Older Siblings May Outcompete Their Younger Siblings But Are Least Likely To Make It To The Competition At All

Main Category: Biology / Biochemistry
Article Date: 11 Mar 2008 - 17:00 PDT

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You are a younger child in your family, and your older siblings use their size and age advantage to exercise their dominance, leaving you, invariably, with the smallest piece of the pie. If this sounds familiar, rest assured that you are not alone in the animal kingdom and, in fact, you should be happy to survive long enough to get any pie at all.

Many organisms, not just humans, produce offspring in series and then rear these dependents simultaneously, setting the stage for a competition-mediated probability of post-natal hardship that depends on the order in which the individual offspring was produced.

Birds are famous for this, whereby the earliest laid eggs hatch first, and hatchlings gain an initial competitive advantage over younger siblings. As a result, the youngest often dies from being out-competed for limited resources provided by the parents. However, many researchers that have documented this phenomenon, whereby probability of mortality increases with laying order within the brood, have not accounted for viability differences during other periods of the life cycle, in particular during the pre- hatching period.

Keith Sockman, an Assistant Professor of Biology at the University of North Carolina at Chapel Hill and the third of four sibling offspring, has discovered that, in Lincoln's sparrows, the first-produced offspring is indeed the most likely to win the battle of post-hatching sibling competition. However, the first-produced are the least likely to make it to the competition at all because they are the least likely to hatch. This may be due to maternal neglect of the first-produced embryos while the mother is busy producing the subsequently laid siblings. Thus, ovulation order mediates a trade- off between pre-hatching and post-hatching viability. This trade-off ultimately results in a relatively even distribution among siblings in their probability of surviving the complete nesting cycle, from laying through incubation, the nestling stage, and finally fledging.

Sockman's findings are published this week in the journal PLoS ONE.

Ovulation Order Mediates a Trade-Off between Pre-Hatching and Post-Hatching Viability in an Altricial Bird.
Sockman KW
PLoS ONE 3(3): e1785. doi:10.1371/journal.pone.0001785
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