How lights can kill turtles and why they cry…

A beautiful green turtle. Image © Debbie Holroyd

In this newsletter, we have a variety of random facts which I found fascinating. To begin, do you know what the difference between a tortoise, a terrapin and a turtle is? Whilst there are many other differentiators, in nutshell tortoises are found on land, terrapins are found in fresh water and turtles are ocean-dwelling creatures. 

ARE TURTLES ENDANGERED?

How rare are these animals? There are various institutions and thus categorisations pertaining to how threatened a species is. As a rule, I work with IUCN definitions. Their classifications are complicated (see info block for a link) and take many factors into account – but whichever definition you choose to use, there can be no doubt that turtles are in trouble! The IUCN listing Vulnerable refers to a species considered to be facing a high risk of extinction in the wild. The Loggerhead Turtle (Caretta caretta) has most recently been assessed for The IUCN Red List of Threatened Species in 2015 and is listed as Vulnerable under criteria A2b (50–80% decline over any 50-year period including the present and future). The Leatherback Turtle (Dermochelys coriacea) has most recently been assessed for The IUCN Red List of Threatened Species in 2013 – it is also listed as Vulnerable under criteria A2bd.

A large hawksbill turtle. Image © Debbie Holroyd

So, the reality is that these animals are threatened with extinction – by us! Their nesting beaches are becoming more and more commercial, eliminating them as feasible sites. The females are being disrupted at night as they’re trying to nest. Beach parties create further disturbances. 4x4s driving in the wrong places (globally, you should only drive between the high-water mark and the ocean, but beach driving is mostly prohibited in South Africa) and carelessly placed beach umbrellas can destroy eggs. Leatherback turtles are often known as gelatinivores, meaning they only eat invertebrates such as jellyfish and sea squirts. Loggerheads also eat jellyfish as part of their diet. Plastic litter, especially plastic bags, food packaging and balloons (which all resemble jellyfish and seaweed), are ingested and obstruct the stomach of the turtle, causing a slow and painful death. Turtles around the world (remembering that “our” turtles travel vast distances and thus become ‘global citizens’) are slaughtered for their eggs, meat, skin, and shells. They also face accidental capture (known as ‘bycatch’) in fishing gear. 

Plastic floating in the ocean looks like a tasty jellyfish to the poor turtles – with fatal outcomes!

NEST TEMPERATURES DETERMINE GENDER

Turtle gender depends entirely upon the surrounding temperature of the sand, where the eggs are incubated. This is called temperature-dependent sex determination, or TSD. Research shows that if a turtle’s eggs incubate below 27.7° C, the turtle hatchlings will be male. If the eggs incubate above 31° C, however, the hatchlings will be female. A clutch that incubates between these thresholds will have a mix of male and female hatchlings. You can remember how temperature affects turtle gender by remembering the phrase “Hot Gals, Cool Dudes!” Once male hatchlings reach the ocean, they will never again come ashore, living out their lives in the open oceans.

Climate change has an impact on turtle nesting sites as it alters sand temperatures, which in turn affects the sex of hatchlings. One study (see info block) on the north of Australia’s Great Barrier Reef found that 99% of immature green turtles were born female, while 87% of adult turtles in the same area were female. This indicated that the proportion of female hatchlings has increased in recent years. In the future, there may not be enough males to fertilize the overabundance of females. This could lead to a decline in genetic diversity within the population. The fact that they only come to sexual maturity over the course of 25-40 years, means that the effects on hatchlings today may not be seen for decades to come. 

One cannot even imagine turtles being able to nest in this environment…

TURTLES AND LIGHT POLLUTION

Light pollution is another seldom-considered threat to turtles. When hatchlings emerge, they orientate themselves towards the brightest horizon, which should be the glow of the moon and stars on the ocean. When there are artificial lights of any kind on the beach, the turtle hatchlings move towards them. They die of exhaustion, dehydration, baking in the sun, and predation by, amongst others, gulls. It is estimated than only one in a thousand hatchlings make it to maturity. Female turtles also navigate towards the shore using the dark outline of vegetated dunes against the night sky.

TURTLE HATCHLING’S HAZARDOUS JOURNEY

After an incubation of approximately 60 days, the hatchlings use a temporary, sharp egg-tooth, called a “caruncle”, to break out of the egg. After they come out of their eggshells, they spend another 3-5 days buried under the sand while they absorb their yolk sac. The yolk sac is attached to their underside and provides the energy they need to swim a long distance offshore to find food. During this time, their carapaces also straighten out and harden. They will then wait for cooler temperatures and the dark of night to break out and head for the ocean. Digging out of the nest is a group effort.

These little leatherback hatchlings weigh about 45 grams – yet reach close to a ton as adults!

FEMALES STORE SPERM

Female turtles have the remarkable ability to store sperm. A female will mate with several males and store the sperm for several months until she is able to fertilize all her eggs and start nesting. It is believed that female turtles may mate with multiple males to ensure fertilization, which will also result in multiple paternity within their nests, providing “fertilization insurance,” so to speak. This behaviour may also help keep genetic diversity high in the population. After a nesting season, the turtles migrate to a foraging area. They generally remain there until their next nesting season, usually one or two years later.

TURTLES AND DRINKING / CRYING

Turtles, like most animals, need to drink water to stay hydrated. Dehydration is a top killer of many of the hatchlings that lose their way. In the oceans, turtles gulp water down with the food they swallow. They have less efficient kidneys than mammals, and they also can’t produce urine with a higher salt concentration than the seawater they drink. To avoid poisoning themselves with salt buildup, they have a gland in each eye that actively pumps salt ions into their tears. These glands need to run continuously to maintain the correct balance of salt in their bodies. People often associate crying with egg laying because that’s the only time they come ashore, but they cry in the sea as well – you just can’t see it. The tears also help flush sand from their eyes whilst on the beach.

DO TURTLES SLEEP?

Turtles have lungs and breathe air. Which means they must surface reasonably regularly to access air. This is why, when they are captured underwater in a fishing net and unable to reach the surface, they will drown. They can, however, hold their breath for four to seven hours while sleeping and resting. When active, they come to the surface to breathe far more frequently. This breath-holding ability allows them to sleep whilst tucked away in sheltered areas on the seabed – which prevents them from drifting too far away from their foraging spots. While migrating across the open oceans, they will sleep floating at the surface. They rely on the fact that the winds and currents will help push them where they want to go.

A green turtle tucked away in a sheltered area. Image © Debbie Holroyd

WHAT CAN WE DO TO HELP TURTLES?

A turtle is least likely to abandon nesting when she is laying her eggs, but some turtles will abort the process if they are harassed or feel they are in danger. It is thus important that, should you choose to go on a ‘turtle nesting adventure’, be sure to choose a reputable company who won’t allow tourists to disturb the turtles.

Picking up hatchlings can transmit bacteria against which they have no defences. They also need to ‘walk’ to the ocean by themselves, as this provides the vital exercise required to strengthen their flippers for swimming. Any lights tourists use should be directly in line with their path to the sea, so as not to misguide them.

Beyond that, the biggest difference we can make to turtles is to stop using plastic bags…

Jacqui Ikin & The Cross Country Team

INFO BLOCK:

Criteria for IUCN classifications:
http://cmsdocs.s3.amazonaws.com/keydocuments/summary_sheet_en_web.pdf

Amazing Sea Turtle Mating Footage:
https://www.youtube.com/watch?v=avGE_OhXplM

Environmental Warming and Feminization of One of the Largest Sea Turtle Populations in the World
https://www.cell.com/current-biology/fulltext/S0960-9822(17)31539-7