Salad, Anyone?

The hoatzin may be a strange-looking bird on the outside, but it's equally a strange bird on the inside.  This is largely due to its unique gut anatomy, which in turn is due to its unique diet.  The hoatzin is unique among birds in being a folivore, or leaf-eater.  It may accidentally peck up an insect now and then, but really, leaves are all that it eats.  This specialized diet, and the challenges of replicating it, are a large part of the reason that the species has typically fared so poorly in zoos.

And it isn't just the hoatzin.  When I look at the list of species that have failed to become established in zoos, leaf-eaters make up a disproportionate number.  If an animal eats grass in the wild, it can often be switched over to hay.  Meat-eaters will eat meat, fish-eaters will eat fish, and insect-eaters will eat insects - the meat, fish, or insects may not be the same as they were in the wild, but the idea is close enough, and the nutritional content generally matches, with some tweaks.  Animals that have diverse, omnivorous diets often adapt very well to zoo-based diets, their biology largely being driven by the mantra, "Everything is edible if you are willing to try."

Many leaf-eaters, in contrast, are very specialized and eat only certain leaves, the nutritional content of which is not easily replicated.  Perhaps the best-known example of this is the koala, which eats only eucalyptus.  The ability of a zoo to house koalas is essentially tied to its ability to provide adequate amounts of the right kinds of eucalyptus, which partially explains why the vast majority of koala holders in the US are in regions where the trees can grow.  (Giant pandas would seem to form a close analog with their attachment to bamboo, but the difference is that the pandas don't need to eat bamboo - they could be fed other food sources and do fine.  Bamboo is just the natural diet, and one which encourages the most natural feeding behaviors).

Unlike eucalyptus, the preferred (or required, rather, as preferred makes it sound like there's a choice) trees of other species are not grown far and wide, around the world.  I'm not saying diet is the only reason, but it's a major contributing factor to why we don't see the indri - the world's largest lemur - in zoos, though I'm sure they'd be extremely popular if they could be maintained.  The longevity record of the species in zoos can be counted in days.  The indri's closest relatives, the sifakas, are also leaf-eaters and can be maintained in zoos, but are considered a far greater challenge than many other, more commonly-kept lemur species.

Other folivores seldom seen in zoos?  Three-toed sloths.  Douc langurs.  Red colobus.  Even species that are kept in zoos that have leaves as a greater proportion of the diet can be much trickier - and more expensive - to maintain than grazers or fruit-eaters.  In some cases, their care may require having regular shipments of browse to provide leaves year round, including working out storage of the browse to keep the animals in the green in the cold winter months.

So, next time you're ordering off the menu, or making dinner at home, never assume that the salad is automatically the cheapest or easiest option.