Reader Dennis Hansen, whose photographs of Aldabra (a large coral atoll that’s part of the Seychelles in the Indian Ocean) and its tortoises and crabs have appeared on this site before (see, for instance, here, here, here, and here), has now sent a video of his work on the island’s tortoises, “Ghosts, giants, and resurrection.” It’s a wonderful video made by a man who truly loves his tortoises. (See also his posts on Mauritius here and here.) Dennis’s notes:
Here’s a slightly unusual set of visuals for your consideration for the Reader’s Wildlife Photograps section – in the form of a video.
The video is from a recent private & annual TED-like event here in Zurich, where business people, artists, and scientists get together and discuss ideas and stories presented by artists and scientists. It’s usually great fun and a very broad mix of topics and people. The curator of the event, Rolf Dobelli, generously provided me with an opportunity to share our work with giant tortoises, resurrection, and islands with the audience.
As the talk contains almost nothing but photos (and a few video clips of the giants & what they’re up to!), I thought it could be an interesting semi-personal way to share some wildlife photos with your & your readers.(like you, though, I absolutely hate listening to my own voice, so I hope I remember what I said in case you or anyone else has questions…).
Afterwards, when discussing tortoises with attendants, a lot of people enjoyed touching and examining the carapace I’d brought along, leading to discussions about tortoises and ecology in general. Turns out that show & tell works even for CEOs
Well done. I’ve had the privilege of seeing these tortoises on Alphonse, another Seychelles island. I’ve seen the copulating. The male is very loud!
A marvelous story! One can certainly see that Dr. Hansen has found his niche, and it is an important one.
So why do islands have tortoises of unusual size?
It’s a Mega-niche! I certainly enjoyed watching that.
“So why do islands have tortoises of unusual size?”
Foster’s rule with little to no predation pressure and relaxed food constraint?
“… he proposed the simple explanation that smaller creatures get larger when predation pressure is relaxed (due to the absence of some of the predators of the mainland) and larger creatures become smaller when food resources are limited (due to land area constraints).[4]”
With a caveat “Case also demonstrated that Foster’s original conjecture for the reason all this happened was oversimplified and not completely true.”
[ https://en.wikipedia.org/wiki/Foster%27s_rule ]
Makes me wonder how that atoll could provide enough food for 100k large individuals and how the population is regulated?
[The shells around the shelters seem to imply the shelter capacity is what sets the population size. Which at a guess could mean they used to starve to death at times (small tortoises) but now they face the luck of the draw (large tortoises).]
I wonder whether it’s even been considered to build artificial shelters.
Thanks Mark! -actually, the story is maybe slightly different than normal island size ‘rules’ – and somewhat simpler: Continents used to have giant tortoises, too, and even much larger ones. However, they died out in the last few million years. To what extent humans/hominids were a causal factor is not certain in many cases, so I won’t try to speculate too much.
What is still typical island-style (evolution-wise) is the loss of anti-predator adaptations: The shell of island giants can be down to only a few mm thick, whereas the shell of comparative (and also of much smaller!) continental tortoises is muc thicker (need to survive teeth & possibly trampling).
Absolutely fabulous! Great talk, fascinating biology. The idea of “rewilding” of Zurich (or actually somewhere out in the suburbs, I suppose) is intriguing. The Chernobyl Exclusion Zone is coming to look more and more like “Pleistocene Park”– wolves, lynx, wisent, true wild horses, red deer, boars, moose (= elk), roe deer, and brown bears now live there– all it lacks are mammoths! Three quick questions for Dennis if he’s following the comments. 1) The Rodrigues tortoise picture made it look like a Galapagos-style ‘saddleback’ tortoise– were they saddlebacks, and will it make a difference that the introduced Aladabrans are dome-dacked (and hence can’t reach high up in the vegetation)? 2) Is there any truth to those stories that Rodrigues or other supposedly extinct Indian ocean tortoises have been found surviving as an individual here or there in somebody’s garden? 3) Have you looked in the Aldabran resting caves for fossil remains, which may allow you to date the usage of the caves, and any changes in the tortoise’s numbers or morphology over time?
Thanks for the kind words, Gregory. Happy to answer: 1) Rodrigues was home to two tortoise species (Cylindraspis spp.) – one dome-shaped, and one saddle-backed. The dome-shaped one was on the smaller side for a giant, and even the saddle-backed one was not as large as the Aldabra or Galapagos ones. When an adult male Aldabra stretches way up, on three legs, can reach higher than 1m. Various lines of evidence suggest that this is comparable to the saddle-backed Rodriguan one – most interestingly that plant species in many different families all have heterophyllous leaves (different shapes of young and adult leaves). Curiously, most of these plants change their leaf shape/colour around 1-1.4 m above ground, sometimes quite dramatically so. Even more notably, while Aldabran tortoises don’t seem to like the young leaves close to the ground, they often much with gusto on the adult ones, higher up, if you feed them to the tortoises directly.
2) I sadly think that all five Cylindraspis species are entirely extinct (those that lived on Réunion, Mauritius, Rodrigues). For the Aldabrachelys group (Madagascar, Aldabra, Granitic Seychelles), it may be a different story for some genes of extinct species/lineages still surviving, but not to my knowledge at the species level. All DNA studies so far have failed to demonstrate any significant difference between wild Aldabra tortoises and the various groups of captive individuals (despite the insistence of some/one biologists to the contrary).
3) We’d planned to dig down into the deposits in the back of the cave. The cave is not very deep, around 5m deep, and only 1-1.5 m tall. However, that year it rained like mad for a week or so, and the upper layer of the deposits turned into a muddy goo. We thus could not dig down and obtain older material (for C14-dating) without too great risk of contamination. Maybe next year…! (cross your fingers).
Dennis– Thanks so much for the answers. Sounds like Aldabrans may be the best single species replacement for the extinct saddle- and dome-backed Rodrigues natives. Sorry the stories of survivors haven’t panned out, and good luck with next year’s excavations!
Greg
I had no idea there were still islands with a large natural population of tortoises. Very encouraging to see. Great show.
He mentioned something about the effect of global warming on the island. I don’t think it was a complete sentence, but I guess he was saying if there is significant sea level rise, the only land left will be the 20 meter sand dune. Rather small for all those tortoises.
That is what I got from it.
Yup, sea-level rise is the main threat to Aldabra in the near future. While conventional wisdom has it that the average height above sea level of the atoll is 8m, I think that quite large swaths of the tortoises’ favoured feeding habitat (their turf; hehe) lies lower than that. We just flew two drones over a large section of this part of the atoll, and will be building a 3D model of the surface of this part quite soon, and will know more then.
Thanks so much, Dennis, for your efforts out there. I had been thinking how hard it’s going to be for humans on island in the future when the sea rises. But, of course, they can move away relatively easily. I hadn’t thought ’till now about the biota. That’s sad. Perhaps conservation efforts can help by moving at risk populations to safer places.
Sub
Great talk. Thank you!
Glad you liked it; thanks.
That was great! It’s wonderful to know there are still such big populations of tortoises out there.
I hadn’t expected to watch right through, but I did. Excellent talk!
What struck me about the tracking of the tortoises, though, was that individuals were very much creatures of habit. They might wander several kilometres, but always along the same route (for each individual).
cr
Exactly – that is certainly very much the case for the three tortoises shown. I’d really like to get a lifetime-track of a tortoise (from hatchling to death). How & when does it’s mental map of such a large area arise? Walking along an almost straight line for 4-5km, the same route, but with more than a year in between – that’s quite impressive for a ‘stupid tortoise’. Some tortoises have annual migrations from the coast to the inland turf, others pretty much move around in the same 300×300 m area in the four years since they were tagged, or move in and out of the mangroves in a monthly pattern (tides?). We have really only started scratching the surface of these magnificent creaures 🙂
Could their navigation and memory be related to the Earth’s magnetic field? A simple experiment would be to mount a simple coil on the animals skull and set up a small EM field and see if the migration is disrupted. Cruel, I know, but curious scientists need to know. 😎
As well as being a fascinating talk, the speed of delivery is a great help in holding one’s attention. I wish more lecturers were aware of this 🙁
There’s a giant African spurred tortoise that walks the streets of shitamachi Tokyo. His owner bought him not expecting him to grow so large, I think. There’s a YouTube film you can Google – well, several, in fact, but Google the following and it should be in English: YouTube.One Man And His Pet Tortoise in Tokyo.
I much enjoyed the video, both for the beasts and the ideas. I have always liked tortoises, and turtles – I remember standing for about an hour watching a green turtle swimming at the aquarium at London Zoo: it was so extraordinarily graceful.