Life History Characteristics of Marine Turtles: What Are They and Why Are Adults and Juveniles So Different?

There is no question that the way living creatures go about the business of reproduction is complicated, so much so that there seems to be no end to how the process is described. Biologists, in an attempt to make some order out of this apparent chaos, have tried over the years to come up with overarching “principles”. These are largely based upon what qualities are characteristic of the two main actors, specifically, males and females.

Darwin, as so often is the case, was the first to do so with competence. His distinctions were based upon behavior. He characterized males as the sex most likely to spend time competing with other males for access to females, using aggression or conspicuous displays both to inhibit other males and to attract as many females as possible for mating. A consequence of those habits, incidentally, was the increased likelihood of risk-taking*. Unfortunately, aggression and other forms of competition often led to injuries and attracted predators, so males invariably suffered an earlier death than females. But for males, success was measured by the number of accomplished matings, not by how long they lived! Females, in contrast, were often drab in color, favored being inconspicuous, chose their mates carefully, stayed safe as long as possible, and used their time to convert food to as many surviving offspring as possible. Thus, as Darwin suggested, each sex had the same goal (maximizing offspring production) but went about the process behaviorally in entirely different ways.

An alternative perspective was developed by ecologists who while interested in behavioral differences between the sexes, were far more concerned with relationships between reproduction, on the one hand, and on the other hand by adaptations shown by populations to favor survival in their environment. They came up with a scheme based upon what they called “life-history characteristics”. According to this idea, “environments” can be characterized as either relatively stable (that is, changing slowly over long time periods) or unstable (subject to frequently unpredictable, sudden and drastic change caused by fire, storms, drought, flooding, etc.).

Organisms living in those contrasting environments exhibited very different reproductive strategies. In stable environments, organisms tend to be large, slow growing, live a long time, exist in populations that show little fluctuation in numbers, and produce only a few offspring over a long reproductive life span. Those youngsters receive lavish parental care and usually survive. In contrast, organisms living in unstable environments showed the antithesis of these characteristics. They tend to be small in size, grow quickly to sexual maturity, and produce many offspring in just one or a few reproductive episodes. Most of their offspring fail to survive. That’s because when conditions suddenly change, both adults and juveniles have to migrate elsewhere in search of another habitat. More often than not, they starve to death or are taken by predators before they can find a new home. But because so many offspring are produced, a few lucky ones usually find a new home so the cycle is repeated once again. Obviously, populations of these animals, again compared to species in stable habitats, fluctuate wildly over time. Those numbers are best described by the phrase “boom or bust”!

Ecologists soon recognized that most species didn’t fall neatly into one or the other category; instead, they exist along a continuum reflecting the extent to which their environment varied in its stability. Perhaps most interesting of all, some creatures, especially those found in marine habitats, show combinations of these life history characteristics, even within a species. Marine turtles are an excellent example (Figure 1).