The Flatback: Australia’s Own Sea Turtle
BY MARK HAMANN, COLIN LIMPUS, KELLIE PENDOLEY, CHLOE SCHAUBLE, JANNIE SPERLING, AND JEANETTE WYNEKEN
A flatback turtle comes ashore to nest on Eighty Mile Beach in Western Australia. Although the flatback is a medium-sized sea turtle, its eggs are the size of the mighty leatherback. © Calen Offield
Until 1988, the Australian flatback was still so poorly understood that it was considered to be a congener of the green turtle, despite what we now know are vast differences between the two species. Following decades of research, Colin Limpus and his research group at Queensland Parks and Wildlife Service, Australia, redescribed the species, giving it a genus of its own, thereafter known as Natator depressus. In the short time since then, science not only has renamed the flatback, but also has seemingly rediscovered Australia’s own sea turtle.
Flatbacks are unique among sea turtles in form and function, in life cycle, and in life history. In morphological terms, their pancaked bodies and flared carapace margins—like hydrofoils—are not observed in any other sea turtle species. Flatback shells are covered with only lightly keratinized scutes that tend to be softer and more susceptible to injury than are those of other hard-shelled turtle species. Thus, the sensitive flatbacks rarely nest at beaches that are fringed with coral reefs, preferring open mainland beaches or inshore continental islands where they are less likely to collide with obstructions or to be scraped by an abrasive substrate. Although they nest on a very diverse range of beach types, many of the largest flatback rookeries occur on low-energy beaches that are often sheltered behind broad intertidal zones.
“Flatbacks are unique among sea turtles in form and function, in life cycle, and in life history.”
Though flatback turtles forage across the northern Australian continental shelf and into the territorial waters of Papua New Guinea and southern Indonesia, the species nests only in Australia, making it a truly Australian animal. Indeed, it is the only sea turtle species able to claim endemism to a single country. Because of their Australia-centric life history, flatbacks have one of the most restricted breeding distributions of all sea turtles, second only to the Kemp’s ridley that nests solely in northeastern Mexico and in Texas, U.S.A. Although small in global terms, flatback distribution still spans three enormous states in northern Australia, with nesting scattered across numerous beaches from Mon Repos in southeast Queensland across the Northern Territory to the Pilbara region of Western Australia (see map of flatback distribution below).
Large segments of the northwestern Australia coastline remain un-surveyed, but major rookeries have been confirmed recently. Among those rookeries, the east coast of Barrow Island has 1,700 nesting females per year, Mundabullangana Beach on the mainland coast has 1,600 nesting females per year, Eighty Mile Beach has hundreds to thousands of nesting females per year, and Cape Dommett has hundreds to thousands of nesting females per year. Furthermore, the Montebello Island group, the Dampier Archipelago island group, and the island chains between Dampier Archipelago and Exmouth Gulf each host flatback breeding populations in the same hundreds to thousands range (see map, pages 24–25).
Despite being intermediate-sized sea turtles as adults, flatbacks lay eggs that match the size of those laid by the mighty leatherback, giving flatbacks the honor of laying the largest eggs and having the largest hatchlings per adult female body size. Genetic and tagging studies have shown that the species comprises at least six genetic stocks, each roughly tied to a distinct nesting area. Across genetic stocks, there are some differences in flatback morphology and in the timing of their breeding. For example, adults from the eastern coast of Queensland (the East Australia Stock) breed through the Australian summer, are larger, lay fewer eggs per clutch, lay larger eggs, and produce larger hatchlings than do those adults in the neighboring Gulf of Carpentaria Stock, which breed in the winter (dry season) months. Across their nesting range, it is not uncommon to observe flatbacks nesting under the blaze of the Australian sun, unlike other large-bodied, nesting sea turtles that typically avoid the burn by nesting under the cover of darkness.
Although there are a few longer-term studies in the Northern Territory and Western Australia, most information about flatback population dynamics comes from Queensland sites. Using long-term tagging studies at Peak Island in central Queensland, we can estimate that adult female flatbacks breed, on average, every two to three years and have a mean reproductive life of 10 years. In addition, new first time breeding females account for approximately 14 percent of the annual nesting population each year. Similar to other sea turtle species, female flatbacks also show strong fidelity to nesting sites, and data from several long-term studies show that only a small percentage of nesting females change nesting beaches within a breeding season or between breeding seasons.
Flatback nesting beaches are not adjacent to major ocean currents that would disperse hatchlings, as occurs with other species. Indeed, in the 1980s, Terry Walker and John Parmenter found that flatback turtle hatchlings lack an oceanic dispersal phase altogether and, instead, disperse through the inshore waters. Thus, hatchling flatbacks may have evolved a swimming behavior that differs markedly from other species that rely on currents for dispersal to open-ocean habitats. Little flatbacks swim consistently during the 24 hours after emergence, and then they gradually switch to a mostly diurnal swimming pattern. They dive frequently, spending little time at the surface, and they tend to spend more time submerged during dives as they age. This behavior suggests that flatback hatchlings, like green and leatherback hatchlings, might undergo physiological shifts (such as a decrease in residual yolk, an increase in blood volume and lung capacity, or a change in oxygen carrying pigments) with increased size and age. Their dispersal patterns and residency in continental shelf waters near the Australian coast make flatbacks the ultimate “homebody” sea turtle.
Although we know that flatbacks spend their adolescent years in nearshore areas, an enormous knowledge gap exists regarding the locations and characteristics of habitats used by flatbacks at all life stages, across the entirety of the species’ distribution. Flatback turtles do not form the highly visible nearshore mating aggregations that are commonly observed in other species. And in much of their distribution, flatbacks swim in the same waters as the saltwater crocodile (Crocodylus porosus), an aggressive and dangerous predator whose presence precludes sensible humans from diving and snorkeling nearby. Because of the crocodile threat, the study of the flatback’s marine habitats and behaviors requires approaches that are creative, opportunistic, and nonlethal (to humans or turtles). In fact, most flatback in-water behavior information has come via satellite, with researchers collecting data from the safety of their offices.
Researchers attach a satellite tag to a flatback turtle on Cemetery Beach in Port Hedland, Western Australia. Satellite tracking studies are shedding new light on the marine habitat use of flatback turtles, the only sea turtle species whose entire life cycle occurs on the continental shelf. © Calen Offield
In a satellite tracking program in the Pilbara region in 2005, 16 satellite tags were deployed on nesting flatback turtles at Barrow Island and at Mundabullangana Beach on the mainland (280 kilometers [174 miles] east of Barrow Island). The Pilbara satellite tracking program has shown that flatback turtles use shallow, nearshore (mainland coast), inter-nesting habitat—regardless of whether they nest on the mainland or offshore on Barrow Island.
In addition, not all of the flatback turtles tracked showed strong foraging-site fidelity. Some traveled constantly along the Pilbara and Kimberley coastlines, stopping briefly for a few days over unidentified seabed features before moving on to other sites. Following the nesting season, satellite tracking results indicated that between 41 and 72 percent of the recorded locations for foraging flatback turtles were in waters 50 to 100 meters (54.5 to 109.0 yards) deep and between 100 and 1,000 kilometers (62 and 620 miles) from the nesting beaches. This range of distances is similar to those recorded in eastern Australia from tag recoveries, which also show migration distances up to 1,300 kilometers (806 miles).
Additional information on foraging flatback turtles has come from trawling bycatch records, limited mainly to eastern and northern Australia, which were collected by the Queensland Department of Primary Industries immediately before the introduction of bycatch reduction devices (the Australian equivalent of the turtle excluder devices of the 1990s). Those records of bycatch of flatback turtles in trawl nets brought to light another interesting observation: researchers working on turtles and bycatch issues in Queensland reported that flatbacks seemed to survive forced submersion better than other sea turtles—twice as well as loggerheads. This finding prompted questions about what makes flatbacks such successful survivors.
A study of flatback diving behavior and respiratory physiology was initiated between 2000 and 2002 to investigate whether flatbacks are particularly well adapted for long dives. The study found that adult flatbacks most frequently dove to the ocean bottom and that they spent 57 percent of their time submerged on the seafloor. During the dives, turtles presumably remained inactive because the dive records showed that they appeared to passively ride the up-and-down cycles of tides, requiring little time at the surface to breathe between prolonged dives. The inactive dives typically lasted nearly an hour (up to 98 minutes), which is unusually long for adult sea turtles; similarly inactive dives to the seafloor by loggerhead turtles typically last only 30 minutes, and rarely in excess of an hour.
The answers to this riddle of prolonged submergence appear to be in the flatback’s blood. In general, air-breathing, diving animals must rely on oxygen that is stored in blood and muscles once the oxygen in their lungs either is consumed or becomes unavailable. However, the capacity for the flatbacks’ blood to store oxygen and to buffer the potentially toxic buildup of carbon dioxide during breath-holding is at the high end of the range in diving reptiles. So, because flatback turtles are rarely found in waters deeper than 45 meters, their respiratory physiology may be suited particularly well to sustaining prolonged dives in shallow habitats. Researchers think that those traits could explain the flatback’s ability to survive the stress of forced submergence in trawl nets better than the less-resilient loggerhead.
Although the scientific world has come to know the flatback only recently, the relationships between sea turtles and indigenous peoples in the region have spanned millennia. Many present-day flatback nesting sites are located on the lands of Aboriginal or Torres Strait Islander people, and partnerships among those indigenous communities, researchers, and government have been critical in improving knowledge of flatback distribution, biology, and conservation. The research and conservation programs have greatly benefited from the inclusion of traditional ecological knowledge and the enthusiastic participation of several indigenous communities in monitoring exercises. Successful management of flatback turtles in northern Australia will ultimately rely on those community collaborations.
The once mysterious flatback turtle has literally been drawn out of obscurity in the past few decades, has been researched and renamed, and has been brought to the attention of the world. In a country known for strange and superlative creatures—from egg laying, poisonous mammals, such as the platypus, to man-eating crocodiles larger than any other reptile on Earth—the flatback turtle has entered the ranks of Australia’s most amazing animals. Whether known for its smallest home-range, longest dives, biggest eggs, or any of the other unique natural history and physiological features highlighted here, the flatback is unquestionably an Australian original and an icon for the country’s unique biodiversity.
SWOT Feature Map Global Biogeography of the Flatback (Natator depressus)
The SWOT feature map below displays the global biogeography of the flatback turtle (Natator depressus) and demonstrates, as described in the preceding article, that flatbacks are undeniably Australia’s own sea turtle. In the map, the relative abundances of nesting rookeries are displayed by site over varying time periods; some rookery sizes are represented by averages over several years, and others are represented by the most recent available year of data at that site (2006, 2007, or 2008). Nesting abundances are indicated by number of clutches. In areas where abundance data were reported as numbers of crawls or numbers of nesting females, conversion factors were used of 70 percent nesting success (numbers of crawls that resulted in successful clutches) and 2.8 clutches per female, respectively (Limpus, C. J. 2007. A biological review of Australian marine turtle species. 5. Flatback turtle, Natator depressus (Garman). The State of Queensland. Environmental Protection Agency, 2007). Altogether, the map displays 290 flatback nesting sites from nine different data providers. Please see the SWOT Data Contributors section here for complete citations of all data points (pages 47–48 of the print version of SWOT Report, Vol. IV).
This map represents the “next generation” of SWOT spatial displays of biological information on sea turtles and is an exciting step forward. In addition to the nesting abundance estimates shown in previous SWOT Report centerpiece maps, the flatback map incorporates, for the first time, data on in-water distribution and known genetic stocks. A special thanks goes to Dr. Nancy FitzSimmons (University of Canberra) for providing the most current genetic stock information (see Data Contributors section for more information on genetic stocks).
This article originally appeared in SWOT Report, vol. 4 (2009). Click here to download the entire article as a PDF.