Mom really did like you best
What insight can we gain from the animal kingdom about family size and sibling dynamics in humans? Scott Forbes, a professor of biology at the University of Winnipeg, tackled this question in his 2005 book, A Natural History of Families. In this e-mail to The Boston Globe Magazine, Forbes writes about birds and mammals that routinely have more offspring than they intend to end up with, as humans did in the days when infant mortality was much higher
In birds there are a number of good examples of species that create surplus offspring: black eagles, white pelicans, and Nazca boobies are all large predatory birds that lay two eggs, but almost never raise two chicks, a phenomenon known as obligate brood reduction. The second egg serves as insurance against the failure of the first to hatch. If both eggs do hatch, the first-hatched chick normally kills its younger nestmate swiftly.
In the red-winged and yellow-headed blackbirds that I work on, the parents often lay clutches of four or five eggs though they do not often raise more than three chicks. The surplus become victims of brood reduction by selective starvation and mainly serve as insurance against the failure of one or more eggs to hatch. When food is very plentiful, however, the extra chicks are occasionally raised.
There are similar examples in mammals. My favorite is the panda. They often produce twins at birth, but routinely abandon one, allowing it to die in favor of the other. (Mom really did like you best.) This is the dark side of panda parental care.
Marsupials (pouched mammals) routinely produce more embryos than there are nipples. A marsupial "mouse" Antechinus agilis (colorfully named the Agile Antechinus) does this, and often eats the surplus babies.
Embryos not securing a nipple are doomed. There is an added twist to this story: These animals appear to use infanticide to select the sex of their progeny, as humans have done in the past, and still do today.
Pigs routinely produce more piglets than they expect to raise, and the surplus is culled by fatal sibling rivalry. The piglets are equipped with weapons for this, milk teeth that are used to slash littermates in competition for nipples. Farmers who are alert to this tend to clip these teeth at birth to prevent this unwanted behavior.
And domestic rabbits and house mice frequently produce surplus babies which then become victims of selective starvation. Female house mice will sometimes cannibalize their offspring, recycling nutrients that would otherwise go to waste.
