Cannibalism in the animal kingdom Bill Schutt
In the deserts of the American Southwest,
spadefoot toad tadpoles
hatch in tiny oases.
Until they develop into toadlets,
they can’t survive outside of water,
but these ponds are transient
and quickly evaporate.
The tadpoles are
in a race against the clock
to grow up before
their nurseries disappear.
So nearly overnight,
some of the brood explode in size.
They use their jack-o-lantern teeth
and huge jaw muscles
to devour their smaller pond mates.
Nourished by this extra fuel,
they develop quicker,
leaving the pond before it can dry out.
The spadefoot toad
is far from the only animal
to eat members of its own species
as a normal part of its life cycle.
All of these animals do.
If that surprises you,
you’re in good company.
Until recently, scientists
thought cannibalism
was a rare response to starvation
or other extreme stress.
Well-known cannibals,
like the praying mantis
and black widow spider,
were considered bizarre exceptions.
But now, we know they more
or less represent the rule.
While it may seem counterproductive
for members of the same species
to eat each other,
cannibalism can promote the survival
of the species as a whole
by reducing competition,
culling the weak,
and bolstering the strong.
Some species, like the spadefoot toad,
cannibalize in response
to environmental pressures.
Their situation is precarious,
but cannibalism for them isn’t
a last-ditch attempt to avoid starvation.
Rather, it’s a way to more quickly
outgrow a stage
where they’re especially vulnerable
to predation
or dangerous environmental conditions.
Other species, including many fish,
indiscriminately cannibalize each other
during foraging behavior.
Fish produce large numbers of tiny young,
and adults exhibit about as much
individual recognition of their offspring
as humans do for a handful of raisins.
Fish eggs, larvae, and juveniles
are easily available, nutrient-rich meals,
and with thousands of eggs in a clutch,
plenty are still available to hatch
after the adults have snacked.
Baby fish aren’t just at risk of being
cannibalized by adults—
siblings eat each other too.
Sand tiger shark eggs develop and hatch
inside their mother’s oviducts
at different times.
When the hatchlings run out of yolk
from their own eggs,
they eat the other eggs and hatchlings
until one baby shark
from each oviduct remains.
When they emerge, the young sharks
are well-nourished, experienced predators
who stand a better chance of surviving.
Even when they aren’t consumed
for nutrition,
young animals are especially
vulnerable to cannibalism.
Hamsters,
rats,
and other rodent mothers
will eat some of their young
if they’re sick,
dead,
or simply too numerous to feed.
In other mammals,
including bears and lions,
males will kill offspring
sired by another.
That’s because childless females become
receptive to mating more quickly
than if they were caring for a cub.
Rather than waste nutritious meat,
the males then eat the dead cubs.
Meanwhile, cannibalism is less common
in birds than in other groups,
but certain species will eat diseased
or dead hatchlings
as a way of disposing of the
bodies before they can attract maggots.
When adults eat each other, males are
cannibalized more often than females,
usually during mating
and generally because they’re smaller.
Male Australian redback spiders
mate with much larger females.
Rather than scrambling away after mating,
the tiny male does a somersault,
bringing his abdomen into contact
with his mate’s mouthparts.
The female showers him with enzyme-rich
gut juice and consumes his abdomen.
Males not killed in the initial mating
crawl back into the fray,
often half-eaten,
to mate again, after which they’re
dispatched to the spider pantry.
So not only does the male provide
the female with his sperm,
but he also provides her with
a nutritious meal
to better ensure that she’ll
survive to pass on his genes.
All in all, it’s clear
that cannibalism is
as much a part of life
in the animal kingdom
as other, better-recognized behaviors.
As we sink our teeth into the evidence
of cannibalism in nature,
we might ask ourselves,
what else have we missed by applying
human standards to the natural world?