The Wonders of Sea Turtle Nesting
Sea turtles may be introverted, preferring to spend their time solely ambling about the ocean, but once you get to know them they’re fascinating. The one time they do meet up and interact with each other occurs during the courting and mating process. However, this does not occur in most species until turtles reach at least 12 years of age (Hawksbill turtles can reach sexual maturity in 3 years), and sometimes longer depending upon the species of sea turtle. Up to this point, the turtles float around seaweed mats in the open ocean, eating, growing, playing sports, and deciding where they want to attend college.
Mating and Nesting
Upon reaching sexual maturity, sea turtles return to the coastal areas around their birthplace during nesting season. Nesting season generally occurs in the 3-4 warmest months of the year with the lone exception being leatherback, which nests in fall and winter. The process of returning home, known as philopatry, occurs through the turtles’ instinctual use of the earth’s magnetic field. Every part of the coast has its own magnetic “signature,” allowing the turtles to find their way back to mate and eventually lay eggs.
Upon returning to the breeding waters, males begin courting females several weeks before nesting season, just offshore. Females will mate with several males over the course of a couple weeks before coming ashore to lay eggs. This ensures that the eggs will have been fertilized by several different males, preserving the genetic diversity of the species.
Females will crawl onto the beach at night during the summer to lay eggs. This most often occurs around high tide as the female crawls past the high tide line. Occasionally, a ‘false crawl’ occurs and the female will return to the ocean without laying eggs. This can happen due to there being too much light or some other disturbance, or for reasons unknown. If she does decide to lay eggs, the mother turtle will dig a pit for herself with her front flippers and a pit for the eggs with her rear flippers. She will then lay between 50 and 200 eggs (sometimes up to 350!) over the course of 1-3 hours. Sea turtles will nest up to 9 times throughout the nesting season.
The eggs are soft and leathery and are surrounded by a soft, clear mucus which prevents them from breaking upon dropping into the nest. When laying is complete, the mother will cover the eggs in sand which protects them from predators, keeps them moist, and insulates them from temperature fluctuations. To further protect the eggs from predators such as foxes, raccoons, dogs, and seabirds, the mother turtle will spread sand around the outside the nest as well in an effort to disguise the nest completely. After laying the eggs, the mother turtle returns to the ocean, exhausted. Experts can identify which species of turtle has just laid eggs depending upon the shape of the nesting pit and the “flipperprints” left in the sand.
The average incubation period for sea turtle eggs is about two months. The rate of embryo development depends upon the temperature of the surrounding sand, with warmer sand leading to faster development. Additionally, the sand temperature also determines the sex of the egg in a phenomenon known as temperature-dependent sex determination. The “pivotal temperature” appears to be 83-85 degrees Fahrenheit (28-29 degrees C), which will produce a mix of male and female hatchlings. Sand temperatures above this threshold will produce more female turtles whereas lower sand temperatures lead to more males. As a result, many of the eggs towards the bottom of the nest will become males due to cooler sand at deeper levels on the beach.
After about two months, the turtles begin to hatch by using a tiny tooth known as a caruncle (good scrabble word!). After hatching, the turtles will remain in the nest for several days, absorbing the yoke from the egg through an umbilical cord. The sand on top of the nest will fall by several inches throughout this process as sand falls in to fill up the empty space created.
Hatch rates fluctuate drastically from year to year and from place to place. It is estimated on average, that 80-90% of sea turtle eggs from a given nest will hatch under optimal conditions. This average drops to 60-70% if the nest is moved through a sea turtle conservation effort.
After nearing the surface of the sand and waiting for the temperature to cool, the turtles will use the broken shells and each other to climb up out of the nest en masse, usually at night. There is less danger of predation at night although the hatchlings’ chance of survival remains slim, often quoted as 1 out of 1,000. After emerging, the baby sea turtles sprint as fast as they can towards the water, using the natural slope of the beach, moonlight, and wave crests as physical/visual guides. Man-made light from development can lead the turtles in the wrong direction so homeowners are often encouraged to limit light emission along the coast during nesting season. Larger turtles will be faster than smaller turtles and have a slightly better chance to survive.
Once reaching the ocean, the turtles attempt to get offshore, remaining in as large a group as possible to further increase chances of survival. Once making it offshore, the turtles will continue to swim out to sea for up to a decade of growth and maturation before returning nearshore upon sexual maturity.
Climate change and sea turtle nesting
As you can see, this is an extremely delicate process with low chances for survival. As a result, any kind of unnatural disturbance could result in grave consequences for sea turtle populations. Possible effects of climate change on sea turtle nesting habits include the following:
- Warmer sand could result in decreased hatching rates. Southern Florida saw a record number of turtle nests in 2016, but only a 40-50% hatch rate, including some nests with zero hatchlings. This followed record heat during the summer of that year but could be attributed partly to other factors as well.
- Warmer sand could result in an increased ratio of females to males, since warmer sand temperatures lead to more more females being born. Although existing male turtles might welcome this development, a large swing in the sexual ratio of sea turtles could jeopardize mating habits and the genetic diversity of the species.
- Warmer oceans mean altered ocean currents. Currents are vital to sea turtles for navigation and food as well. Any alterations in these large-scale currents could result in less food for the turtles as well as complicating mating rituals. In this scenario, turtles could be found more north or south, which could affect mating and nesting rituals negatively.
- Warmer oceans mean coral bleaching. As ocean temperatures rise, coral reefs die in a process known as coral bleaching. Many sea turtles depend on the vitality of coral reefs for food and mating habits and any alteration in the health of the reefs may negatively impact the local sea turtle population.
- Warmer oceans mean more tropical storms. Tropical storms and hurricanes cause erosion and destruction of nesting sites and warmer sea surface temperatures lead to an increase in the quantity and strength of storms. In a given year, 90% of sea turtle nests in the United States will be found in Florida, which is particularly susceptible to tropical storms and hurricanes.
Sea turtles are some of the ocean’s most adored creatures, and for good reason. The difficulties sea turtles encounter on the journey to adulthood and reproduction indicate just how fragile life and ecosystems can be. There are hundreds of sea turtle conservation organizations and efforts along coastlines throughout the world if you’d like to get involved. Fanimal works with a number of animal organizations (link) including those dedicated to sea turtles. Some ideas for getting involved can be found here (link). Additionally, climate change poses a huge threat to the long-term survival of sea turtles and we should all take steps to reduce our ecological footprint accordingly. Fanimal has some suggestions on this as well (link to page that has easy, moderate, and more intense actions to take.)