Saturday, 24 March 2012

Turtles and Tortoises 2: Not just on Galapagos

Female D. dussimieri, Bristol
Today, when the phrase giant tortoise is heard, people immediately think of the giant tortoises of the Galapagos. That group of species however is just the largest surviving group of species in a world which once had giant tortoises almost everywhere, from South America to Southeast Asia. Away from the Galapagos, the largest tortoises alive today are the African Spurred Tortoise, Geochelone sulcata, a mainland species that can grow to 90cm long and a weight of 90kg, and the giant tortoises we have at Bristol, a group of seven Aldabra Giant Tortoises Dipsochelys dussumieri.

When human beings first colonised the islands of the Indian ocean, giant tortoises could be found almost everywhere. Species of Dipsochelys could be found on Madagascar, the Comores, and the Seychelles, and species of Cylindraspis lived on Mauritius, Reunion, Rodrigues and many of the smaller islands of the Mascarenes as well. As a result of human predation, fires, and introduced predators, especially pigs (which ate eggs), cats (which ate hatchlings) and rats (which ate everything) today the only place where giant tortoises dominate island ecosystems is the large atoll of Aldabra.

Male Galapgos tortoises displaying
Aldabra is around 34km long by 14km wide, with a central lagoon, and a total land area of 155km2. On this fairly small area live over 100,000 giant tortoises, which understandably dominate the entire ecosystem. Aldabra tortoises are grazers for the most part, and the result is a short ‘tortoise turf’ where any grasses grow on the islands. The climate of Aldabra is heavily influenced by the monsoon rains, and in the dry season the main cause of mortality is starvation and overheating in animals that cannot get to the central lagoon to wallow. They have a unique adaptation that enables them to suck up water through their nostrils from even the shallowest puddle, a very useful asset on an island with little standing fresh water.

It was thought at one time that tortoises grew so large on islands because of the lack of predators, but this is plainly not the case. Very large fossil tortoises are well known from many continental contexts, which had a full suite of predators, including mainland Europe and Africa. The reason that they are associated with islands today is simply because people are very fond of eating tortoises, and the large continental forms were mostly wiped out many thousands of years ago. Even where giant tortoises are found on islands, they did not necessarily evolve there – Aldabra has been completely submerged on several occasions and each time was re-colonised by giant tortoises from other islands when it re-emerged, probably from the Seychelles or Comoros islands.
Young G.sulcata. Note the spiky forelimbs
So why did they survive on Aldabra when their relatives on other islands were being eaten by hungry sailors? Probably the most significant cause is the lack of decent fresh water on the island – sailors preferred other islands with better supplies and the lack of water meant that pigs and cats had trouble surviving. It was a very close run thing however, and today a new threat has appeared in the form of climate change. Aldabra is only 8m above sea level at its highest point, and any rise in sea level could submerge most or all of the land, this time with no other source of giant tortoises to recolonise when it dropped again.

Tortoises are actually very good at dispersing across oceans, and the process still goes on. A few years ago an Aldabra tortoise washed up, still alive, on the coast of Tanzania, and judging by the barnacles on its shell it had been floating in the open ocean for several weeks. In the fluctuations of sea level during the last few million years, numerous islands have appeared above the waves and then disappeared again, and each time the land was probably covered in tortoises within a fairly short period. It would only take one gravid female surviving every thousand years or so to account for the recolonisation frequency that has been observed.

Today, Aldabra tortoises are key players in a major ecological experiment in Mauritius. Although the native Cylindraspis tortoises are extinct, the Mauritius Wildlife Foundation has been using them as ecosystem substitutes on Isle Aux Aigrettes, and now on Round Island, which are both close to Mauritius. There were actually two Cylindraspis species on Mauritius, one with a dome shaped shell and the other with a saddle back, and the Round Island experiment used both Aldabrans and Madagascar Radiated Tortoises, Astrochelys radiata, to see which would work best. The studies are in their early stages, but initial results are very encouraging, and Aldabrans have bred naturally or Isle Aux Aigrettes and will probably do so soon on Round island.

Given the history of what non-native animals can do to an ecosystem, the concept of using taxon substitutes for extinct fauna is understandably controversial. Management of reserves using domestic animals – for example grazing on nature reserves – is well established, but using wild animals for a similar purpose is potentially troublesome. Tortoises however are easy to control, can be readily removed if required, and perform many useful functions in an ecosystem. The most important is probably seed dispersal – many Mauritius plants produce their seeds at the base of the trunk at tortoise height, so they can be eaten and moved (slowly it is true) to another suitable growing spot. Tortoises also trample plants, and non-native plants often have much less resistance to such treatment than the native vegetation, which is also often less palatable than introduced weeds. However, tortoises can also disperse the seeds of non-native plants and invasive weeds, so the process needs to be carefully monitored.

For more on this topic, check out the MWF link on the list at the right and read their newsletters – well worth reading.

So what of the tortoises at Bristol? Our oldest two, a male and a female called Biggy and Twiggy, came to us in the early seventies from the university. Their exact age is unknown, but Biggy is probably at least 80 years old. Tortoises are famously long-lived, and one, a male called Adwaita, died in 2006 at the reputed age of 255. He alledgedly once belonged to Clive of India, and even if this is legend he had been at Alipore Zoological Gardens in Kolkata, India, since 1875. Our youngest tortoise, called Matilda, is only around 13 years old, and if she lives as long should see in the 22nd, possibly even the 23rd, century.
Adwaita, 2005
Although they live along time in captivity, giant tortoises, whether Galapagos or Aldabrans have as far as I know never bred in the UK. The problem appears to be the climate – breeding behaviour is triggered by the rains, and our climate does not provide the right triggers for breeding. Having said that, in some hot summer periods we have seen attempted mating by the two males we now have, so if we get the right kind of summer we may yet succeed in breeding them. Bristol has a good track record with breeding other species, so we live in hope.

Next week, another group of tortoises in trouble – the box tortoises of Asia.

(images from Bristol Zoo, wikipedia)

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