Unequal provisioning of offspring – benefits via ‘bet hedging’ or niche partitioning?

Commonly observed variations in offspring size within broods of the same female are usually attributed to ‘bet hedging’. Bet hedging theory expects that in unpredictable environments, mothers trade-off the benefits of producing offspring of the same ‘optimal’ size (will usually perform best within a generation), by producing variable offspring; this ensures that at least some of these offspring survive the prevailing conditions. In other words, bet hedging theory suggests that variation in offspring size will provide benefits in fitness among generations, although the trade-off is reduced fitness within a generation.

Results from Hayley Cameron’s latest PhD research do not support the assumptions of bet hedging theory, and are soon to be published in the influential journal, Ecology Letters.

In this study, Hayley used the model species Bugula neritina, to directly manipulate within-brood variance in offspring size for broods of siblings that had the same mean offspring size. In particular, Hayley experimentally tested the fitness consequences of variable broods when siblings compete, and how the fitness of individual offspring of a particular size are affected by such sibling competition. Importantly, existing bet-hedging theory ignores the potential for such interactions among siblings.

Hayley found that performance of all offspring (irrespective of their size) was enhanced in broods that had greater initial variation in offspring size. This novel finding may be a result of within-brood variation decreasing competition among siblings via ‘niche partitioning’, whereby the differences in offspring size allow siblings to access different resource-pools, reducing niche overlap within these broods.

This novel explanation may also apply to other systems, especially systems where siblings remain in close proximity and therefore directly compete for resources. While theory does not yet account for such effects, Hayley’s PhD research is an important step towards generating new theoretical models to explain why mothers might unequally provision their offspring.

Hayley’s PhD is supervised by Keyne Monro and Dustin Marshall.