Ex ovo ad mortem: Life as an egg

Many insects live a long time as larvae and only briefly as sexually mature adults — extreme examples include the mayfly, the cicada, and some crane flies, though there are countless others. In most cases, the brevity of adult life is not because of rapid onset of decrepitude but rather because the adult morph lacks some essential tool (like a mouth).

Such life histories are vanishingly rare among vertebrates — though they do exist, as revealed by this fascinating (and, to my mind, somewhat poignant) tale from Karsten et al., in which the short-lived adult does appear to be undergoing accelerated senescence:

A unique life history among tetrapods: An annual chameleon living mostly as an egg

The ≈28,300 species of tetrapods (four-limbed vertebrates) almost exclusively have perennial life spans. Here, we report the discovery of a remarkable annual tetrapod from the arid southwest of Madagascar: the chameleon Furcifer labordi, with a posthatching life span of just 4–5 months. At the start of the active season (November), an age cohort of hatchlings emerges; larger juveniles or adults are not present. These hatchlings grow rapidly, reach sexual maturity in less than 2 months, and reproduce in January–February. After reproduction, senescence appears, and the active season concludes with population-wide adult death. Consequently, during the dry season, the entire population is represented by developing eggs that incubate for 8–9 months before synchronously hatching at the onset of the following rainy season. Remarkably, this chameleon spends more of its short annual life cycle inside the egg than outside of it. Our review of tetrapod longevity (>1,700 species) finds no others with such a short life span. These findings suggest that the notorious rapid death of chameleons in captivity may, for some species, actually represent the natural adult life span. Consequently, a new appraisal may be warranted concerning the viability of chameleon breeding programs, which could have special significance for species of conservation concern. Additionally, because F. labordi is closely related to other perennial species, this chameleon group may prove also to be especially well suited for comparative studies that focus on life history evolution and the ecological, genetic, and/or hormonal determinants of aging, longevity, and senescence.

If nothing else, an apt reminder of the crazy games evolution plays in determining the genetic control of lifespan.

But examples like this are more than curiosities — per the final sentence of the abstract (emphasis mine), the vast diversity of life histories generated over the course of evolution provide an ideal laboratory in which to investigate the determinants of lifespan. Specifically, what can we learn from organisms with similar body plans and overall metabolism but significantly different lifespans? At least one project that will explore and exploit the longevity differences between related species is already underway.


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