The prenatal period is a crucial time for offspring development. Mothers are in a unique position to influence this development, through the resources she invests. In the majority of species, this development occurs in an egg (or seed), allowing us a glimpse at this process. However, although eggs are packed full of nutrients, it is actually incredibly hard to demonstrate that mothers who produce bigger eggs (and so invest more into the development of their offspring) benefit through the increased growth and survival of their offspring. This difficulty arises because offspring not only receive resources from their mother, but they also inherit genes. If the genes that influence offspring growth are the same as those that influenced the size of the egg that the mother produced, then it would appear that the prenatal investment that a mother made influenced an offspring’s growth. Furthermore, other components, e.g. mitochondria, are entirely maternally inherited, and these too could influence both growth and egg investment. Therefore isolating the effects of prenatal maternal investment is extremely difficult.

Over the last few years, we have been working on a solution to this long-standing problem. We created selection lines for high and low maternal investment (measured as the size of the eggs) in the Japanese quail. Whilst comparing offspring directly between these lines would not separate maternal from genetic effects, by reciprocally crossing the lines, we could compare the growth and survival of hybrids that had a similar genetic background, but differed in the amount of maternal investment they received during development (i.e. according to their maternal line). Furthermore through matching offspring to the specific egg that they hatched from, we could further separate the effect of the egg from other maternally inherited components (e.g. mitochondria).

Maternal investment indeed had strong positive effects on offspring growth and survival, that were not caused by correlated genetic effects. Moreover, they could show that these maternal effects were due entirely to maternal egg investment through to independence. At this stage additional to these egg effects, there was an additional effect on offspring size, most likely due to mitochondrial variation, that is previously unreported. Although these distinctions may seem arbitrary, they are extremely important for understanding how both offspring growth and maternal investment evolve, as well as a starting point from which to understand why some mothers invest more than others. For more details see our paper in published in The American Naturalist.