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Short-lived chameleon may further research into aging Newly discovered species has shortest life span of any four-legged animal mongabay.com June 30, 2008 Rare color-changing lizard lives an insect-like existence but may help researchers better understand the process of aging.
Labord's Chameleon (Furcifer labordi), a lizard found only on the island of Madagascar, is the first reptile species known to have an annual lifecycle. Among the world's 28,300 species of tetrapods — four-limbed vertebrates — only a handful share this trait, but Furcifer labordi exhibits the shortest observed life span of any documented tetrapod species. "Remarkably, this chameleon spends more of its short annual life cycle inside the egg than outside of it," note the authors. The species' post-hatching life span is just 4 to 5 months. The researchers say the species discovery may not only explain why pet chameleons die notoriously quickly, but also shed light on hormonal determinants of aging, longevity, and senescence. Radio-tracking chameleons
The findings were quite unexpected lead author Kristopher B. Karsten, a zoologist at Oklahoma State University, told mongabay.com via email. "I actually set out to study the social behavior of F. labordi and also the perennial F. verrucosus initially," wrote Karsten. "But for my first season, I arrived fairly late in the active season (late January) and I noticed something really bizarre — I could only find adults but not juveniles in F. labordi. At this point, I thought that either I wasn't very good at spotting the juveniles or else they simply weren't there, which would indicate that the entire population is of the same age. Then at the end of the season, we saw this huge drop off in the population quite abruptly, but no signs of this species going into aestivation. I started to really have a hunch that it could be an annual species. "For the second year, I arrived much earlier, in mid-December," he continued. "At this time, we found only older juveniles and young adults. Throughout the season we saw the population aging at the same rate so that once again they were all adults with no juveniles at the end. And again, we saw the big drop in population numbers, but this time, we were able to document that it was due to some type of unexplained mortality (that is, it wasn't due to predation or other physical injury). I was starting to be convinced by now, but we collected two more years worth of data, including better sampling from the very beginning of the active season in November, and then the picture became very clear. Out of a sample size of nearly 400 individuals, there was not one outlier to contradict our hypothesis." Short life-cycle an adaptation to an arid environment? The researchers don't yet know why the species exhibits such an unusual life cycle, but they propose two possibilities that are "not mutually exclusive": (1) an adaptation to extreme environmental conditions and (2) a hormonal-driven evolutionary tradeoff between adult mortality rates and rapid growth and earlier age of reproduction. F. labordi lives in an area characterized by extreme seasonality, including a long, harsh dry season punctuated by a short, but unpredictable, period of rain. The authors note that mammals in Madagascar show two opposing "solutions" to these climate fluctuations: extreme versions of either "short-lived" or "long-lived" life histories. The survival strategy employed depends on whether environmental variability has a greater effect on juvenile or adult survivorship: "Among several Malagasy mammals (carnivores, primates, tenrecs, and rodents), reduced juvenile survivorship due to environmental variability resulted in the evolution of longer life spans, whereas stochastic climatic variables that reduced adult survivorship resulted in the evolution of shorter life spans," write the authors. "If environmental unpredictability differentially affected age-specific survivorship in chameleons, this may help explain why F. labordi is annual whereas other sympatric chameleons are perennial." An alternative theory is that "high adult mortality rates can drive the evolution of rapid growth and earlier age of reproduction, with the cost being decreased longevity." The authors note this trade-off, between resources allocated to somatic cell maintenance compared with reproduction, is regulated by hormones and can be correlated with behavioral attributes that affect reproductive success and adult mortality. "For example, increased androgens [hormones] in both natural populations and by experimental manipulation can be correlated with mating success but are also known to contribute to traits typically associated with increased adult mortality rates (e.g., reduced survival, increased parasite loads, increased energetic expenditure)," write the authors. "It seems possible that a change in the social structure in ancestral F. labordi, to a social system characterized by increased androgen levels or sensitivity, could contribute to increased intrinsic and/or extrinsic adult mortality. Indeed, F. labordi is characterized by physically intense combat and agonistic courtship. Accounting for hormonal regulation of physiology and behavior is critical to a comprehensive understanding of life history evolution. Although our hypothesis is plausible, the role of hormones, and even behavior to a lesser extent, is unexplored in chameleons. Our hypothesis can be tested by quantifying seasonal hormone profiles, social systems, and sexual selection within a phylogenetic comparative framework." Lead author Karsten says the species could eventually helping scientists better understand the process of aging. "We've found this tetrapod that doesn't live like all the others," he said. "This provides a great opportunity to answer some good questions on how organisms age, but also what are the historical, evolutionary forces that have shaped such a weird way of life." Kristopher B. Karsten, Laza N. Andriamandimbiarisoa, Stanley F. Fox, and Christopher J. Raxworthy (2008). A unique life history among tetrapods: An annual chameleon living mostly as an egg. PNAS July 8, 2008 vol. 105 no. 27 8980-8984.
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