Metabolic theory: how does the cost of development scale allometrically with offspring size?

One of the most fundamental patterns studied in life-history theory is how offspring size links to performance of an individual. Within species, larger offspring generally have higher survival, reproductive output and growth, and lower risk of predation and starvation. One key question that remains is why larger offspring outperform smaller offspring.

The Centre’s Amanda Pettersen and colleagues Craig White, Robert Bryson-Richardson and Dustin Marshall explored one potentially widespread mechanism: how the costs of development scale with offspring size, using metabolic theory. Metabolic theory proposes that there is an allometric relationship between energy use (metabolic rate) and body size, where on a log-log scale, the slope of this relationship is less than one.

Amanda and colleagues sought to explore whether the same pattern (i.e allometric scaling) occurs with offspring size, in order to understand how size affects the relative use of energy reserves throughout a critical life period. They measured embryo mass and metabolic rate throughout development, from fertilisation to hatching, in the freshwater fish, Danio rerio.

3-hour old embryos of the tropical freshwater zebrafish, Danio rerio.

The team found an allometric relationship between embryo mass and metabolic rate – while larger offspring use absolutely more energy, they also use relatively less energy to reach the end of development, than smaller offspring. Larger offspring use proportionally less of their supplied energy to reach the end of development than smaller offspring. These findings are supported by the observation that hatchlings from larger embryos are both disproportionately heavier and retain relatively more of their initial energy reserves than smaller embryos. These findings mean that the same allometric scaling relationships that are found for adult body size also apply for offspring size. But they also may explain a fundamental pattern in life-history theory: allometric scaling with offspring size may serve as a widely applicable explanation for why larger offspring often perform better than smaller offspring.

This research in published in the journal Functional Ecology.