29 August 2008

The evolution of offspring size

ResearchBlogging.orgThe question of offspring size - that is, how big a child is relative to its parent - can seem downright absurd. In fact, it was the subject of the only paper (to my knowledge) ever published in the journal Evolution that ends with a punch line. That piece, written by Ellstrand in 1983, pretended to seriously address the question of why juveniles are smaller than their parents [$-a]. It was basically pointing out the absurdity of assuming that every trait we observe in a living organism has evolved adaptively, or specifically because natural selection favors it. Clearly, not all traits are adaptive - juveniles are smaller than their parents because, universally, a child has to emerge from its mother. It's a basic fact of the conservation of mass-energy.

Photo by TIO....
On the other hand, the size of a child as a proportion of its mother's body size varies tremendously in the natural world. Mushrooms release nearly-invisible spores, while kiwis lay eggs equal to a quarter of their body mass. There seem to be clear benefits to making bigger offspring - they should be better competitors against their peers, they may be more likely to survive to reproduce, and they may reach reproductive maturity faster. But there are also costs, in terms of the energy a parent uses to either produce an egg with a bigger yolk, or to provision a bigger embryo in the uterus, or to feed a juvenile in the nest.

The classic description of this trade-off is a mathematical model developed by Smith and Fretwell in 1974 [$-a]. But the Smith-Fretwell model doesn't explain the wide variety of offspring sizes we see in nature, especially among species that seem to have more or less the same ecological requirements. In the current issue of The American Naturalist, Falster et al. propose an extension of Smith-Fretwell to better capture this variation, which follows juveniles from the moment they leave their parents, through a phase of establishment and growth, and then through a period of competition with their peers. Which juveniles survive to adulthood is determined by body size - in the final period of competition, big individuals win.

Falster et al. then use the parameters of the model - adult body size, total adult lifespan, and energy used for reproduction - to predict juvenile body sizes for mammals and plants. The model seems to predict the relationship between parental size and offspring size pretty well for mammals, not so much for plants. Which is actually not all that surprising. In mammals, adult and juvenile body sizes probably have a lot less "wiggle room" relative to each other; but adult plants can be multiple orders of magnitude bigger than the seeds they produce. So there's just a lot more variation to try and explain in plants than there is in mammals.


N.C. Ellstrand (1983). Why are juveniles smaller than their parents? Evolution, 37, 1091-4 URL: http://www.jstor.org/pss/2408423

D.S. Falster, A.T. Moles, M.Westoby (2008). A General Model for the Scaling of Offspring Size and Adult Size. The American Naturalist, 172 (3), 299-317 DOI: 10.1086/589889

C.C. Smith, S.D. Fretwell (1974). The optimal balance between size and number of offspring The American Naturalist, 108, 499-506 URL: http://www.jstor.org/stable/2459681

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