Offspring size affects all aspects of an organism’s life, from birth through to reproduction, and studies show that larger offspring do better overall.
Despite the long standing interest in the drivers of differences in offspring size, most studies focus only on one particular taxon or system.
Dustin Marshall, Amanda Pettersen and Hayley Cameron were interested in looking at offspring size across all taxa and at different levels of organisation – within a brood, between individuals and across different species and environments – to see if this wider scope could help them better understand the causes and consequences of variation in offspring size.
They started by looking at a pattern that will be familiar to many ecologists and bio-geographers; offspring size tends to get bigger as you move from the tropics to the poles. They found that this was true for practically all species they compiled data for, with the notable exception of turtles and plants.
Dustin, Amanda and Hayley suspect that the difference in the patterns they recorded relates to the way offspring size and temperature affects development. Small increases in temperature are known to yield large increases in development rate. The lower number of warmer days in higher latitudes might just mean that there just isn’t time for larger seeds or turtle eggs to complete development. Importantly, turtles don’t incubate their eggs and so will be more susceptible to environmental temperature than taxa that do (birds for example). Collecting data on egg size variation in other reptiles would help to test this theory.
For taxa such as fish, amphibians and invertebrates the overall smaller egg size in comparison to seeds and other vertebrates might preclude development time as a limitation on size.
Offspring size also varies across populations and within broods from the same females. Dustin and colleagues highlight that sources of variation might be external whereby mothers buffer their offspring from harsher environments by making them bigger, or choose to maximise numbers in more benign environments, meaning that offspring are smaller. Mothers might also provision offspring unequally within a brood to ensure that whatever environment the offspring find themselves in, some at least, will do well.
Hayley’s PhD work however, suggests that variation in size within a brood reduces competition between siblings and all offspring, regardless of their size, do better.
Finally the team considered the question as to why larger offspring generally tend to perform better than smaller offspring. They were interested in understanding the costs and benefits of a larger size to the energy available for fitness-enhancing functions such as growth and reproduction.
It seems that larger offspring often access more energy resources than smaller offspring. In plants, seed size likely affects photosynthetic capacity, in certain fish and snakes, a larger gape size at birth allows for more efficient energy acquisition and, in filter feeding invertebrates, larger offspring initially produce more or larger feeding structures. In addition, larger offspring should expend relatively less energy than smaller offspring and complete energetically costly developmental stages with more energy reserves intact.