In the wild, family bonds can be tricky. When sharing a nest, burrow or even a womb is more about competition than companionship, only the strongest will survive.
Food scarcity and slim survival odds can drive one of evolution’s most brutal solutions: removing the competition entirely. From birds and mammals to insects and fish, siblicide (and sometimes cannibalism) is a surprisingly common strategy.
Deadliest siblings on the planet
Sand tiger shark (Carcharias taurus)
In quite possibly the fiercest example of sibling cannibalism, sand tiger sharks begin vying for the position of top pup whilst still in the womb.
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These shark embryos hatch inside the uterus and the first to emerge promptly eats its siblings, along with any unfertilised eggs for extra protein. Sand tiger sharks have two wombs and by the time the mother gives birth, just two – one per uterus – well-fed pups remain.
Golden eagle (Aquila chrysaetos)
Golden eagle parents often lay two eggs, but only one chick is likely to survive, especially if food supplies are running low. The older (or stronger) eaglet may attack its sibling and claim all food deliveries for itself until the weaker chick starves or is killed outright.
This strategy is referred to as obligate siblicide in some populations, where the stronger sibling instinctually kills the weaker in order to secure necessary resources.
Blue-footed booby (Sula nebouxii)
Demonstrating similar behaviour but usually only when food is truly scarce, these charismatic seabirds offer an example of facultative siblicide. If there’s enough food to go around, all chicks might survive – but if there isn’t, the eldest chick will aggressively peck or push the younger out of the nest. Blue-footed booby parents are unlikely to intervene in these cases, allowing the victorious chick to claim its rightful sustenance.
Spotted hyena (Crocuta crocuta)
Hyena cubs are born with sharp teeth, open eyes and an innate tenacity. Littermates may quarrel shortly after birth, and these fights can turn deadly for the weaker cub.
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As with other sibling-killing species, there’s a higher likelihood of this happening amongst hyenas in the face of a food shortage, when the dominant cub will monopolise milk or meat at the nutritional expense of their sibling.
Tiger salamander (Ambystoma tigrinum)
For these amphibians, sibling cannibalism occurs in the larvae. In crowded ponds or food-poor conditions, tiger salamander tadpoles can develop a specialised “cannibal morph” with enlarged heads and teeth to facilitate the eating of other larvae – often siblings from the same clutch. In turn, the cannibal tadpoles gain strength and grow faster towards their metamorphose.
Polar bear (Ursus maritimus)
True siblicide is uncommon in polar bears, but opportunistic cannibalism is sadly on the rise as Arctic habitats suffer the consequences of climate change and bears go hungry.
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Cubs that die of malnourishment may become a food source for their sibling and mum alike, and male polar bears have reportedly attacked females with cubs during seasons with especially poor hunting conditions.
Komodo dragon (Varanus komodoensis)
Komodo dragons may engage in cannibalism when resources are scarce, but sometimes it’s a matter of dominance.
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Larger Komodo dragons may assert their dominance by killing and eating a subordinate as an act of swift competition elimination. Though young Komodo dragons aren’t likely to go after siblings deliberately, island life means doing whatever it takes to survive.
Black widow spider (Latrodectus spp.)
These fearsome spiders have no hesitation about snacking on siblings for strength. Young black widows often go after one another shortly after hatching, consuming the slowest and weakest and ensuring that only the most robust make it to adulthood.
Great white shark (Carcharodon carcharias)
White sharks are known for their practice of oophagy, or egg-eating, wherein developing embryos eat unfertilised eggs to gain strength in the womb. Sometimes this results in the consumption of smaller siblings, but intrauterine cannibalism in white sharks isn’t as extreme as that of the sand tiger shark.
Domestic pig (Sus scrofa domesticus)
Large litters come at a cost, and domestic piglets are often born into intense competition for both milk and personal space. Weaker piglets may be killed by stronger siblings as a means of reducing competition, and in especially dire circumstances – usually perpetuated by overcrowding – pigs may also turn to cannibalism.
