by Greg Mayer
Last month Matthew asked me about herbivory in reptiles, and part of my reply was that there are few or no reptiles that are exclusively herbivorous. The ones that came closest that I could think of were the true land tortoises (family Testudinidae, sensu stricto). I wrote, “some true tortoises are pretty close to vegetarian, but I’d still say they are at least facultatively omnivorous.” And sure enough, shortly after I ran into the following:
The video, of a giant tortoise (Aldabrachelys giganteus) on the island of Fregate in the Seychelles eating a noddy (Anous tenuirostris, a member of a genus of common tropical terns), accompanied a paper on the incident published last August by Anna Zora and Justin Gerlach in Current Biology. Wikipedia makes an amusing observation about noddies:
Anous is Ancient Greek for “stupid” or “foolish”. Noddies are often unwary and were well known to sailors for their apparent indifference to hunters or predators.
The sailors were right– the noddy, it seemed to me, though not fledged, could have gotten away. Perhaps it had a strong aversion to moving away from the immediate vicinity of its nest.
We’ve encountered island biologist Justin Gerlach before here at WEIT, where I noted his paper on an Aldabran tortoise’s ocean journey.
We’ve encountered Justin Gerlach before here at WEIT, where I noted his paper on an Aldabran tortoise’s ocean journey (picture just above). The Fregate tortoises are probably introduced from Aldabra, but the systematics of Indian Ocean tortoises is not entirely settled, and there have been a number of claims of tortoises surviving from the Seychelles populations that are usually thought extinct.
As far as reptile feeding in general goes, snakes, crocodilians, and the tuatara are exclusively carnivorous (in the broad sense of feeding on any kind of animals, including carrion), lizards range from carnivorous to omnivorous with a large herbivorous component (e.g. iguanas), and turtles are omnivorous, ranging from mostly carnivorous (e.g. snapping turtles) to mostly herbivorous (e.g. true tortoises).
Gerlach, J., C. Muir and M.D. Richmond. 2006. The first substantiated case of trans-oceanic tortoise dispersal. Journal of Natural History 40(41ā43): 2403ā2408.
Zora, A. and J. Gerlach. 2021. Giant tortoises hunt and consume birds. Current Biology 31: R989-R990.
Desert tortoises are nearly entirely herbivorous, but will go for certain caterpillars in season. Also occasionally bones or even carrion.
It would be foolish for an ostensibly herbivorous reptile to pass up some easy protein.
We think of our farm animals (pigs and chickens) as herbivorous, however, if given the opportunity they will eat just about anything.
I mentioned the bird-eating tortoise to a colleague at the Field Museum yesterday, and he mentioned that cows and deer eat birds, too. I’d never heard of cows eating birds, but knew that deer around the Great Lakes eat alewives, so I wasn’t that surprised. You Tube readily confirms both sorts of hoofed-animal bird eating.
GCM
Wasn’t the Andrewsarchus, about the most formidable mammalian predator, an artiodactyl? (Not to mention orcas and spermwhales). And the Entelodonts were also the type of animal you would not want to cross paths with. IIRC the Daeodon stood 2m (6.6 ft) at the shoulder.
I would be interested in seeing a tortoise eat a hare.
Everything is on YouTube so it’s worth a look. Might be fake though. š
The similarity of these giant tortoises to those on the Galapagos is striking. So I wonder if that is a case of convergent evolution, or maybe there were lucky tortoises that floated from one island to a far-flung island. It seems possible that the Seychelle and Aldabran tortoises are related since those are both east of Africa.
Iām pretty certain that there have been detailed DNA and anatomical studies showing that the Galapagos Tortoises are members of the South American clade, and the Indian Ocean tortoises of the African (or perhaps Asian) clade.
That’s correct. An excellent book on the subject is A Sheltered Life, by Paul Chambers (2006)
Are there any records of marine iguanas feeding on anything other than marine algae?
I checked some reliable sources, and they concur that marine iguanas also eat small invertebrates (crustaceans and grasshoppers being mentioned), feces (sea lion and their own), and sea lion afterbirths. Along with marine algae, a rather eclectic mix! Large males are the ones that feed on marine algae subtidally, while smaller iguanas feed more intertidally, and probbaly account for most of the non-algal food.
GCM
Thanks Greg. The more one learns about nature the more amazing it is!
Are there any records of marine iguanas feeding on anything other than marine algae.
(Apologies if this appears twice – the first attempt to post seemed to disappear into the ether).
A lot of reptiles exclusively eat insects (chameleons?), is that diet put under carnivorous?
Broadly, yes, but you usually see ‘insectivorous’ (or, as a colleague whose study lizards also ate spiders insisted, ‘arthropodivorous’).
Yeah, I was wondering why insectivorous (I thought arthropodivorous was good!) wasn’t listed in the last paragraph describing reptiles’ diets. Still don’t know why that type of diet was not mentioned. No biggy, just a curious query.
I used carnivorous to include insectivorous– “in the broad sense of feeding on any kind of animals, including carrion”: insects are animals. Most lizards are largely insectivorous (in the sense that includes among the prey spiders, etc., as well). There are reptiles that specialize on mollusks (molluscivores) and eggs (ovivores), but these would all be carnivores as defined.
GCM
I’ve seen giant tortoises in the Seychelles, on Alphonse Island. One time I heard them before I saw them, and on further inspection found a pair in the act of copulating. The male was literally bellowing. I wondered how they could get from island to island, and my fishing guide told me they can swim, and that he’d seen them in the open ocean. The photo of the tortoise with barnacles is confirmation.
I’m wondering why these tortoises make their perilous seafaring journeys. Most of them must die en route. Are they accidentally washed out with the tide, maybe while feeding near the surf? Are they driven by population pressure? Is there an adaptive, selfish-gene advantage to setting out? They’re evidently adapted to survive long times afloat — long enough to grow barnacles. That must be part of their niche.
I think islands consist of a subset of the set of nearby continental organisms, filtered by their ability to disperse over salt water. That’s how the organisms got there in the first place. So the plants and animals of an island would all be very good at dispersing across water (compared to the average continental organism), even if this ability played no role in their current lives; though I suppose if the traits that made them good travelers were expensive and had no benefit in thier current lives, they might eventually be lost.
That still doesn’t explain why the tortoises would actually want to swim. But the trait of not being afraid of swimming is also one of the traits favored by the “island filter”. Animals that found themselves washed out to sea frequently (whether voluntarily or not) should be better represented on islands than animals that are afraid of the water, even if that trait plays no role at all in the aniimals’ current ecological niche.
That makes sense, and explains why the tortoises are absurdly unfit to thrive on the continents.
Another curious fact is that they have massive carapaces that presumably evolved as armor in their ancestors, but they have no use for them as armor on the islands, except maybe for fighting other tortoises.
Are they ever seen swimming from the shore to nearby, visible islets to forage on the vegetation there? That would be one way in which they could end up in the sea and then if they misjudge a current perhaps they could find themselves on the open sea. Of course another likely way in which they might end up at sea would be as a result of a tropical cyclone flooding them out.
Didn’t you know that tortoises have another way of knowing, like the Polynesian navigators?
One might suppose the common snapping turtle to be the most carnivorous conceivable tortugan, yet i had one scale a fence intended to keep other turtles in. When it discovered the watermelon that I was feeding the sliders as a treat it self domesticated and would beg for more melon. Even after climbing back out of the turtle pen and having been gone for half a year it would return to beg for more.
I’m guessing you didn’t try to train it to take water melon from your hand!
A number of supposedly herbivorous tortoises will eat animal matter if they get the chance. Hermann;s Tortoise (Testudo hermanni) from southern Europe is one example. In captivity people starting feeding them on canned dog food but the growth rate outdoes the calcium supply for the carapace and when they have too much protein they develop a deformed shell. The Red-footed Tortoise (Chelonoidis carbonarius) of South America will also has a mixed diet including carrion.
I missed this post – really interesting especially the tortoise from Aldabra fetching up in Tanzania!