FAQ: How Many Eggs Does It Take to Make an Adult Turtle?
With their specialized biology and their unique behaviors, sea turtles tend to provoke a lot of questions. Spend an hour with someone who is watching a turtle nest for the first time, and inevitably the questions will come: How old do they get? Where will she go after she leaves the beach? Where did she mate? When will she come back? How long until the babies become adults? And so on.
When it comes to turtles, however, the answers to such seemingly simple questions can be surprisingly elusive. Those of us who work with turtles have therefore grown accustomed to answering with phrases such as “We don’t really know, but ...” or “Our best guess is that ....” Although the lack of concise answers to basic questions about sea turtle biology can be frustrating, that lack is precisely what makes sea turtles so interesting to study. After 60 years of science, sea turtles are still mysterious in many ways.
Increasingly, however, advancements in technology and the results of long-term studies are giving scientists the information they need to answer with increasing certainty some age-old questions about turtles. Some mysteries are being solved, and yet others are still answerable only with our best guess. With such continuing mysteries in mind, we thought it would be fun to invite three experts to weigh in with current perspectives about three of the most frequently asked questions concerning sea turtles, and here is what they had to say.
How Many Eggs Does It Take to Make an Adult Turtle?
By PILAR SANTIDRIÁN-TOMILLO
A loggerhead hatchling begins its journey to sea. So little has been known about the early part of sea turtles’ lives that this period is often called the “lost years.” © KATE L. MANSFIELD
Shortly after arriving at our project in Pacific Costa Rica, volunteers go on beach patrol and excitedly anticipate their first encounter with a nesting leatherback turtle. Walking along the beach in the middle of the night or watching a turtle lay eggs in the beam of a red light will make anybody wonder about sea turtle biology. Not surprisingly, many excellent questions arise. A common one we hear is “How many eggs make an adult turtle?” The answer to this apparently simple question is actually very complex and requires many assumptions. In fact, there is not one magical number that serves as the correct answer.
A female of any species in a stable population (and this is the first assumption) produces enough offspring to replace herself and her male partner. Because we know very little about male turtles and because natural sex ratios are complex (they are normally female biased as hatchlings but are possibly more evenly balanced among reproductive animals), we make a second assumption that there are 1:1 sex ratios (i.e., a female needs to reproduce herself and one male in her lifetime). We could make the problem more complicated,
because sea turtles exhibit both polyandry and polygyny, but to answer this question succinctly, we’ll keep it simple.
Next, we also need to know how many eggs a female will lay, on average, during her lifetime to be able to replace herself and a male. This question implies knowing how many eggs she lays per clutch, how many clutches she has per season, how frequently she reproduces, and how long her reproductive lifespan lasts. The last piece is especially difficult, because most projects haven’t been around for long enough to exceed the reproductive lifespan of a long-lived sea turtle, but some projects have observed that turtles can reproduce over a period of 20–30 years.
So, let’s say that an average female leatherback in Pacific Costa Rica lays 66 eggs per clutch, lays 6 clutches per season, reproduces every 3.7 years, and has a reproductive life of 20 years. Such a female will lay 2,141 eggs in her lifetime to replace two adults (herself and her partner), which yields an estimate of about 1,000 eggs to make one adult turtle.
To arrive at our estimate, we have made a rough, but educated, guess. It is based on the best available information from a single population of a single species. Although it is by no means accurate, nevertheless, it gives us an idea of the effort that it takes to keep sea turtle populations stable. From this calculation, we can see that turtles must make a huge investment in reproduction because many eggs, hatchlings, and juveniles die before reaching maturity. The investment needed to keep the population stable varies among species, across populations, and with changes in survival of the different age classes within the same population. Moreover, populations are not stable; they are dynamic and change over time. The best way to approach this question, therefore, is to look at the unique characteristics of each nesting population and to make the calculations using population-specific numbers.
In the end, finding an exact number to this elusive question may be less valuable than the thinking that is stimulated by simply asking it.
This article originally appeared in SWOT Report, vol. 14 (2019). Click here to download the complete article as a PDF.