Dustin J. Marshall, Amanda K. Pettersen & Hayley Cameron
The size mothers make their offspring has profound consequences for the fitness of both mothers and offspring. All multicellular organisms must trade-off the number versus size of offspring that they produce and so the challenges facing mothers at reproduction are universal. Despite this ubiquity, reviews of offspring size that cover more than a single taxonomic group are remarkably rare. We explore offspring size variation and its consequences across a wide variety of organisms, from marine invertebrates to plants through to birds. We show systematic patterns in offspring size, with most groups making smaller offspring in the tropics relative to the poles, and offer general explanations for these patterns – selection for larger offspring is greater at higher latitudes. We found two groups, plants and turtles, in which offspring size was actually larger in the tropics – we suspect that temperature-mediated developmental constraints drive these exceptions. We also provide evidence that larger offspring perform better than smaller offspring because they have more net energy for performance-enhancing functions due to more efficient development. We show that mothers often manipulate the size of their offspring in order to maximise their own fitness – for example, butterflies produce smaller, more numerous offspring when laying eggs on more nutritious plants. Nevertheless more tests are needed that provide realistic and reliable cues to mothers about the offspring environment. Finally we identify key knowledge gaps in our understanding of offspring size – despite years of intense study, we still lack estimates of the genetic covariance in offspring size across generations, yet such estimates are essential for determining the extent to which parents and offspring are in conflict over provisioning.