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Infanticide and adoption are two behaviors seen in a variety of animals that are initially hard to explain by natural selection. These behaviors are two courses of action an adult can take when faced with someone else’s hungry young. The idea, though, that an individual would take the time and energy to feed and defend these offspring when they could instead ignore or eat them seems maladaptive. However, studies of birds show that adoptions and infanticides do occur and shed light on how these behaviors have been selected and when each action is advantageous. In European starlings, a replacement male’s decision to adopt an entire brood or get rid of it corresponds to their expected paternity. Meanwhile, in tree swallows, males will generally adopt an entire brood as long as they father a threshold number of young. Male yellow-headed blackbird territory owners demonstrate their fatherly skills to females by adopting unrelated broods. In species that live in areas with few nesting sites, such house sparrows, females compete for nesting sites much like males compete for territories in other species. Due to this competition, when females replace other females at a nest, the replacement females kill the brood. Finally, infanticide also occurs in communal breeders, such as the guira cuckoo. Individuals that did not reproduce in a given reproductive cycle force the entire group to renest when they kill the young hatchlings, thereby giving themselves a chance to reproduce. By studying the variations in adoption and infanticide behaviors in birds, the adaptive value of each can be determined.

Author: Brittany Hodgson

Introduction

A male barn swallow about to commit infanticide.
Male barn swallow about to commit infanticide by throwing the non-related baby bird in his beak out of the nest.
Courtesy of Tico_Bassie and Flickr.

Natural selection causes behaviors that maintain or increase the fitness of an individual to spread in a population and behaviors that decrease fitness to fade away. This idea implies that behaviors now seen in animals must be adaptive. However, behaviors like adopting offspring, which costs energy and time helping them to survive, or like committing infanticide, which wastes energy killing offspring, do not obviously increase the fitness of the actor. Why then would adoptive and infanticidal behaviors be found in so many organisms?

Siblicide, a type of infanticide

Birds perform infanticide on their own young, not just on other’s young. Although they may not be the actors in the death of their young, they may be passive watchers, allowing another individual to kill their young. One example of this is manifest in siblicide, where adults watch as one of their chicks is repeatedly pecked and abused until it dies. Usually, this occurs in species that live in variable environments. The mothers lay more eggs than she can raise with the resources in the area. Because she cannot support all of these eggs, she allows the biggest and strongest to survive. Sometimes, the mothers may even stack the cards in favor of the oldest. By infusing the first egg with more testosterone and by laying it before laying others, the oldest chick is favored to win because of its big size when the younger chicks hatch. (Hillstron et al. 2006 and Fujioka 1985).

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Source:  OpenStax, Mockingbird tales: readings in animal behavior. OpenStax CNX. Jan 12, 2011 Download for free at http://cnx.org/content/col11211/1.5
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