The most obvious potential downside, and the one that concerns visitors the most, is the
prospect of one animal eating another. Apart from shark and crocodilian
exhibits, where an occasional fish disappears down the maw of the predator,
this is pretty rare. I have seen displays where predators and potential
prey are housed together, but always in circumstances where the prey is safe
(for example, the alligator exhibit at the Salisbury Zoo also features herons
and egrets, but the smaller, faster birds fly circles around the
largely-indifferent alligators... I just wouldn't try this combination with
ducks).
Aggression doesn't have
to come from a predator - some animals, to put it nicely, are just jerks.
Zebras are a prime example, and will kill smaller ungulates, just
because. Many zoos display their zebras alongside giraffes or rhinos, or
among larger antelope which can hold their own, but not with smaller ones.
Primates also can be problematic in mixed species displays; much like
people, they are sometimes possessed with a sadistic curiosity which can have
unfortunate consequences for smaller animals. Among birds or hoofstock,
if two species are close enough to each other in appearance and behavior, they
may see each other as rivals and fight... or as mates, and produce hybrid
offspring. For this reason, it usually works best to display unrelated species - a parrot, a pigeon, and a duck, for instance, rather than three species of parrot.
A lack of aggression in
the wild where two species coexist doesn't translate into those same species
doing fine together in a zoo. In the wild, two animals may share a
landscape, may even walk within a few feet of each other once in a rare while,
but may never interact. In a zoo, where the quarters are close and mutual
avoidance is much harder, conflicts may brew. In the wild, a lion may ignore a very small mammal, like a mongoose - too small, too hard to catch, not worth in the effort. In a zoo setting, where the mongoose cannot escape, things may work out differently.
Cases of flat-out
aggression are easy to identify, but there are far more subtle problems that
can arise in mixed-exhibits. Chief among them is stress. Just
because two animals aren't physically fighting, or even looking at each other,
the weaker of the two can still be stressed, sometimes to the point of illness,
anorexia, or death, by the presence of what it perceives to be a threat. Keep in mind, no one is observing these animals 24/7 - just because you don't see aggression or bullying while you are there doesn't mean it might not be happening after hours, or when no one is around.
Speaking of anorexia, then there is the problem of feeding. Do all
of the animals in the exhibit eat the same food? If so, how do you make
sure everyone gets enough? If they don't, how do you make sure everyone
eats their diet and not that of an exhibit mate? I was caring for an
exhibit that featured flamingos, pheasants, parrots, and sloths. The
flamingos ate the pheasant food, the pheasants at the flamingo food, the sloths
ate the parrot food, and the parrots ate everything. Which, in this case,
wasn't a disaster... but what if the food of one species is toxic to another?
Disease transmission is
also a risk... some animals are just germier than others. Turtles and
tortoises come to mind, readily. They carry diseases which can be lethal
to snakes and lizards; when I first started working with reptiles, the turtles
were all kept in one section of the building, and we were instructed to do them last everyday. We didn't have a single exhibit of turtles
displayed with lizards or snakes. Crocodilians, on the other hand, seem
immune to the diseases of turtles and do just fine being housed with them.
Every exhibit
combination is a matter not just of species, but of individuals. When you add
or subtract individuals, you change the dynamic of the exhibit, and anything
can happen. Two factors which cause the most change are breeding males
(which can be super-aggressive to their exhibit mates) and offspring (which
exhibit mates may view as prey or toys, with disastrous results). To this
end, many mixed-species exhibits become non-breeding situations with single-sex
groups. Cool for display purposes, but not super-helpful for the survival
of the species or the captive population.
Mixed-species displays
have become the norm in the modern zoo, and some combinations have become
tried-and-true features at many zoos. Other zoos seemed determined to
create new, exciting combinations, including some that might surprise visitors:
bears and wolves... rhinoceroses and cheetahs... crocodilians and deer.
There have been many surprising triumphs... as well as some failures.
What is important to remember is that every combination comes down to the interplay of the species involved,
the individual animals, and the exhibit itself. A combination that works
beautifully at one zoo may fail dismally at another.
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