SOMETHING TO GET BIG-HEADED ABOUT

How keepers at London Zoo successfully bred Platysternon megacephalum.

The big-headed turtle (P. megacephalum) is a Critically Endangered species of freshwater turtle from East and Southeast Asia. Although there are three distinct subspecies of big-headed turtle, the ancient, monotypic genus Platysternon dates back some 75 million years, making it a remarkably unique species of turtle. Because of this, it is ranked number 19 on the ZSL EDGE list as it is “Evolutionary Distinct and Globally Endangered” and the species remains on the edge of existence. In 2022, keepers at London Zoo became the first in Europe to breed the species, marking the first step in a potentially monumental captive-breeding programme.

In-turt-duction

Big-headed turtles are freshwater, semi-aquatic turtles from China, Vietnam and northern Thailand (as well as a small distribution in Cambodia and Myanmar). They can be found at elevations up to 800m and in a variety of tropical and subtropical climates. Adults are typically very drab in colouration, with menacingly powerful jaws and flat carapaces. They are also solitary and reasonably territorial turtles that will occupy individual sections of rivers or streams. Being predominantly aquatic and growing to an impressive size of around 40cm, they are frequently targeted by poachers for meat. In some parts of their range, they are also smuggled into the illegal pet trade.

Back across the pond

The four big-headed turtles at London Zoo arrived with keepers in the winter of 2018, after being seized from smugglers who had exported them as “children’s toys”. They are thought to have originated in China but were destined for Canada before being intercepted at customs and held temporarily at Toronto Zoo.

“When they came in, they were in a bit of a rough way” explained Kimberley Carter, Reptiles Keeper at London Zoo. “They couldn’t shed that well, so they had a lot of retained scutes and because of that, they had shell lesions. It’s been an ongoing process trying to get them back into a healthy state as each time they would shed, it would reveal a new lesion. Thankfully, we’re pretty much sorted with that now and they’re all looking amazing.”

The team at London Zoo did a genetic analysis on the turtles and found that they are of the subspecies megacephalum megacephalum. “Because animals are often harvested from different areas, it is really difficult to tell exactly where they’re from” adds Kimberley. “They all came in as adults, so they’re also of unknown ages. The big male was probably fully-grown, but the smallest female is likely to be a sub-adult. She’s grown a lot since 2018.”

As well as being harvested for meat, big-headed turtles are also prized by poachers due to demand from the illegal pet trade, potentially fuelled by their unique appearance. Although they may look very similar to some of the snapping turtles of North America, they are of a completely different lineage. Kimberley continued: “They are extremely unique. For a start, they can’t retract their heads into their skull. Because of this, they have evolved really strong armour and really strong jaws. This is partly because of the prey they eat, but also as a defence. Big-headed turtles are mostly recorded in rocky montane streams so they use their flat carapaces to wedge into hiding places. Surprisingly, they are also fantastic climbers!

In the wild, they climb over rocks and up streams but they are capable of climbing almost vertical surfaces – I’ve had this problem a few times! Normally it’s the one big male who will climb during the breeding season, but I have also had it happen with the juveniles too!”

The big-headed turtles are housed in large semi-aquatic enclosures that incorporate a water section with significant movement and several hides, as well as a land area with loose substrate and a drainage layer. Although only one individual is on show at the zoo’s Reptile House, husbandry remains the same for both adults and juveniles, across all enclosures. They are provided with a basking spot of around 26-30°C and a UVI of 4. “Their basking spot is perhaps a little cooler than you would provide other turtles” added Kimberley. “They do bask fairly frequently, but they are a predominantly aquatic species.”

Big-headed turtles spend much of their time submerged in fast-flowing, cool water. At London Zoo the water is run through a biological filter, water chiller and is also treated with a UV steriliser as well as being changed three times a week to prevent bacterial growth that could encourage shell lesions.

Kimberley continued: “Where they come from, it’s really high quality, fast flowing water, so it’s important that we replicate that in our care. You can quickly notice if the water is not perfect as big-headed turtles will soon show signs of superficial health problems. Treating the shell lesions was a bit of a problem. We had to ensure that the water was perfect, but then we tried various ways to treat them. We started dry docking and using topical treatments but soon found that a hands-off approach seemed to be more effective. We added salt to the water also, which seemed to really help. Three parts per thousand of aquarium salt creates a bad environment for bacteria to grow without harming the turtle. It helps to fight bacterial problems.”

A shell-thy lifestyle

Keepers at London Zoo used information collected in big-headed turtle habitat by our colleagues from the Asian Turtle Program of Indo-Myanmar Conservation, to provide seasonality within their husbandry practices. Not only did the replication of wild seasons in a captive environment encourage the development of healthier animals but it was also crucial to the breeding process. “We used a whole mix of knowledge to breed the big-headed turtles” explains Kimberley. “We have used information from Prospect Zoo, as well as private keepers, but we also have a strong relationship with conservationists in the field. This helped us fine-tune our husbandry. For example, initially, they were just fed shellfish, fish and other high-protein foods. However, looking into their wild diets, they actually eat a lot of plant material. About 50% of their diet is made up of plants. The main plant they are known to consume in Hong Kong is a round fruit that looks almost like a conker/ avocado mix (Machilus spp.). It has a small outer fruit that is really tough. They are actually very good seed dispersers for this particular species of plant. There is also some seasonality in their diet. In the wet season they will eat more fish, whereas, in the dry season, they will go for more plant materials. Males also tend to eat more mammals and other reptiles too, whereas juveniles will eat a greater amount of plant matter. I didn’t expect them to be so omnivorous!”

Kimberley and the team at London Zoo produce a special “turtle jelly” that contains varying amounts of nutritionally dense plant matter. “They’re not very good at eating their vegetables” added Kimberley. The turtle jelly is a mix of shrimp, pumpkin, carrots and additional dietary supplements such as Nutrobal and Aquavits. This allows the team to provide more nutrients for the females in the run-up to breeding season.

After the animals have been conditioned to hibernate, the keepers must carefully induce a winter “cool” period. By using a chiller, the water temperature is gradually lowered from a summertime average of 25°C to a frigid 8°C. “Part of the way to encourage breeding is to hibernate them” added Kimberley. “This helps with sperm production, but it also helps the females regulate their follicles. Then, the breeding is super quick but super exciting! They’re very territorial and because of their really big jaws, they can cause a lot of damage. So, we clear out all the furnishings in their tank, then introduce the male to the female. The male will do a little head bob and circle the female. She will waggle her tail if she’s receptive. Then, the male will jump on her and flip them both on their side. During this time, she normally tries to bite him a lot so he will stretch his head out of reach but occasionally bite too. Then, every time we’ve done it, it’s taken them about 8 minutes, so it’s really quick! The big male is always straight on it, but the smaller male is a bit more hesitant. I try every combination that I can, but the smallest female has been a little too small so we’ll probably try that combination this year. I repeat until there are signs that she is not receptive. Anecdotally, a bit of aggression from the female is a good sign that she is no longer receptive and the breeding was probably successful.”

Soon after breeding, the female will begin to search for a location to lay her eggs. In the wild, big-headed turtles will rarely venture more than one meter from the water's edge and therefore, they have adopted a method of burying their eggs remarkably shallowly. This may be because of the high-drainage soil that surrounds the stream. By burying the clutch in shallow soil, they are less likely to become damaged by excess moisture in the nesting medium. “Clutches seem to be size-dependent” added Kimberley. “So, bigger females tend to lay more eggs and they can have anywhere from one to nine eggs in a clutch. Our female has had 3-4 eggs each time, which, based on her size, seems about right.”

Un-be-leaf-able youngsters

Adult big-headed turtles can reach sizes of up to 40cm in length, although most of this is head and tail. Adults are robust, drab creatures whereas juveniles are shockingly ornate. They typically exhibit bright colouration, with distinct carapace patterns. The youngsters still have a proportionately enormous head, but this is adorned with two post-ocular stripes, dividing their olive skin from their peach-coloured underside. Both adults and juveniles have remarkably flat shells, but this is perhaps more noticeable in the hatchlings. Interestingly, Kimberley has noticed several behavioural differences that may tell us more about the species’ natural history.

Kimberley told us: “The juveniles are quite similar to the adults, but there are a few differences… Initially, they would cling to the rock faces and sort of dangle in the stream but at a certain point, they become more like adults and walk along the bottom. I am hoping to do some research into this with our next offspring, to track exactly when this change happens. When the babies first hatch, they’re so brightly coloured and they tend to dangle from rocks, which I expect is them mimicking leaves. Then, after a certain point, they lose their bright green and start walking on the bottom a lot more. Perhaps that’s to look more like a rock?”

A world of turt

The big-headed turtle used to be common in certain parts of its distribution, but it is now rare throughout its range. It is assessed as Critically Endangered on the IUCN Red List and has been included on CITES Appendix 1 for over a decade. They are consumed by people in China and are regularly harvested for food as well as for the international pet trade. Populations in Thailand and Hong Kong are presumed stable, but the species is undergoing rapid declines in population sizes across other parts of its distribution. It is suspected that the population of bigheaded turtles has been reduced by over 90% in the past three generations (90 years).

A site comparison study in 2009 – 2011 of freshwater lakes, rivers and streams in Hong Kong (Sung et al. 2013) found that big-headed turtles Platysternon megacephalum grew larger in a stream inside a fenced, patrolled, protected area without turtle harvesting compared to turtles in four national park sites where illegal harvesting is believed to take place. The study also found that the growth rates were consistent across both males and females. This suggests that harvesting may be impacting turtles other than just causing population to declines. Individuals in exploited populations are on average, smaller than non-exploited populations and this may be detrimental to these populations.

London Zoo also works with the Asian Turtle Program of Indo-Myanmar Conservation, an NGO that has invested a lot of time and resources in understanding the threats facing big-headed turtles. ZSL’s Curator of Reptiles and Amphibians, Dr. Benjamin Tapley, explained: “The Asian Turtle Program of Indo-Myanmar Conservation help to rehabilitate the hundreds of big-headed turtles seized by local authorities and housed at the Turtle Conservation Center in Cuc Phuong National Park, Vietnam. This collaborative project, implemented by the Asian Turtle Program has implemented a vital health and genetic screening protocol at the centre, helping to safeguard wildlife at reintroduction sites from the spread of any pathogens that may be carried by the rescued reptiles when they are released - this has resulted in more than 350 rescued turtles having been safely released into protected areas over the past three years. Our international team is also working with local communities to help them understand that it is illegal to hunt and collect them, how vitally important these amazing animals are, and that together we can bring them back from the brink.”

The turtles that were hatched at London Zoo have been named after three colleagues from the Asian Turtle Program in Vietnam, in recognition of the collaborative efforts taking place to save this unique species: Thuy Thu Nguyen, Tim McCormack and Ha Hoang. Ha is a ZSL EDGE Fellow who led on the translocation of rescued Bigheaded turtles in Vietnam said: “Having these turtles hatch at the Zoo not only adds to the global numbers of this unique species but allows us to learn more about them and their breeding habits. This is vital information shared between colleagues that informs the turtles’ rescue and rehabilitation.”

Pooling resources

Perhaps surprisingly, a lot of what zoologists know about the captive management of big-headed turtles comes from the private trade. Although they are a target for unscrupulous smugglers, Platysternon megacephalum was only listed under appendix 1 of CITES in 2013, suggesting that there could be many breeding adults already within the trade that were legally imported decades ago. “There are some fantastic keepers that have done it properly, especially in the United States” Kimberley explained. “I have actually spoken to a lot of them who have really helped in what we’re doing here, but sadly the illegal pet trade is still a thing. If we can create a good captive population in zoos and the really good private keepers can produce a captive population for the trade, then I think that’s a good way of driving down demand.”

Although the species has been bred in several North American institutions and in large numbers at rehabilitation centres in Asia, the successful hatching of three turtles in an EU zoo marks an important development for their captive care. The team at ZSL may have reached an important milestone in establishing a healthy genetically diverse captive population in the UK, but they are excited for what is to come. Kimberley concluded: “I absolutely love these turtles and now I kind of feel like a bit of a pioneer! It’s really early days but eventually, I hope we can create a good breeding population of these animals in UK zoos. One of our animals from last year has gone to Newquay. We have three more that are going to Whipsnade and next year we will probably export and import animals to keep a good genetic variety. It’s exciting to be the first ones in Europe to push it!”