In Indonesia's Komodo National Park the world's largest lizard takes down prey with bacteria-laden toxic saliva and jaws that can snap a person's leg in half. Yet for all their ferocity, Komodo dragons (Varanus komodoensis) face hardships of their own. Especially the females: new research shows that the struggle to usher in the next generation of dragons most likely costs these monitor lizard mothers years of their lives.
"We were surprised to find the sex-specific differences, especially in longevity, between males and females," says Tim Jessop, a zoologist at the University of Melbourne in Australia and one of the co-authors of the PLoS One paper. "There's no way we could have worked that out without doing a long-term field study."
Although Komodo dragons are well-known icons of nature's extreme side and are protected under the International Union for Conservation of Nature, conservationists do not know much about the species's growth rate, population differences or life span. To begin teasing out some of these factors, Jessop and a team of international researchers followed 400 Komodo dragons found in 10 populations around the national park's islands. For eight years the researchers collected data about the animals' growth and survival using a capture–mark–recapture method. After collecting those data, they used statistical models to examine how the dragons varied depending on age and sex, taking into account island location, prey availability and lizard population density.
Their results revealed gross disparities between the sexes, with females living, on average, 32 years, compared with 60 for the males. Moreover, the former grew to around 1.2 meters long and weighed about 22 kilograms whereas the latter measured in at more than 1.5 meters and nearly 70 kilograms.
Although Jessop and his colleagues cannot prove a cause-and-effect relationship without performing manipulations in the field, they suspect that reproduction accounts for the differences between the genders because all other factors appear equal in the animals' ecology. Females breed as often as once a year, a process that costs them threefold: They spend energy producing eggs, manually digging nests in which they lay their eggs and, most important, guarding the nest for up to five months against hungry males or other predators. During the nest-guarding stage, the females reduce feeding or fast altogether, and emerge from the ordeal in poor physical condition, meaning they are more susceptible to diseases or attacks by cannibalistic conspecifics.
Males, on the other hand, must compete for the few reproductively available females. This competitive pressure further exacerbates the sex-based skew, the researchers think, selectively pushing males toward progressively larger body sizes to give them the advantage for winning mates.
"It sounds horrible that these differences exist, but it's a strategy that works for the species," Jessop says. "On the evolutionary side of things, Komodo dragons have been using this strategy for countless millennia on these small islands, so obviously it works, or else they would be extinct."
But like any animal confined to a small territory and unable to mingle with outside populations, he points out, Komodo dragons remain particularly vulnerable to environmental flux. Climate change that increases rainfall, for example, threatens to inundate some of the lizards' island habitat, which may shift vegetation away from the current savanna grasslands that support the dragons' prey: predominantly water buffalo and deer. In addition to habitat degradation, however, the more immediate threats include humans poaching prey species and, occasionally, the dragons themselves.
To best prepare for these scenarios and predict how the monitors may respond, Jessop and his colleagues plan to further tease out differences between isolated dragon populations living on separate islands. "If dispersal is not taking place, then those classic harbingers of extinction could take over and mandate the fate of those populations," he says.
The researchers will share their results and recommendations with the national park and Indonesian government in the hopes of designing management schemes best suited to individual dragon populations. "We really need to be very careful about how we're going to look after these animals in the future," Jessop says.