Siblicide
Note: Varies by jurisdiction Note: Varies by jurisdiction Siblicide (attributed by behavioural ecologist Doug Mock to Barbara M. Braun) is the killing of an infant individual by its close relatives (full or half siblings).
[2] In facultative siblicidal animals, fighting is frequent, but does not always lead to death of a sibling; this type of behavior often exists in patterns for different species.
For instance, in the blue-footed booby, a sibling may be hit by a nest mate only once a day for a couple of weeks and then attacked at random, leading to its death.
Originally proposed by Dorward (1962) harvp error: no target: CITEREFDorward1962 (help), the insurance egg hypothesis (IEH) has quickly become the most widely supported explanation for avian siblicide as well as the overproduction of eggs in siblicidal birds.
In the context of Hamilton's inclusive fitness theory, the marginal chick increases the total number of offspring successfully produced by the parent and therefore adds to the gene pool that the parent bird passes to the next generation.
This is one of the driving forces of siblicide because it increases the fitness of the offspring by decreasing the amount of competition they have.
[4] Parents may either discourage or accept siblicide, depending on whether it increases the probability of their offspring surviving to reproduce.
[4] The cost and effect siblicide has on a brood's reproductive success can be broken down into an algebraic equation:
units of care in the current batch of offspring can expect a future reproductive success
to give their offspring any non-zero chance of their brood / litter maturing to themselves become breeding adults.
where the parents raise as many offspring as possible, with some risk to their own future, but not so much that they completely squander their own chance of breeding again.
Cattle egrets, Bubulcus ibis, exhibit asynchronous hatching and androgen loading in the first two eggs of their normal three-egg clutch.
The extra "excess" egg is possibly laid either due to exploit the possibility of elevated food abundance (as seen in the blue-footed booby, Sula nebouxii) or due to the chance of sterility in one egg.
This is suggested by studies into the common grackle, Quiscalus quiscula[5] and the masked booby, Sula dactylatra.
The fact that such a sacrifice occurs indicates an evolutionary tendency in some taxa toward improved vertical gene transmission in families or a higher percentage of the unit in reaching a reproductive age in a resource-limited environment.
Therefore, when the junior chick (B-chick) hatches, there is a significant disparity in size and strength between it and its older sibling.
[10] Masked booby and Nazca booby dominant A-chicks always begin pecking their younger sibling(s) as soon as they hatch;[10] moreover, assuming it is healthy, the A-chick usually pecks its younger sibling to death or pushes it out of the nest scrape within the first two days that the junior chick is alive.
However, unlike the obligately siblicidal masked and Nazca booby chicks, their behavior is not always lethal.
[8] A study by Lougheed and Anderson (1999) reveals that blue-footed booby senior chicks only kill their siblings in times of food shortage.
According to Anderson, the average age of death of the junior chick in a masked booby brood is 1.8 days, while the average age of death of the junior chick in a blue-footed booby brood may be as high as 18 days.
Facultatively siblicidal blue-footed booby A-chicks only kill their nest mate(s) when necessary.
In a study, the chicks of blue-footed and masked boobies were switched to see if the rates of siblicide would be affected by the foster parents.
It was found that the synchronous brood fought more, was less likely to survive than the control group, and resulted in lower parental efficiency.
[citation needed] Siblicide (brood reduction) in spotted hyenas (Crocuta crocuta) resulted in the champions achieving a long-term growth rate similar to that of singletons and thus significantly increased their expected survival.
Some mammals sometimes commit siblicide for the purpose of gaining a larger portion of the parent's care.
Sex ratios may be manipulated in this way and the dominant status of a female and transmission of genes may be ensured through a son or daughter which inherits this solely, receiving much more parental nursing and decreased sexual competition.
Siblicidal "survival of the fittest" is also exhibited in parasitic wasps, which lay multiple eggs in a host, after which the strongest larva kills its rival sibling.
[13] In sand tiger sharks, the first embryo to hatch from its egg capsule kills and consumes its younger siblings while still in the womb.
While one of the twins may not die because of these factors, it is entirely possible that their health will be compromised and lead to complications after their birth.
Genetic relatedness may be an important moderator of conflict and homicide among family members, including siblings.
