Dances with Alligator Pears


"Most of us live in impoverished places. Each year throngs of visitors ogle the wildlife in America's national parks, unaware that for tens of millions of years the land has never been so bereft of big creatures. The bison and elk and moose of Yellowstone are a remnant of what came before. We can still experience the great beasts that were lost, however, if we learn to read the plants."

-Connie Barlow, "The Ghosts of Evolution"

A staple in restaurants and homes worldwide, the "alligator pear" or avocado (from the Spanish "aguacate" which was itself derived from the Aztec word "ahuacatl", literally translated as "testicle") is a large berry native to the modern-day Mexican state of Puebla.  The avocado's creamy, fat-rich flesh, and pleasant taste (the Indian word for avocado means "butter fruit") makes it popular with humans, but, as described in The Encyclopedia Britannica, "the principal botanical purpose of the fruit is the protection and dissemination of the seed".

In this mutualistic relationship, the animal, enticed by bright colors, aromatic scent, and concentrated nutrients, will consume the fruit and subsequently pay the plant back by excreting the indigestible seed in a packet of fertilizer (aka "poo").  This strategy is so effective that, according to 2003 figures, fruit-bearing species of plants have enticed humans to produce approximately 380 million metric tons of fruit each year.  (The idea that plants have been "pulling the strings" is presented in great detail in Michael Pollan's though provoking book "The Botany of Desire".)  Humans are not the only animal species enamored with fruit, however, and prior to our arrival on the ecological scene, the relationship between fruit-bearer and fruit-eater was well established.

The circle of poo.

Let's consider for a moment the common grape.  As any grape farmer can attest, birds are voracious consumers of grapes and, as noted in an 1995 LA Times article, "Massive flocks of grape-eating birds are among the most aggressive enemies of vines."  It is worth noting, however, that birds are only "enemies of vines" from the perspective of humans who hope to profit from producing wine and other products derived from grapes.  A more accurate statement would therefore be "birds are friends of vines and competitors with humans."  To this end, humans employ such diverse tactics as protective netting, scare-tactics (i.e. the iconic "scare-crow"), and even explosives.  So, while it is clear that grapes have comfortably co-evolved with birds, who co-evolved with the "testicle"?  We can start with a process of elimination.

From Wikipedia.com:

"Avocado leaves, bark, skin, or pit are documented to be harmful to animals; cats, dogs, cattle, goats, rabbits, rats, birds, fish, and horses can be severely harmed or even killed when they consume them. The avocado fruit is poisonous to some birds, and the American Society for the Prevention of Cruelty to Animals (ASPCA) lists it as toxic to many animals including cats, dogs, and horses."

In a 1928 presentation to the California Avocado Society titled "Pests and Diseases of the Avocado" some of the animals that consume avocado fruits are described, but none of them provide the useful service of seed dispersal.

"On account of the high fat content, most small animals are very fond of avocados. Skunks, coyotes, and dogs prefer to eat fruit which has fallen to the ground and has softened, although the writer once observed two bull-terriers in the act of pulling Fuerte avocados from the trees and burying them like bones for future use.

Grey squirrels, opossums, and field mice, all climb the trees and eat the fruit while it is still firm. Where there are many grey squirrels in an adjoining wood-lot they may do a great amount of damage by cutting small holes in a great number of fruits."

Lacking a useful seed-dispersing partner, the avocado, like the mango of Asia, is considered to be an "evolutionary anachronism".  It was precisely these sorts of anachronisms that captured the attention of science writer Connie Barlow and ecologist Dan Janzen and inspired them to collaborate on a book devoted to the subject. In "The Ghosts of Evolution - Nonsensical Fruit, Missing Partners, and other Ecological Anachronisms" they provide the following definition:

"The term applies to plants that some time in the past thirty or forty million years evolved fruits intended to attract very large mammals. The big beasts are gone, but the fruits remain. Year after year in the American tropics (and temperate climes too), trees and vines produce fruits that make little sense today. Some fruits simply rot on the ground beneath the parent plant. Others are raided by seed predators or plundered by pulp thieves. Whether rotted, raided, or plundered, viable seeds are rarely dispersed."

Specifically addressing the avocado, the authors not that while the pulp of modern avocado varieties represents a sizable increase over ancestral forms, the seed itself has remained "virtually unchanged in girth." Therefore, only an animal with a "massive gullet" could swallow the seed, as well as the surrounding pulp, whole, thereby satisfying the avocado plants desire to "have it's seeds plopped into the world within steaming heaps of dung."

In imagining the type of Cenozoic megafauna that helped propagate avocados for millions of years, the authors describe the giant ground sloth or Eremotherium who, possessing a "friendly mouth" and "friendly gut", would have feasted upon fruiting avocados.

"The visitors are ground sloths , whose closest living relatives are South American tree sloths, anteaters, and armadillos. Eremotherium looks like nothing alive today. Think of a bear crossed with a prairie dog or marmot and endowed with the bulk of an elephant. The adult sloth begins to sniff the carpet of fruits for the ripest specimens. Her agile offspring climbs a nearby tree for safety and also because, at this age, climbing is not only possible but irresistible. In a few years, the young sloth's tree - climbing days will be over. By then, an enormous bulk and powerful clawed forelimbs will suffice to ward off all but the most determined predators.

The mother finds a fruit that smells acceptable and tests it for softness between frontally toothless jaws. The whole fruit is then mashed between tongue and palate. The slippery seed slides easily down the animal's gullet, along with the nutritious pulp. Laxatives in the pulp ensure that the seed will complete its dark journey before digestive juices do it harm.

Other seeds follow. Before she is satiated, the sloth and her young depart. The adults loth will balance the oily meal with leafy browse, thus keeping microbes happy in the vast fermentation vat of her gut. Tomorrow the pair will return to the same tree, dispersing seeds along the way."

Reconstructed skeleton of the a fellow Megatheriidae

Following the extinction of the great mammals, the avocado continued producing fruits for its dead partners, unaware that they had gone the way of the dinosaurs.  In the 13,00 years hence, the odd jaguar or rodent-like seed thief (who buried the fruit and later forgot about it) likely saved the avocado from becoming extinct itself.  When humans arrived in South America, they took on the role of seed distributors and have exceeded even the megafauna in this role.

Procured from the grocery store or backyard orchard (avocados have are limited in their ability to self-pollinate, so two or more trees must be planted adjacent to each other in order to bear fruit), we use our senses of touch and site to identify perfectly ripe fruits (this clue in particular is quite helpful) rather than the sense of smell that previously drew gomphotheres and ground sloths.  With our mind focused on the delectable pulp, we quickly dissect the flesh and extract the seed, scarcely pausing to consider why it is so large.

The avocado is an echo of the past, an organic memory of a forgotten age.


Alligator Pear Salsa

Ingredients:

3 ripe Hass avocados (mashed)
2-3 cloves garlic (minced)
2-3 jalapeno peppers (minced)
1/4 white onion (grated)
Juice from 1/2 of a lime
Sea salt to taste

Directions:

In a large bowl, combine all ingredients and mix thoroughly.  Serve with plenty of pork rinds or crispy bacon for dipping.
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1 comments:

  1. What an interesting post Tony! I've read "The Botany of Desire" but just added "The Ghost of Evolution" to my list. Thanks!

    ReplyDelete