EMPEROR penguins endure months of subzero temperatures without eating, all to incubate their eggs. Wolf spiders carry their offspring – hundreds of them – around after they hatch. Poison dart frogs carry each of their tadpoles to its own little pond and return to feed it every few days.
Octopuses go even further, making the ultimate sacrifice for their offspring. After females lay their eggs, they spend the next month or two looking after them, cleaning them and fending off predators. By the time the eggs hatch, the mothers are so exhausted that they usually die or succumb to predators.
Not all parents in the animal world are so devoted. Among reptiles, especially, “lay and go away” is the norm. But have we been misjudging these animals? Out of South America have come the first hints that some turtles look after their young.
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Canoe quietly along an Amazonian river at low water, and on rocks and logs you might see the smooth dark-grey shells of basking arrau river turtles (Podocnemis expansa), also known as giant Amazon river turtles. In favoured spots, where space is tight, they stack themselves like plates on a drying rack, forming a slanted series of shelly sun-worshippers. As one of their names suggests, they are the largest turtles in the Amazon. The shells of females can be nearly a metre long, and individuals can weigh up to 45 kilograms.
Female arrau turtles lay eggs during the dry season, when the low water exposes banks of sand for a few months. The turtles were once so numerous that Victorian naturalist estimated that 400,000 clutches – each consisting of 100 or so eggs – were pulped annually for their oil. Even more were dug up to be eaten. It’s no surprise that arrau turtle populations have collapsed.
Conservationists now dig up eggs to protect them from people and predators. It was while doing this that turtle biologist Camila Ferrara, of the National Institute for Amazonian Research (INPA) in Manaus, Brazil, got thinking. Adult turtles make sounds, so could hatchlings do so too? If they do, it could explain some puzzling observations. For instance, it has been shown that eggs from a clutch all hatch at the same time, even if some are incubated at different temperatures, but no one knew how the baby turtles coordinate their emergence. Might they talk to each before they hatch, Ferrara wondered?
This opened up another possibility, too. Radio-tracking studies by Richard C. Vogt, a veteran biologist, also at INPA, who wrote , show that females often hang around in the water near their nesting beaches around the time the eggs hatch. What if the turtlets also talked to mum?
This, after all, is what caimans and other members of the crocodile family do. The females guard their eggs, waiting for from their offspring. When they hear it, they carry the hatchlings to water in their jaws and continue looking after them there, responding to distress calls and sometimes moving the young to a new nursery. Male crocs have sometimes been seen helping too.
The only member of the turtle order known to show any form of parental care is the yellow mud turtle (Kinosternon flavescens), and its efforts are not exactly spectacular. Females simply stay by their newly laid eggs for a few days, occasionally urinating on them if the weather is dry.
So Ferrara, aided by Vogt and another INPA colleague, Renata Sousa-Lima, set out to see if arrau turtles went further. The team’s three-year study included putting microphones near captive and wild adults and hatchlings. The results surprised everyone. In 308 hours, Ferrera recorded 2128 calls of 11 distinct kinds. Some were short pulses, others single calls, with frequencies that varied from 36.8 to 4500 hertz. Excitingly, some were the short and high-frequency vocalisations that the biologists recognised as characteristic of animal contact calls ().
Hear three (very short) turtle recordings: The first is an adult female making what is the commonest call in both hatchlings and adults. Hatchlings use it both within the egg and in the nest to call to adults; both hatchlings and adults make it underwater and at the surface. The second call is made only by hatchlings, in the same environments; the third is made between hatchlings within eggs and on the water surface, and by adults underwater.
Baby turtles started making noises around a day before hatching. When the team released hatchlings into the wild close to females, the females appeared to respond to their calls and approached them. Similarly, when an adult female was put into a tank with recent hatchlings, the hatchlings made calls not heard under other circumstances and actually climbed onto her. When she swam, the hatchlings swam beneath her.
This co-swimming happens in the wild, too. By attaching radio transmitters to females, the researchers found that they leave the vicinity of nesting beaches only after the eggs have hatched and the hatchlings have entered the water. What’s more, the hatchlings swim away with them.
Hatchlings with radio transmitters have been found swimming with adults several kilometres from the nest site. Most couldn’t be relocated after the first sighting, either because the transmitter failed or because they were eaten, but one remained with a group of adults for at least 10 days. “We think the females are guiding young to safe feeding sites,” says Vogt, who is planning fieldwork to study this.
“The hatchlings swim with females. We think the females are guiding them to safe feeding sites”
All this suggests that female arrau turtles do care for their young after they hatch, which is extremely rare in reptiles. Although a few snakes and lizards look after their eggs, crocodiles are the only reptiles known to care for the hatchlings as well. “The study revolutionises how we look at these turtles,” says Ferrara. “They show social behaviour, including complex parental care, which was previously totally unknown in turtles.”
But there is something odd about the turtles’ case. Crocodiles nest alone and can be sure the hatchlings they care for are their own, but turtles nest en masse and the females don’t appear to discriminate between their own offspring and those of others. Why should this be? Vogt thinks he has the answer. “Beaches are used by generations of females,” he says. “Hatchlings and females may be close relatives.”
So what do other turtle biologists make of all this? “That adult Podocnemis care for hatchlings is truly news,” says Gerald Kuchling of the University of Western Australia in Crawley. “Until this study, we thought hatchlings were on their own, and navigated alone to the specific habitat where they can grow up.” Yet although the study proves that adults and hatchlings interact after the young reach the river, Kuchling isn’t sure whether this can really be called parenting.
Whatever you call it, the discovery is of more than academic interest, because of the practice of collecting newly laid eggs and incubating them elsewhere to protect them from people and predators. The hatchlings are often released far from the capture site to try to establish new breeding populations. But such actions could be depriving the young of adult assistance. “Instead of reducing hatchling mortality, well-intentioned conservationists may actually be increasing it,” Vogt says.
It wouldn’t be the first time. In the 1970s, Vogt found that turtles’ sex-determination is temperature dependent. By keeping eggs in controlled environments, some conservation programmes inadvertently raised batches with highly skewed sex ratios. “One programme spent 20 years releasing nearly all males,” recalls Vogt, who is convinced that breaking mother-hatchling vocal bonds could also harm conservation efforts.
That makes it important to find out if other species behave this way towards their young. Kuchling thinks such behaviour may well occur in other river turtles, especially in species that nest communally on sandbanks like arrau turtles. This could include some critically endangered turtle species in Asia.
Vogt thinks even some sea turtles could care for their young. His team has recorded calls in olive ridley sea turtles, both by hatchlings on the beach and by adults in the sea. “I think all turtles vocalise to some extent. This does not mean they have parental care, however,” he says. “This remains to be studied.”
Others are sceptical. T. Todd Jones of the Pacific Islands Fisheries Science Center in Hawaii thinks it very unlikely. Adult marine turtles swim a lot faster than their hatchlings, he points out, have different temperature requirements and often feed far from the nursery areas where hatchlings spend their early years.
Only further studies will settle the issue. But if many turtles do care for their young, some of the strategies intended to save them could be casting them alone and adrift from an early age. And what would a mother turtle have to say about that?
How the turtle got its shell – and learned to hide in it
The protective shell of turtles, terrapins and tortoises is unique. No other vertebrates have anything like it. But how did it evolve?
For more than a century, the earliest known ancestor of this group of animals was the 210-million-year-old Proganochelys, and it answered few questions because it looked remarkably like a modern turtle. It had already evolved a complete shell, although it still had some primitive features such as a few small teeth rather than the horny beak found in modern turtles and tortoises.
Then, in 2008, a was uncovered in China. The 220-million-year-old Odontochelys was a lizard-shaped creature that had evolved the bottom half of the shell – the plastron – but not the top half. However, it did show signs of the development of a carapace, in the form of broad flattened ribs and some bony plates in the skin. It also had a full set of teeth. Odontochelys is thought to have lived in shallow seas, so the plastron probably evolved to protect against attacks from below.
For the half-shell of Odontochelys to evolve into the complete shell of Proganochelys just 10 million years later, a pretty spectacular piece of internal rearranging had to take place. The shoulder blades moved from being outside the ribcage to inside it, allowing turtles to move with a leverage that would be hard to obtain if this muscle-anchoring plate was fused to the bony carapace. Studies of turtle embryos have revealed how this probably happened, with a fold in the body wall early in development preventing the uppermost ribs from forming at the same time as the others, and pulling the shoulder blades through the resulting gap into the ribcage.
Proganochelys had a formidable shell, but the best it could do when feeling threatened was simply fold its neck sideways. This offered some protection under the shell’s lip, true, but still left it vulnerable to a persistent predator. Some of its descendants, the pleurodirans, still fold their necks sideways. This group includes arrau turtles (see main story) and their extinct relatives Carbonemys and Stupendemys. These South American turtles lived around 80 million years ago, and at 2 metres long were the largest-ever freshwater turtles.
Around 190 to 150 million years ago, however, a different lineage evolved some rather kinky neck vertebrae that allowed them to bend their neck in an S-shape and thus retract their head completely inside their shell. These S-benders, or cryptodirans, were more successful than the side-necks and gave rise to most living members of the group today, from Galapagos tortoises to leatherback turtles.
This article appeared in print under the headline “I’m coming, mum”
