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 415 KB doi:10.1002/ecy.1590

Deconstructing environmental predictability: seasonality, environmental colour and the biogeography of marine life histories

Authors: Dustin J Marshall and Scott C Burgess

Published in: Ecology Letters, volume 18, issue 2 (February 2015)


Environmental predictability is predicted to shape the evolution of life histories. Two key types of environmental predictability, seasonality and environmental colour, may influence life-history evolution independently but formal considerations of both and how they relate to life history are exceedingly rare.

Here, in a global biogeographical analysis of over 800 marine invertebrates, we explore the relationships between both forms of environmental predictability and three fundamental life-history traits: location of larval development (aplanktonic vs. planktonic), larval develop- mental mode (feeding vs. non-feeding) and offspring size.

We found that both dispersal potential and offspring size related to environmental predictability, but the relationships depended on both the environmental factor as well as the type of predictability. Environments that were more seasonal in food availability had a higher prevalence of species with a planktonic larval stage.

Future studies should consider both types of environmental predictability as each can strongly affect life-history evolution.


Marshall DJ, Burgess SC (2015) Deconstructing environmental predictability: seasonality, environmental color and the biogeography of marine life histories, Ecology Letters, 18, 174–181 PDF 1.2 MB doi:10.1111/ele.12402

Offspring size in a resident species affects community assembly

Authors: Kurt Davis and Dustin J Marshall

Published in: Journal of Animal Ecology, volume 83, issue 2 (March 2014)


Offspring size is a trait of fundamental importance that affects the ecology and evolution of a range of organisms. Despite the pervasive impact of offspring size for those offspring, the influence of offspring size on other species in the broader community remains unexplored. Such community-wide effects of offspring size are likely, but they have not been anticipated by theory or explored empirically.

For a marine invertebrate community, we manipulated the size and density of offspring of a resident species (Watersipora subtorquata) in the field and examined subsequent community assembly around that resident species.

Communities that assembled around larger offspring were denser and less diverse than communities that assembled around smaller offspring. Differences in niche usage by colonies from smaller and larger offspring may be driving these community-level effects.

Our results suggest that offspring size is an important but unexplored source of ecological variation and that life-history theory must accommodate the effects of offspring size on com- munity assembly. Life-history theory often assumes that environmental variation drives intra- specific variation in offspring size, and our results show that the converse can also occur.

Full paper

Davis K, Marshall DJ (2014) Offspring size in a resident species affects community assembly. Journal of Animal Ecology, 83, 322–331 PDF PDF 274 KB doi:10.1111/1365-2656.12136

Fertilisation is not a new beginning: sperm environment affects offspring developmental success

Authors: Hannah Ritchie and Dustin J Marshall

Published in: The Journal of Experimental Biology, volume 216 (August 2013)


For organisms with complex life histories, the direction and magnitude of phenotypic links among life-history stages can have important ecological and evolutionary effects.

While the phenotypic links between mothers and offspring, as well as between larvae and adults, are well recognised, the links between sperm phenotype and offspring phenotype have been less well explored.

Here, we used a split-clutch / split-ejaculate design to examine whether the environment that sperm experience affects the subsequent performance of larvae in the broadcast spawning marine invertebrate Galeolaria gemineoa. The environment that sperm experienced affected the developmental success of larvae sired by these sperm; larvae sired by sperm that experienced low salinities had poorer developmental success than larvae sired by sperm that experienced a normal salinity.

When we explored the interactive effects of the sperm environment and the larval environment with an orthogonal design, we found an interaction; when sperm and larvae experienced the same environment, performance was generally higher than when the sperm and larval environments differed. These effects could be due to selection on specific sperm phenotypes, phenotypic modification of the sperm or both.

Together, our results challenge the traditional notion that sperm are merely transporters of genetic material; instead, significant covariance between sperm and offspring phenotypes exists. Our study adds to a growing list that demonstrates that fertilisation does have a homogenising effect on the phenotype of the zygote, and that events before fertilisation during the gamete phase can carry through to affect performance in later life-history stages.

Full paper

Ritchie H, Marshall DJ (2013) Fertilisation is not a new beginning: sperm environment affects offspring developmental success. The Journal of Experimental Biology, 216 (16), 3104–3109  PDFPDF 254 KB doi:10.1242/jeb.087221