Nesting of Sea Turtles – Mohd Uzair Rusli

Female sea turtles have a unique characteristic of returning to the same beaches on which they were born in order to nest (Brothers and Lohmann 2018). Upon leaving the nest early in life (after about 2 months in-nest incubation), sea turtle hatchlings will swim vigorously to escape predator-rich shallow waters and will never return until they reach sexual maturity about 20 years later.

Sea turtles are generally solitary, and in most species, individual female turtles nest on their own. However, out of the seven species of sea turtles worldwide, two species – the olive ridley (Lepidochelys olivacea) and Kemp’s sea turtle (Lepidochelys kempii) – exhibit mass nesting, called arribada, the Spanish word for “arrivals.”

Nesting activity typically happens on sandy beaches with sediment types ranging from fine to coarse sands (Kelly et al. 2017). Most of the time, the nest is constructed above the high tide waterline to secure the incubating eggs for the approximately 2 month-long (range, 45–70 days) incubation period. The incubation process depends on temperature where the higher incubation temperature will shorten the time to hatching (Booth and Astill 2001; Jong et al. 2009). However, incubation temperature above 33–35 °C of sea turtle eggs might be causing low-quality and impaired locomotion of hatchlings produced to escape from their nests (Segura and Cajade 2010). Therefore, the behavior of sea turtles constructed their nest under shaded or open area will result in variation in phenotype and performance of an individual.

Sea turtles are known as a highly reproductive species as one female can produce several clutches within a nesting season (Miller 1997). However, nesting activity seldom occurs annually. Typically, the interval between nesting seasons is between 2 and 4 years (Broderick et al. 2001; Chaloupka 2001; Chan 2010; Sims et al. 2008). Some scholars believe that this interval period occurs because sea turtles spend so much time in the open ocean, foraging and migrating from one feeding ground to another.

Within any particular nesting season, a female turtle will remain within the vicinity of a nesting beach and spend several months there in order to deposit multiple clutches. A single female may nest up to six times with interval of 10–14 days before the next nesting activity (Hart et al. 2010). The nesting period for a typical female lasts about 3 months before she heads back out to the sea.

Nesting activity typically occurs at night as sea turtles are very sensitive to disturbance from light and potential predators. Night nesting also helps turtles avoid dehydration from the long arduous crawl from the water to the beach and back again. The complete process may take between 2 and 4 h depending on the species and beach characteristics. Occasionally females also crawl out from the water but do not nest. This phenomenon is known as a “false crawl” (Talbert et al. 1980).

Sea turtles are likely to nest during high tide, probably to reduce the crawling distance from shoreline to the nesting spot. Due to the high drag of her heavy body weight (60–600 kg depends on species), nesting is energetically costly and seems exhausting to human observers. Teardrops can often be seen during the nesting process as a mechanism to excrete excess salt in blood (Lutz 1997).

Nesting activity starts when a mature female emerges from the water and crawls up to her nesting spot. When the appropriate spot is reached above the high tide level, a turtle will move both left and right flippers synchronously back and forth and start lowering her body until it reaches the same level of beach surface. This sand throwing behavior, termed “body pitting” may later help rear-flippers to reach deeper level. Hence, the depth of nest chambers is dependent on the size of a female’s rear-flippers.

When the turtle has stopped using its front-flippers, she begins to use her rear-flippers, which act like shovels to scoop the sand out of the nest cavity. This is known as “egg chambering.” During this activity, the turtle moves the rear-flippers alternately between left and right. The turtle is typically seen trying to maximize the depth of nest chamber by lifting up her body with the support from her front-flippers. The nest chamber is constructed in a shape similar to a flask, which is thought to equalize temperatures among all of the incubating eggs. Depending on the species, the nest chambers typically highly related to maternal morphology and the reproductive output (Bjorndal and Carr 1989).

Sea turtles often abort the nesting process if they found any hard surface such as roots, dead coral, or gravel while chambering. She may then move to another spot and start with body pitting again as described above or may return back to the water immediately, likely returning to the same beach the next night.

Once the nest chamber construction is completed, turtle lays eggs through ovipositor by dropping two to three at a time until there are about a hundred eggs per clutch. It is believed that when the eggs touch the tail of the turtle, this serves as a cue that the nest chamber is full enough. However, Hailman and Elowson (1992) suggested that this tactile feedback is not the only cue for termination of eggs laying as some of turtle had stop earlier. The whole process takes around 10 min, depending on the species.

Upon completion of egg deposition, a turtle will conceal the nest cavity with her rear-flippers. Then, the turtle will rest for about 5 min. During this short window, conservation workers can safely measure morphological characteristics and attach identification tags, before the turtle starts using both front flippers to continue covering the nest. While dispersing the sand over the covered egg chamber by using their front flippers, turtles moved gradually a few meters forward. This camouflaging behavior created an elongated mound of sand on the origin nest chamber hence will confuse potential predators (this is typically known as creating a “false nest”). When the nest is covered, the turtle will crawl immediately back to the sea.