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Parent-Offspring Conflict: Evolution, Weaning & Significance

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Have you ever thought your parents were paying more attention to your brother or sister than you? Well you're not alone. This lesson will explore the parent-offspring conflict in animals, looking at why it occurs while highlighting the weaning conflict and siblicide.

What is Parent-Offspring Conflict?

There are some amazing animal parents. Take the orangutan mom, who is so devoted to her offspring, she only has one every eight years. In the first four months, the two are inseparable and she even nurses until her baby is five years old. Or the highly dedicated emperor penguin parents, with the male incubating the eggs in frigid temperatures and the female going long distances to find food.

But sometimes all of this parental investment, which is anything a parent does to increase an offspring's chances of survival, while impeding the parent's ability to invest in future offspring, isn't enough. Well, at least in the eyes of the offspring. This tug-of-war between what a parent is willing to give and what an offspring wants is termed parent-offspring conflict.

Why does this tug-of-war exist? Well, we need to understand about diploid, sexually reproducing critters. Wait, diploid? Diploid is a term used to describe an organism that has one set of chromosomes from each parent.

In diploid (sexually reproducing) organisms, the parents have a 50% relation to each of their offspring. On the other hand, the offspring is 100% related to itself, 50% related to its full-siblings, and 25% related to its half-siblings. This difference in relations creates a conflict.

It's the biological goal of an organism to pass on its genes, and since a parent shares 50% of its genetics with each of its offspring, it's in the parent's best interests to invest in each offspring equally. However, the offspring is 100% related to itself, so it's in its best interest to have all of the parental investment.

The more parental investment, the more likely the offspring will survive into adulthood and be able to reproduce, passing on its own genetic material. Ahhh, do you see the conflict?

Weaning Conflict

An example of parent-offspring conflict is the weaning conflict, which exists in mammals where the mother wants to stops nursing but her offspring wants to continue. The sooner the mother weans her baby, the sooner she can reproduce again, thus having more offspring.

The weaning conflict is not only seen in primates. It is seen in many mammals species including cats, dogs and rats.
Dog nursing

The baby, on the flipside, wants to nurse as long as possible so it can get optimal nutrients and be protected by the mother. The weaning conflict is common in several mammal species, including primates.

For example, in baboons, the weaning process can last months while the mom bites, pushes and hits her offspring in order to keep it away, while the baby cries and screams. So, while the mother rejects it, the baby has all sorts of tactics to try to get the mother to continue nursing. This arguing leads to a lengthy weaning process.

Siblicide

Let's take this whole parent-sibling conflict to the next level. Sometimes half- or full-siblings will kill each other in what is termed siblicide. This can be seen in different species, but is more common in birds, especially those with asynchronous hatching, meaning all of the eggs do not hatch at once. The older fledging will either take all of the food, leaving the younger sibling to starve to death, or, in some species, one fledging will kill the other.

A study of the Nazca Booby, an eastern Pacific seabird with a catchy name, found that one sibling killed the other even when the parents could feasibly support both. Scientists speculated that this evolved because, at one time, the environment could not support two offspring, so the form of the gene that promoted siblicide became prominent. Now, even though the environment could support two offspring, the other form of the gene isn't found in the population anymore.

In some species, siblicide occurs when there aren't enough resources to support both, so the older, stronger sibling gets all of the food and the younger sibling starves to death. In these cases, both will survive if there are enough resources.

Unlike some other species, the osprey fledglings will only fight when food resources are low
Osprey

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