Social Behavior
In the wild, piranha's from the genus Pygocentrus live in large
shoals, roaming the South American rivers. This situation is impossible to
imitate in captivity, but even in a tank they will show some traits of their
wild behavior, provided they are kept under proper conditions.
First of all, most shoals will have one or more dominant animals, depending on
the size of the shoal: the leader(s) of the pack. Even though the fish will
often hang out together, in crucial moments the dominant fish will show its
might. In most cases, the alpha-animal will be the largest, most aggressive and
bold specimen, first at feeding sessions and owning and guarding the best spots
in the tank (the spot with the best view, the best place during feeding time,
in the current from a powerhead). Any unwilling 'servants' will be corrected
instantly by aggressive behavior, chasing or even inflicting wounds.
A group of Pygocentrus piranha's lives in a state of constant fear and mutual
mistrust, even when all seems calm: the animals are all capable of severely
wounding or even killing each other. To survive, the fish must always know
where the others are, in what their states of mind they are, and how they might
act the next moment. Letting your guard down may turn out fatal.
This behavior may be a partial explanation why many captive pirana's are
relatively skittish and nervous, despite their fiercesome reputation. One
moment of carelessness may mean the end of your life, even more in the wild,
where piranha's are part of the diet of many predators, like jaguars, caimans,
boto's (freshwater dolphins) and other, larger predatory fish, which live there
in abundance. And piranha's are even preyed upon by their own relatives,
especially amongst the young and during the dry season, when the amount of food
is increasingly limited, and the fish are frequently trapped in increasingly
small puddles. Juvenile piranha's have to face even more natural predators, and
are even targeted by large insects and crustaceans. Most captive piranha's are
less skittish and shy when they live in a tank with enough places to hide and
dimmed lights. It makes them feel more at ease and secure, which will be
reflected in their behavior: the fish will be more active, swim around more
freely , and behaving in a more 'natural' way.
In the wild, the staple diet of the carnivorous Pygocentrus piranha species
consists of fish. They catch their prey by active chasing, or by ambushing.
Besides fish, they also eat insects, crustaceans, birds that have fallen into
the water, and sometimes even mammals, reptiles or amphibians: basically
anything that has attracted the attention of a hungry shoal, and is unable to
leave the water in time. It is observed that shoals of Pygocentrus cariba
congregate under trees where groups of birds are nesting. Somehow, the fish know
when the young birds have hatched: they patiently wait under the trees for
chicks falling in the water behavior that is remarkably similar to what
Alligators do in the Everglades. And in times when prey animals are hard to
find, piranha's will even supplement their diet with fruits, nuts and seeds
when prey animals are hard to find, highlighting that these fish are very well
adapted to cope with whatever circumstances their natural habitat throws at
them.
Most larger animals that are attacked, are weakened, sick or injured: even a
large shoal of piranha's will only attack a healthy animal on very rare
occasions: during the dry season, when moving around freely is as good as
impossible and food is scarce. Cannibalism amongst piranha's is not unusual during
this period, and many others fall victim to predators looking for an easy meal.
And perhaps also during the mating season, when aggression levels are at their
highest. By weeding out the weaker animals, piranha's have the same task as the
vultures or hyena's of the savannah: they are the health police of their
habitat. Piranha's are attracted by splashing or the erratic movement of an
animal in distress. Once they start feasting on their prey, other piranha's
rush to the scene, drawn by the splashing of the victim, the blood and the
noises and disturbance caused by the frenzied piranha's that are already
feasting.
With the exception of a few species (like Serrasalmus spilopleura, Serrasalmus
maculatus en Serrasalmus geryi), piranha's from the genus Serrasalmus
are solitary fish. In general, they will not tolerate other fish in their tank,
and are very aggressive and territorial. Their behavior in the wild is, due to
lack of research, largely unknown. What we know is that Serrasalmus rhombeus
sometimes travels and feeds in loose shoals in the wild, but prefer a solitary
lifestyle. This may be due to environmental factors (such as the drying up of
rivers during the dry season forcing the fish to share their constantly
decreasing living space), and/or during the mating season.
Also known is the fact that juveniles from many Serrasalmus-species look very
different from their parents, and this has a specific reason: cannibalism is
very common amongst piranha's, and the younger they are, the more species prey on
them. Many juveniles look like adult Pygocentrus piranha's (in particular: a
red coloration of the lower body, ie. Serrasalmus medinai en Serrasalmus
sanchezi), and live in their shoals. This behavior is called mimicry.
Living in the middle of a shoal of larger, similar fish has certain advantages
for the juveniles: not only does a large shoal provide protection against
predators, it also offers a steady supply of food to the growing up piranha's.
Young piranha's are also parasitic fin nippers, and will not hesitate to eat
the fins of the fish in their "host" shoal to supplement their diet.
When they get older, they usually leave the shoal and start living solitarily.
