large branches, or maybe they somehow managed to attack other animals, kill them, and crush their bodies with their skulls. That’s Probably not true at all, but it’s just a thought. Or maybe their heads were just for defending themselves against terrestrial crocodiles and the 23 foot, 1,500 pound Varanus priscus. Chelydridae have large heads to act both as a defense and an offense, maybe meiolaniids needed more defense than their ankylosaur-like tails against prehistoric Australian predators
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