Karsten, K.B., Andriamandimbiarisoa, L.N., Fox, S.F., Raxworthy, C.J. (2008). A unique life history among tetrapods: An annual chameleon living mostly as an egg. Proceedings of the National Academy of Sciences, 105(26), 8980-8984. DOI: 10.1073/pnas.0802468105.
If we are talking about the Madagascarian chameleon Furcifer labordi, then the answer is the egg. According to Karsten et al., the eggs of F. labordi incubate for 8-9 months before synchronously hatching at the onset of the rainy season. The hatchlings grow rapidly, reach sexual maturity in less than 2 months, reproduce in January–February and then start dying off only 4-5 months after hatching. Their annual lifestyle is apparently unique or almost so among the ~28,300 species of four-limbed vertebrates (tetrapods) that normally have perennial life spans.
There is a distinct contrast between the population demographies of F. labordi and the perennial chameleon F. verrucosus.
Table 1 from Karsten et al.
From spring thru November, there are only eggs of F. labordi around; otherwise, there are no juveniles, no adults. Whereas, in the case of F. verrucosus, there is never a period during a year when one can’t find a mixture of different age groups of chameleons.
Interestingly, the 2 chameleon species are sympatric, that is, they can be found together in the same habitat. What factors may have been responsible for the evolution of such different lifestyles?
The demographic and body size comparisons of the 2 species suggest one possible answer. During the period from late December thru early March, F. labordi is present only as adults (and eggs), while F. verrucosus is present mostly as juveniles and some adults. This situation may give the adult F. labordi, whose maximum snout-vent length is about 110 mm (Fig. 2 in the paper), a better chance to compete against the smaller number of adult F. verrucosus, whose maximum SVL may reach 200 mm.
Karsten et al., do not discuss this possibility, at least not directly. Is something wrong with my reasoning?