Mathematical representation

The cost and effect siblicide has on a brood's reproductive success can be broken down into an algebraic equation. ${\displaystyle M}$ is the level of parental investment in the entire brood, with an absolute maximum value MH(0 ≤M ≤M"H)." A parent investing ${\displaystyle M}$ units of parental investment (PI) in its current brood can expect a future reproductive success ${\displaystyle f(M)}$ given by

${\displaystyle fH}$ if M ≤ 0

f(M)= { fH[1-(M/MH)^θ ] if ${\displaystyle 0<M<MH}$

if MH≤M,

${\displaystyle fH}$ is the parents' future reproductive success, if it makes no reproductive attempt. The parameter ^θ determines the relationship between parental investment and the cost of reproduction. The equation indicates that as ${\displaystyle M}$ increases, the future reproductive success of the parent decreases.

The probability p(m) that a chick joins the breeding population after receiving M units of PI is

${\displaystyle P(m)=1-Mv/m}$ if ${\displaystyle m>mv}$

if m ≤ mv

In birds

Cattle egrets, Bubulcus ibis, exhibit asynchronous hatching and androgen loading in the first two eggs of their normal three-egg clutch. This results in older chicks being more aggressive and having a developmental head start. If food is scarce the third chick often dies or is killed by the larger siblings and so parental effort is distributed between the remaining chicks, which are hence more likely to survive to reproduce. 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.^[3]

The theory of kin selection may be seen as a genetically mediated altruistic response within closely related individuals whereby the fitness conferred by the altruist to the recipient outweighs the cost to itself or the sibling/parent group. 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.

The closely related masked and Nazca boobies are both obligately siblicidal species, while the blue-footed booby is a facultatively siblicidal species.^[6] In a facultatively siblicidal species, aggression occurs between siblings but is not always lethal, whereas in an obligately siblicidal species, aggression between siblings always leads to the death of one of the offspring.^[2][3] All three species have an average brood size of two eggs,^[6][7] which are laid within approximately four days of each other.^[7][8] In the few days before the second egg hatches, the first-born chick, known as the senior chick or A-chick, enjoys a period of growth and development during which it has full access to resources provided by the parent bird. Therefore, when the junior chick (B-chick) hatches, there is a significant disparity in size and strength between it and its older sibling.^[9][10]

In these three booby species, hatching order indicates chick hierarchy in the nest.^[2][9] The A-chick is dominant to the B-chick, which in turn is dominant to the C chick, etc. (when there are more than two chicks per brood).^[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.^[11] Blue-footed booby A-chicks also express their dominance by pecking their younger sibling. 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. Furthermore, even when junior chicks are killed, it does not happen immediately. 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.^[6] The difference in age of death in the junior chick in each booby species is indicative of the type of siblicide that the species practices. Facultatively siblicidal blue-footed booby A-chicks only kill their nest mate(s) when necessary. Obligately siblicidal masked and Nazca booby A-chicks kill their sibling no matter if resources are plentiful or not; in other words, siblicidal behavior occurs independently of environmental factors.

Blue-footed boobies are less likely to commit siblicide and if they do, they commit it later after hatching than masked boobies. 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 turns out that the masked boobies that were placed under the care of blue-footed booby parents committed siblicide less often than they would normally. Similarly, the blue-footed booby chicks placed with the masked booby parents committed siblicide more often than they normally did, indicating that parental intervention also affects the offspring's behavior.^[8]

In another experiment which tested the effect of a synchronous brood on siblicide, three groups were created: one in which all the eggs were synchronous, one in which the eggs hatched asynchronously, and one in which asynchronous hatching was exaggerated. It was found that the synchronous brood fought more, was less likely to survive than the control group, and resulted in lower parental efficiency. The exaggerated asynchronous brood also had a lower survivorship rate than the control brood and forced parents to bring more food to the nest each day, even though not as many offspring survived.

In other animals

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. The incidence of siblicide increased as the average cohort growth rate declined. When both cubs were alive, total maternal input in siblicidal litters was significantly lower than in non-siblicidal litters.^[12] Once siblicide has occurred, the growth rates of siblicide survivors substantially increased, indicating that mothers don't reduce their maternal input after siblicide has occurred. Also, facultative siblicide can evolve when the fitness benefits gained after the removal of a sibling by the dominant offspring, exceeds the costs acquired in terms of decreasing that sibling's inclusive fitness from the death of its sibling.

Some mammals sometimes commit siblicide for the purpose of gaining a larger portion of the parent's care. In spotted hyenas, pups of the same sex exhibit siblicide more often than male-female twins. 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. Another example is when mourning cloak larvae will eat non-hatched eggs.^[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.^[14]

In humans

Siblicide can also be seen in humans in the form of twins in the mother's womb. One twin may grow to be an average weight, while the other is underweight. This is a result of one twin taking more nutrients from the mother than the other twin. In cases of identical twins, they may even have twin-to-twin transfusion syndrome (TTTS).^[15] This means that the twins share the same placenta and blood and nutrients can then move between twins. The twins may also be suffering from intrauterine growth restriction (IUGR), meaning that there is not enough room for both of the twins to grow.^[16] All of these factors can limit the growth of one of the twins while promoting the growth of the other. 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.

Siblicide in humans can also manifest itself in the form of murder. This type of killing (siblicide) is rarer than other types of killings. Genetic relatedness may be an important moderator of conflict and homicide among family members, including siblings. Siblings may be less likely to kill a full sibling because that would be a decrease in their own fitness. The cost of killing a sibling is much higher than the fitness costs associated with the death of a sibling-in-law because the killer wouldn't be losing 50% of their genes.^[17] Siblicide was found to be more common in early to middle adulthood as opposed to adolescence.^[18] However, there is still a tendency for the killer to be the younger party when the victim and killer were of the same sex. The older individual was most likely to be the killer if the incident were to occur at a younger age.