Shoebills, the Second Born Gets the Boot

The documentary Africa (Episode 2, Savannah) depicts a scene that is not easy for most people to watch. A Shoebill hatchling, born just three days earlier, continuously picks on its younger sibling. Moreover, the parents allow this to happen and only feed the older chick, thereby ensuring the demise of one of their own offspring. If the name of the game of evolution by natural selection is passing on your genes as much as you can, why in certain populations do the Shoebills practice siblicide?

The most important contributor to the number of offspring that a family of Shoebills (or many other species for that matter) can raise comes from the resources available in the environment. As it turns out, the less food there is available in an ecosystem, the more likely there is to be siblicide [1]. This then begs the question, why even lay a second egg if it is just going to be abandoned by the family? Evolution has led these birds to lay what is known as an insurance egg in case the first born does not survive. This trait is certainly not random, as it actually increases the overall fitness of the Shoebills. The relatively small cost of a second egg is outweighed by the potential benefit of that second egg stepping in if the first one fails [2].

Why exactly then is the sibling the one that ostracizes its nestling? This comes down to the overall fitness of an individual. This fitness encompasses both its own survival, as well as the success of its family. A key point of evolution, however, is passing on one’s own genes. When evaluating the DNA of an individual relative to its parents or siblings, it is easy to see that an individual is wholly related to itself, while on average half as related to its siblings or parents [3]. This causes conflicts known as “sibling rivalry” and “parent-offspring conflict” [4]. The older Shoebill would rather guarantee its own survival than risk sharing food with its sibling and not having enough for itself. This ultimately allows the highest net benefit of fitness to the surviving Shoebill sibling.

Although this scene is heart-wrenching, it goes to show that evolution will nudge species to maximize their fitness in any way that they can.

Africa, Season 1, Episode 2, Savannah, starting at approximately 15:54

by Ben Gabanic, Gautam Nayyar and Varsha Chiruvella

References

  1.  Creighton JC & GD Schnell. 1996. Proximate control of siblicide in cattle egrets: a test of the food-amount hypothesis. Behavioral Ecology and Sociobiology 38.6: 371–377.
  2. Anderson DJ. 1990. Evolution of obligate siblicide in boobies. 1. A test of the insurance-egg hypothesis. The American Naturalist 135.3: 334-350.
  3. Queller DC & KF Goodnight. 1989. Estimating relatedness using genetic markers. Evolution 43.2: 258-275.
  4. Godfray HCJ. 1995. Evolutionary theory of parent–offspring conflict. Nature 376.6536: 133-138.

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