The benefits of big neighbours

Larger offspring typically have higher survival, growth and reproduction than smaller offspring. So why then, do we see such a range in offspring size? PhD student Hayley Cameron tackles this conundrum and the results of her latest experimental study contradict accepted theoretical models by showing that bigger is not always better.

Classic life-history models assume a trade-off in the investment mothers make in the next generation; large offspring perform better but smaller offspring are ‘cheaper’ to make and so mothers make them in large numbers. These models predict that a single offspring size will maximise reproductive success in a particular environment. But, we don’t see single offspring sizes, we see a range of sizes.

Game-theory takes the models further and explains the variation in offspring size by generating a ‘competition-colonisation’ trade-off.  In these scenarios, larger offspring will win contests over smaller offspring, but smaller offspring are better able to colonise unoccupied areas because they are more abundant. This means, no single offspring size will maximise reproductive success for any given population and so variation in offspring size is maintained.

Hayley and her supervisor Dustin Marshall test the idea that larger offspring will out-compete smaller offspring in a well-studied model organism, the invertebrate Bugula neritina. This idea has received surprisingly little testing.

To do this Hayley collected larvae and measured each one before settling them on to acetate squares. She glued these acetate squares, with their newly settled offspring, onto PVC plates in pairs of different sizes. These plates were deployed at a field site and every week Hayley measured survival, growth and number of developing larvae for 336 individuals of known offspring size.

To their surprise Hayley and Dustin found, instead of being out-competed as predicted, small offspring received benefits from having larger offspring as neighbours. Large offspring did compete with large neighbours though, and these bigger offspring did best on their own.

The graph and table both show how reproduction (measured by the number of ovicells) in big and small Bugula changed depending on the size of its neighbour. Hayley found that when small Bugula were paired with big neighbours then the overall reproductive output of the small Bugula increased. In contrast, big Bugula did best with no neighbours at all.

Why did this happen? In this study, larger offspring grew into larger colonies and Hayley and Dustin think these larger colonies disrupt the flow which affects the supply of resources (food and oxygen) available to their neighbours.  A slower flow is likely to benefit smaller colonies which tend to be less efficient at capturing resources in high flows. Conversely, larger, more efficient, colonies may deplete the resources available for their large neighbours.

So, while life history theory has traditionally viewed offspring interactions through the lens of competition, Hayley’s PhD work suggests facilitation might also be important in maintaining variation in offspring size.

More of Hayley’s PhD work on variation in offspring size: Should mothers provision their offspring equally? A manipulative field test

This research is published in the journal Evolution.