Why do larger mothers produce larger offspring? A test of classic theory

Authors: Hayley Cameron, Keyne Monro, Martino Malerba, Stephan Munch and Dustin Marshall

Published in: Ecology, volume 97, issue 12 (December 2016)


Across a wide range of taxa, larger mothers produce larger offspring.

Theory assumes that larger, more fecund mothers create higher local densities of siblings, and so larger mothers produce larger offspring to offset sibling competition. This assumption has been debated for over 30 years, but direct empirical tests are surprisingly rare.

Here, we test two key assumptions of classic theories that predict sibling competition drives maternal-size–offspring-size (MSOS) correlations:

  1. independent effects of offspring size and sibling density on offspring performance or
  2. as a product of an interaction between these two factors.

To simultaneously test these alternative assumptions, we manipulate offspring size and sibling density in the marine invertebrate, Bugula neritina, and monitor offspring performance in the field.

We found that, depending on the fitness metric being considered, offspring size and sibling density can either independently or interactively affect offspring performance. Yet sibling density did not affect offspring performance in the ways that classic theories assume.

Given our results, it is unlikely that sibling competition drives the positive MSOS correlation observed in this species. Empirical support for these classic theories remains lacking, suggesting alternative explanations are necessary.


Cameron H, Monro K, Malerba M, Munch S, Marshall DJ (2016) Why do larger mothers produce larger offspring? A test of classic theory. Ecology, 97: 3452–3459. PDF 430 KB doi:10.1002/ecy.1590

Fertilization is not a new beginning: the relationship between sperm longevity and offspring performance

Authors: Angela J Crean, John M Dwyer and Dustin J Marshall

Published in: PLoS ONE, volume 7, issue 11, doi: 10.1371/journal.pone.0049167


Styela plicata larvae

Styela plicata larvae. Image courtesy of Bronwyn Galletly.

Sperm are the most diverse cell type known: varying not only among- and within- species, but also among- and within- ejaculates of a single male.

Recently, the causes and consequences of variability in sperm phenotypes have received much attention, but the importance of within-ejaculate variability remains largely unknown.

Correlative evidence suggests that reduced within-ejaculate variation in sperm phenotype increases a male’s fertilization success in competitive conditions; but the transgenerational consequences of within-ejaculate variation in sperm phenotype remain relatively unexplored.

Here we examine the relationship between sperm longevity and offspring performance in a marine invertebrate with external fertilization, Styela plicata. Offspring sired by longer-lived sperm had higher performance compared to offspring sired by freshly-extracted sperm of the same ejaculate, both in the laboratory and the field. This indicates that within-ejaculate differences in sperm longevity can influence offspring fitness – a source of variability in offspring phenotypes that has not previously been considered. Links between sperm phenotype and offspring performance may constrain responses to selection on either sperm or offspring traits, with broad ecological and evolutionary implications.

Full paper

Crean AJ, Dwyer JM, Marshall DJ (2012) Fertilization is not a new beginning: the relationship between sperm longevity and offspring performance. PLoS ONE 7 (11) PDFPDF 254 KB External linkFull text, doi: 10.1371/journal.pone.0049167