What Happens to a Sea Turtle Nest that Doesn’t Hatch?

Not all sea turtle nests hatch. Kathy Ho, blog author and 9-12 English, Math, History, and Science teacher at Lucile Packard Children’s Hospital School got first hand experience investigating this phenomenon. With support from The Giles W. and Elise G. Mead Foundation and the Earthwatch Institute, Kathy and three other teachers from Bay Area schools traveled to The Leatherback Trust’s Goldring-Gund Marine Biology Station in Costa Rica. The teachers worked with biologists from The Leatherback Trust to help excavate unhatched nests in Las Baulas National Park.

Nests can fail to hatch for many reasons: raccoons or unleashed dogs sometimes dig up to eat the eggs, visitors might unknowingly plunge a beach umbrella into a leatherback nest deep in the sand or crush a shallow nest cavity dug by an olive ridley, or perhaps the turtle laid infertile eggs. Sometimes the composition of the sand can affect nest outcomes. Rising temperatures can make the sand too hot or too dry, sapping the nest of vital moisture. Changing humidity levels can affect gas exchange and potentially increase bacteria growth, threatening the eggs with infection.

Leatherback eggs incubate for 55-60 days before hatching. Although hotter sand can develop embryos faster, there is a fine line between warm sand simply speeding up the incubation period, and sand being so hot that it dries out the eggs. The Leatherback Trust surveys the beach every night to identify leatherback nest sites and one of our team members walks the length of Las Baulas National Park every morning looking for hatchling tracks. Our biologists calculate when all marked nests are due to hatch, and plan excavations accordingly.

Kathy and her fellow teachers joined our biologists to excavate an unhatched nest in Las Baulas National Park. Our scientists gave this particular nest a 10-day grace period after the regular incubation period before coming to the conclusion that no little turtles would be emerging to make a run for the ocean. Their next step was to excavate the nest and see why it was not successful in producing hatchlings.

In Las Baulas National Park and the other nesting beaches we monitor, The Leatherback Trust follows a set scientific excavation protocol. When excavating hatched nests, our biologists first feel in the sand for any live hatchlings that may not have made it out of the nest with their siblings. Live hatchlings are rehydrated in a bucket of wet sand and released at night to improve their chances of outpacing predators.

To investigate an unhatched nest, a biologist from The Leatherback Trust digs deep in the sand to locate the nest cavity and measure how far down the eggs are. If the eggs were too close to the surface, they may have dried out before the full incubation period due to hotter sand near the surface. Once the nest cavity is identified eggs have been uncovered, we carefully open each egg and register its stage of development.

There are 4 stages of development: stage 0 is no development, stage 1 shows signs of blood vessels forming at stage 2 the beginning of an embryo with pigmented eyes is evident, and stage 3 is a fully pigmented embryo. Once the excavation is finished and all data is collected, the pieces of the egg and insides are placed back in their nest and recovered with sand. Leaving unhatched turtle eggs to break down in the sand provides vital nutrients to the coastal ecosystem.

After Kathy excavated the unhatched nest with our team, she wrote on her blog, “The majority (66) of the eggs were fairly far along in their development, meaning they had reached stage 3. It was just a formed embryonic hatchling.” With this finding, the team postulated “one possible reason for their demise was the temperature was too high.”

El Niño conditions have resulted in a severe 3-year drought within Costa Rica’s dry forest ecosystems on the Pacific coast, bringing a shorter rainy season and higher temperatures (with this year’s temperatures being the highest on record since 2010). Kathy saw the effects of El Niño during her visit in February and witnessed the impact of rising temperatures and shorter rains on hatchlings first-hand as she excavated an unhatched nest. Rising temperatures and lower humidity levels in sand on nesting beaches is yet another way global climate change negatively impacts sea turtle populations. Other impacts include rising sea levels and more frequent coastal flooding events, which can inundate nests. Warmer ocean temperatures under El Niño can also reduce the availability of food for sea turtles, resulting in fewer nesting turtles and, therefore, fewer nests..