by Matthew Norton
The short answer is yes and no. Adult green turtles (Chelonia mydas) are indeed herbivorous, feeding on seagrass, seaweed and pieces of mangrove trees, but in their younger years they are carnivores, eating a range of animals including jellyfish, worms, shellfish and fish eggs. As they grow they gradually include seaweed and plants into their diet until they become entirely reliant on them. Changes in diet during an animal’s lifetime, especially as they develop and grow towards adulthood, is common in nature, but this extreme shift between two very different types of food is less common.
For young green turtles, an animal based diet actually makes sense because they are weaker swimmers compared to the adults. Studies that have monitored green turtle diving behaviour have found that the dives of the younger turtles were shorter and shallower, which suggest they haven’t quite mastered the skill of diving, or the ability to properly control their buoyancy. This makes it difficult to reach patches of seaweed and seagrass because they grow on the seafloor, but near the ocean surface there are small, floating animals that the younger turtles can feed on.
Given that younger green turtles can survive on floating animal food, maybe we should be focussing on why they don’t continue eating animals into adulthood. This is a good question for two reasons.
Firstly, all other sea turtle species, including the green turtle’s close relatives the flatback turtle (Natator depressus) and hawksbill turtle (Eretmochelys imbricata), are either carnivores, or omnivores (they eat both plants and animals).
Secondly, plants are generally harder for animals to eat because they contain, among other things, a lot of fibre and cellulose, a complicated sugar that plants use to build walls around their cells and is hard for animals to break down into the smaller sugars that they actually need. The fruits and vegetables that we eat are only edible because plants ‘design’ them to be eaten by animals for the purposes of seed dispersal.
Herbivorous animals play an important role in the food chain by supporting larger (in most cases) carnivorous animals. However, evolution does not really work towards setting up these food chains and so a more immediate explanation is needed. One possibility is that by adapting to survive on a more difficult food, such as plants, adult green turtles avoid competing for food with other sea turtles who still rely on animal prey. Such adaptations include their serrated jaws which acts like a saw scraping up plants and seaweed from surfaces and tearing them into manageable pieces. Their stomachs also digest plants, especially cellulose, better than most other reptile herbivores thanks to a rich culture of bacteria that are particularly effective at breaking down plant cell walls.
The way that green turtles eat seagrass and seaweed also helps them to make the most of this resource. For example they mainly graze the tips of seagrass, encouraging the growth of new plant material which, if they are quick enough, they can pick off before the fibres, cellulose and other harder digest material can build up. They can also be flexible with their diet depending on what is available with some populations eating mainly seagrass, while others eat more seaweed. Some have even suggested that, in rare cases, they may still be omnivorous into adulthood.
The act of consuming other organisms is essential to the survival of any animal (with some exceptions) and regardless of what species they target there will be challenges for them to overcome. These include catching enough food, digesting, or getting rid of the hard parts and competing with other animals for a particular food source. In the case of green turtles they use different strategies depending on their age and where they live and so make good use of the most convenient and profitable food resource they can find.
From a human perspective
Sea turtles face numerous threats in today’s world as a consequence of our actions, they get tangled in our fish nets, they accidently eat our plastic rubbish and they lose their eggs, shells and lives to poachers. In other scenarios the impacts we have are more subtle, but no less devastating. In particular the rising temperatures on green turtle nesting beach, as a result of human induced climate change, can cause numerous problems for the developing embryos. These include burning through their energy reserves (contained in the egg yolk) too fast during development, as well as abnormalities and weaker swimming ability in the hatchlings. Even more worrying is how increasing temperatures can play havoc with the birth rates of males and females.
Determining whether an animal is born as a male, or a female is a more complex procedure than most of us realise, with different groups doing this in different ways. In some animals gender is determined by how many copies of their entire gene set they have, in others it’s determined by the combinations of specific chromosomes (bundles of genetic material) they inherit from their parents. In turtles, and other reptiles, genes play no role at all with gender being determined by the temperature at which the egg is incubated. Eggs incubated at 26-28oC produce male hatchlings and eggs at 30oC produce females.
Normally this would produce a healthy ratio of males and females, but with increasing temperatures, both globally and on particular nesting beaches, there is a very real risk of these ratios being skewed towards producing too many females and too few males. In some areas this has already yielded extreme results with over 99% of the population being female. The repercussions of this are pretty clear, without enough males to reproduce with the females the green turtle as a species is at risk of extinction.
There may be some nesting sites that can still produce male offspring due to the widespread range of green turtles, which exposes them to a range of temperatures despite the general trend of increasing temperature. Even within a single nesting beach, nesting temperature can still vary due to various factors, such as how high on the beach the nest is and how close it is to shade. This might be wishful thinking, but if green turtles are to survive in the future, then male producing nesting beaches should be prioritised for protection from poaching and other human threats.
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