Siblicide is the killing of an infant individual by its close relatives. It occurs in some mammal, fish, and bird species when one or more siblings kill another sibling shortly after birth. Siblicide is most common among animals that have what is called “asynchronous hatching” where eggs hatch over a period of days or weeks. This results in offspring of different sizes and levels of development. The older, larger siblings often exhibit aggressive behaviors towards their younger, smaller siblings resulting in siblicide.
What animals exhibit siblicide?
Siblicide is seen in a variety of animal species including:
- Birds – Blue-footed booby, great egret, cattle egret, black-crowned night heron
- Mammals – Spotted hyena, brown fur seal, grey seal, steelhead trout
- Fish – Cannibalistic fish like tiger sharks, sand tiger sharks, cloudy catsharks
- Insects – Bark beetles, burying beetles, flour beetles
Siblicide is most prevalent among birds, especially raptors, seabirds, and waterfowl. Up to 95% of blue-footed booby broods experience siblicide. Siblicide is also common in fish when adults are not available to feed the young. Among mammals, siblicide is most frequently seen among carnivores and marine mammals.
Why does siblicide occur?
There are several reasons why siblicide occurs in animal species:
- Limited resources – When food resources are scarce, older siblings will kill younger ones to reduce competition for the available food. This increases their own chance of survival.
- Size disparity – Older, larger siblings have a significant size advantage over smaller, younger ones. This makes it easier for them to physically dominate and kill the younger siblings.
- Birth order – Later-hatching chicks or pups are at greatest risk since they are smaller. Being born first provides a competitive edge.
- Parental favoritism – In some species, parents may encourage siblicide by only feeding the oldest, largest offspring. This promotes their survival over weaker siblings.
The combined pressures of these factors drive siblings to kill one another shortly after birth in order to maximize their own chance of survival. Siblicide may be the best strategy for the older sibling even if it seems ruthless at the expense of a brother or sister.
How does siblicide happen?
Siblicide can occur through a variety of methods including:
- Direct attack – The older sibling directly attacks the younger one. They may bite, crush, or throw them from the nest.
- Kicking out – Older siblings force younger ones out of the nest where they die from exposure, starvation, or predators.
- Withholding food – Older siblings hoard food supplies, preventing younger ones from eating.
- Neglect – Parents only provide food to older offspring, neglecting smaller, weaker ones.
The exact method depends on the species. In birds, direct attacks and kicking from the nest are common. Among spotted hyenas, older siblings attack and kill younger ones with their jaws. Sand tiger sharks devour their siblings in the womb before they are born. Regardless of the method, the end result is that the younger sibling dies so the older one has a better chance to survive.
Does the mother intervene?
In most cases, the mother does little to nothing to intervene and prevent siblicide. There are several reasons for this:
- The mother lacks sufficient energy or resources to feed all her offspring, so she prioritizes the older, stronger ones.
- Intervention would put the mother at physical risk of retaliation from older offspring.
- Siblicide increases the mother’s overall reproductive success by focusing investment on the strongest offspring, so she allows it to happen.
- The mother encourages siblicide by only provisioning the older sibling(s).
Since the mother’s fitness revolves around producing viable offspring, she generally favors those most likely to survive, even at the expense of weaker siblings. However, in some species like fur seals, mothers will actively intervene to protect younger pups, even injuring older ones that attack their siblings.
Does the father intervene?
Paternal intervention in siblicide is extremely rare since male parents are not usually around to participate in brood care. Reasons for lack of paternal involvement include:
- Many species exhibit little or no paternal care at all.
- In polygamous mating systems, males mate with multiple females and provide no offspring care.
- Males abandon the female shortly after mating and play no role in raising offspring.
- Even in biparental care species, males contribute less than females do to protecting or feeding offspring.
While females are intrinsically invested in their offspring’s survival due to gestational investment, males of most species have little incentive to protect offspring at the expense of their own fitness. The only real exception is in strictly monogamous mating systems where males can be certain of paternity. Even so, paternal intervention in siblicide is uncommon in most species.
How does the victim respond?
Younger siblings are essentially helpless to prevent siblicide due to their smaller size and underdeveloped defenses. Strategies they may employ, often fruitlessly, to improve survival include:
- Fleeing – Trying to escape by running or swimming away from the nest. Often ineffective since they have limited mobility.
- Hiding – Attempting to hide from the older sibling, though this rarely works.
- Freezing – Remaining immobile to avoid detection. Not very successful.
- Crouching – Getting as low as possible and making themselves look small to avoid provocation.
- Fighting back – Pitifully trying to fight the larger attacker. Usually just accelerates their demise.
Victim responses are largely determined by physiology. Altricial species with closed eyes and featherless bodies at birth like boobies have almost no ability to flee or defend themselves. In precocial birds like gulls, victims may be able to run away or resist, but the size difference still dooms them in most cases. Despite their efforts, victims seldom escape their fate due to the stacked odds against them.
Does the victim ever survive?
Survival of the victim is possible but extremely rare. A few circumstances where victims may survive include:
- Intervention by a parent as discussed earlier.
- The victim happens to be unusually large and mature for its age compared to the attacker.
- An absent or inattentive attacker who does not notice the victim.
- The attack was incomplete or half-hearted.
- Random chance that the victim was able to flee at the right moment.
However, most instances of siblicide are initiated when the size disparity between siblings gives the attacker an insurmountable advantage. And attackers are usually highly motivated to kill the victim, recognizing them as a direct threat to resources. So while victims may occasionally slip through the cracks, the overwhelming majority do not survive attacks from their siblings.
Does being victimized affect later survival?
For victims that do somehow manage to survive siblicide attempts, their future prospects remain poor. This stems from two key factors:
- They were already the weakest and lowest priority offspring, making them less fit for survival.
- Any sustained injuries or trauma from the attacks further decrease their chances.
Among birds, surviving victims may be permanently impaired by damage to eyes, beak, wings, or legs from being grasped or thrown from the nest by siblings. Close calls also deprive them of food as they recover. All these impacts compound to leave survivors even worse off than before. As a result, victims that initially endure siblicide often fail to thrive and die prematurely afterwards.
Are victims ever consumed after death?
Consumption of siblings after siblicide (cannibalism) does sometimes occur. This is most common among:
- Carnivorous fish like catsharks.
- Predatory insects like ladybugs and lacewing larvae.
- Developing eggs of some reptiles and caecilians.
- Several bird species including boobies, egrets, and storks.
In these cases, siblicide serves the dual purpose of eliminating a competitor while also providing nutrition to the attacker. However, cannibalism of deceased siblings is relatively rare in mammals and many other animal taxa. The risks of consuming close kin likely outweigh any potential food benefits for most species.
Does siblicide always happen?
No, siblicide does not always occur in the species where it has been observed. Whether or not it happens depends on multiple factors:
- Food availability – Siblicide is more frequent when resources are limited.
- Brood size – Larger broods are more prone to siblicide than smaller ones.
- Hatching asynchrony – The greater the size disparity between siblings, the more likely siblicide is.
- Physical environment – Dangerous or high-stress conditions promote siblicide.
- Health of siblings – Weaker or defective siblings are killed first.
When conditions are highly favorable, such as abundant food and a healthy environment, siblicide may not occur at all. Parents can also sometimes prevent it by intervening or providing equitable care. So while siblicide is a common reproductive strategy in certain species, it is not an absolute certainty and depends greatly on context.
Does siblicide provide an evolutionary advantage?
Siblicide does appear to provide an evolutionary advantage in the species where it is commonly practiced. Killing siblings reduces competition for limited parental resources and allows the remaining offspring to be raised more successfully. For species living in extreme or unpredictable environments, siblicide may improve the chances that at least one offspring survives. This improves the parents’ lifetime reproductive success.
Mathematical modeling suggests siblicide is an optimal strategy when resources are scarce relative to the number of offspring. Siblicide essentially culls the brood down to a number the parents can successfully raise. So while it seems gruesome, siblicide is likely an adaptive behavior maintained by natural selection in the species that need it most.
Are any species evolving to stop siblicide?
There are some indications that certain species may be evolving away from siblicide:
- Some bird species have shorter hatching periods so siblings are closer in size and ability.
- Parental intervention is increasing in some animals like fur seals.
- Smaller brood sizes reduce competition among siblings for resources.
- More monogamous mating systems correlate with paternal care and protection of young.
However, siblicide still persists in many species and habitats where resources are limited. Complete evolutionary abolition of siblicide seems unlikely since the selective pressures that favor it remain. But some incremental steps towards reducing it do appear underway in a few species and contexts.
Conclusion
Siblicide, while gruesome, is an important survival strategy for many animal species. The evolutionary drivers that lead siblings to kill one another stem from competition for scarce resources and parental investment. By eliminating rivals early on, animals that practice siblicide increase their chances of living to adulthood and continuing the species. Though difficult to observe, siblicide offers unique insights into the reproductive challenges faced by animals from insects to mammals to birds under natural conditions. Continued research will shed light on this intriguing and extreme behavior.