Not Exactly Rocket Science

The mayfly is known for its incredibly short adult life. After spending months as larvae, the adults finally hatches only to fly, mate and die within the space of a day. Now, in the dry south-west corner of Madagascar, scientists have discovered the lizard equivalent of the mayfly - Labord's chameleon (Furcifer Labordi).

The lifespan of Labord's chameleon is hardly as compressed as that of a mayfly, but it is extraordinarily short for a tetrapod (an animal with four legs and a backbone). From laying of egg to kicking of bucket, the lizard's entire life is played out in a year and 7 months of that is spent inside the egg. The adult chameleons hatch in unison in November and in April, the entire population dies en masse. We know the lifespans of over 1,700 species of tetrapods and none are as short as the Labord's chameleon's.

In fact, the vast majority live for several years, if not decades, and extreme longevity is fairly common. Whales, giant tortoises, some parrots and indeed, some humans only pop their clogs after more than a century of life. In contrast, very few tetrapods have adopted strategies at the other extreme, where life involves a rapid race to maturity and death within less than a year. Until now, the only tetrapods known to do so were a handful of marsupial mice and opposums, and even then, only the males.

Kristopher Karsten from Oklahoma State University has changed all that by spending four years studying Labord's chameleon in Madagascar's dry south-west corner. The wet season begins in November as tropical storms sweep in from the Indian Ocean, and it's then that the first chameleon hatchlings emerge. Most share the same birthday and mature at the same pace, which means that during these months, every single living Labord's chameleon is the same age.

Life of a Lobard

The lizards grow quickly, packing on about 2-4% of their body mass every day. By early January, they are sexually mature adults and by February, females start to lay eggs, just as the wet season draws to a close. This brief window, when both adults and eggs co-exist is the only point in the year when two generations of Labord's chameleon can be found on Madagascar.

After their eggs are laid, the adults' health rapidly worsens, they lose weight, their grip weakens and Karsten saw many of them falling from the trees. By April, all the adults are dead. The eggs remain in a state of arrested development for most of their 8 months of incubation, until the arrival of the rains the following November triggers another round of hatching.

No other tetrapod has a life cycle quite this short, and no other spends such a comparatively large amount of time in the egg. It's unclear why this species in particular has evolved in such an extreme way, but Karsten suggests that Madagascar's harsh and highly seasonal environment may have been a contributing factor. In response to these unpredictable conditions, Labord's chameleon appears to have compressed the majority of its life into a much stabler environment - its own egg.

By all accounts, Labord's chameleons live brutal and difficult adult lives. Even their sex lives is harsh, with males competing violently and intensely for mates, and sex itself being fairly aggressive. In general, species that run high risks of being killed as adults tend to grow quickly, mature early and die young - Labord's chameleon is clearly no exception.

The fact that the unusual life cycle of Labord's chameleon has only just been discovered shows how little we know about these apparently familiar creatures. And for good reason - the majority of them live in Madagascar where they are difficult to find, not least because of their vaunted ability to match their surroundings.

For the moment, Karsten's findings have direct implications for conservationists. Chameleons are notorious for dying rapidly in captivity, and this work suggests that this mortality might simply represent a very short, but entirely natural, adult lifespan.

Reference: PNAS doi:10.1073/pnas.0802468105

Images by Nick Garbutt and Ken Preston-Mafham, taken from ARKive