The avocado.

You may think you have a good relationship with the avocado. The buttery fruit of this plant may regularly accompany your turkey sandwich, sliced and fanned out across the bread. Or, it may serve as a hearty dip in the form of guacamole. More recently, avocado is seemingly being utilized in a greater variety of ways, being deep-fried, thrown in macaroni and cheese, and finding its way onto burgers, Subway sandwiches, and even into ice cream. You’d expect that with all this attention, our green-fleshed, knobby-skinned friend, the avocado, would be content with its current role in human culinary efforts.

However, the avocado may very well be lonely.

Despite all our affection, this loneliness stems from the avocado potentially having eyes for another. I mean this, of course, in the sense of the concept of co-evolution (which I examined in a previous post), which is directly tied to the reproductive role of the fruit itself. The main function of a fruit (a botanical fruit, typically meaning the structure derived from the reproductive tissues of the flower housing the seed(s)), is to move seeds away from the parent plant and into areas that promote growth and survival. Many types of adaptations in fruits exist to help achieve this goal of seed dispersal; from the wind-catching dual blades of maple tree fruits (known as samara), to exploiting the appetites of the ubiquitous (and notably mobile) animals in the neighborhood. This latter evolutionary strategy involves the development of fruits that enrapture animal taste buds and provide irresistible caloric value, allowing consumed seeds to travel safely inside the gut of an unwitting, far-traveling chauffeur until being excreted away from the crippling shade of the parent plant. This is called “endozoochory.”

Most endozoochorous fruits have evolved to be eaten by fairly specific animals. Predictably, fruits adapted to be taken by songbirds are going to have different physical attributes than those associated with insects or elephants. You can try and fit a peach pit through the body of a sparrow, but you aren’t going to get very far. Similarly, expecting tiny, thin-walled seeds to withstand an elephant’s battery of grinding teeth isn’t realistic either. The suite of fruit traits evolved for dispersal by a given group of animals roughly categorize into “seed dispersal syndromes.” By interpreting these syndromes, we can often get a good idea of what the primary dispersing animal, the other partner in a co-evolved relationship, is likely to be.

In light of this, it becomes obvious that despite our love of the avocado (specifically, domesticated cultivars with lots of flesh; wild avocado fruits have a thinner layer of green deliciousness surrounding that pit), it is not “meant” for human consumption and seed dispersal. Any attempt to chew up the whole fruit and swallow the massive pit is bound to land your asphyxiating ass in the cemetery. However, the situation for avocado’s seed dispersal isn’t much better in its wild Neotropical range. Many smaller animals (like monkeys) that partake in avocado consumption are “pulp thieves”, ingesting the oily layer and tossing the seed at the base of the parent tree. In fact, no native animal is known to consistently and effectively disperse wild avocado. Why then does the avocado make a big, energetically expensive fruit that doesn’t cut the mustard on dispersal? Also, who is the true “buyer” of avocado’s product?

The answer to both those questions may be that the avocado’s chief dispersal agent is extinct. Kaput. Gone. Effectively an “ex-animal.” This would mean that the avocado fruit is an evolutionary anachronism, equipped with traits fine-tuned by evolution for interaction with a species that has quite suddenly disappeared, leaving the once perfectly capable seed vessel under-appreciated and inadequately used.

“Fruit’s almost ripe, guys. Come and get it! Hello? ….Guys?!”

It’s a very serious case of being all dressed up with nowhere to go.

The ideas of a “megafaunal dispersal syndrome” (megafauna referring to beasties people-sized and larger) and evolutionary anachronisms have their genesis in the research of Dan Janzen, whose observations of abandoned piles of giant, tough fruits all over the Neotropics first sparked thoughts of the elephant-like (and recently extinct) gomphotheres acting as the proper seed vehicle. The theory is a fascinating departure from how ecological interactions and adaptations are typically thought of, and it has been met with opposition from within the discipline over the past few decades.If these megafauna-plant anachronisms do indeed exist in nature, recent paleontological events have certainly provided quite the opportunity for them to develop. Earth experienced a catastrophic loss of megafaunal biodiversity over the last few tens of thousands of years. In the Americas, it happened particularly recently; only about ten thousand years ago. If you go back just half a hiccup in geologic time, the New World was quite a different place, and was awash in very large numbers of big, furry critters. Megafauna like camels, mastodons, mammoths, rhinoceroses, horses, gomphotheres, and giant ground sloths strode across North America from Oregon to the panhandle of Florida. In South America, tank-like glyptodonts (huge cousins of armadillos) and an odd, extinct group of hoofed animals called notoungulates dominated the landscape. They were all gone by about eight thousand years ago, leaving behind ecosystems abruptly impoverished of the ample appetites of bulky mammalian bodies. Given this prehistoric context, imagining the nutritive avocado sustaining elephantine mastodons or gomphotheres up until their sudden demise, leaving the avocado to “catch up” evolutionarily, may not be much of a stretch. Or perhaps the avocado found a home in the bowels of another animal, like the towering ground sloth, Eremotherium, which weighed more than a UPS truck and could reach nearly three stories up into the trees with its clawed forelimbs.

Ground sloths: evolution’s answer to construction machinery

Over the years, as the idea spread, other candidates for evolutionary anachronisms were proposed (a significant proportion of which are outlined in author Connie Barlow’s comprehensive examination of the theory in her book, “The Ghosts of Evolution”).

A fairly extreme example might be found in a plant native to a small area of the south-eastern U.S., the osage-orange (Maclura pomifera). The plant has been spread across the continent by humans seeking its unique utility as fence post material, but without human intervention, osage-orange is miserably shitty at seed dispersal. It produces a Nickelodeon slime-green, brain-like fruit the size of a softball, which is full of runny latex that is an industrial adhesive-level of sticky, and nothing in nature, including domesticated animals, seems to have much interest in touching the stuff. This means that the fruit, after falling, is staying put, save for a slight downhill roll or a kick from a frustrated park visitor. This results in the fruit piling up underneath the tree and densely adorning the ground as if a nearby tennis ball factory had exploded.

Fruit not falling far from the tree, and not being able to do a damn thing about it

Without its hungry, hairy “partners in time” around to gobble up these succulent globes, Maclura pomifera may be at the mercy of gravity and the swift onset of rot.

Another oft-cited example of anachronism that might be familiar to many readers is the honey locust (Gleditsia triacanthos), a common ornamental species found in city parks. The plant, being in the legume family, produces long, multi-seeded pods as fruit. However, these are no diminutive pea pods; the gently spiraling fruit of honey locust can be as long as your forearm. The honey locust has evolved big fruit that requires a big mouth to take it in without seed loss…a big mouth that no longer exists in its native North America. Cattle and horses will eat the pods off the ground, and assist in dispersing seeds that way, but it’s been offered that perhaps a high-reaching Pleistocene browser, like a mammoth or long-necked camel, is the plant’s intended target.

Anachronistic traits may not be restricted to just fruit. Many plants have evolved armaments (spines, toxins, etc.) as deterrents to overzealous browsing by herbivores. Honey locust is no different, being equipped with pencil-thick thorns encircling the trunk and branches. These sharp-as-hell, dense clusters of thorns are ridiculously excessive for modern browsers like deer, and occur on the tree far above their reach. They seem to do little else these days but strongly discourage tree house construction, crushing the dreams of children everywhere.

This is not a climbing tree

In Pleistocene Arkansas, these lovely daggers would have been “persuasive” tools against the tender trunks, tongues, and lips of much larger creatures, who may have been eager to strip the tree of its precious foliage (perhaps as a side dish to those crunchy pods). Honey locust may be somewhat of a Hiroo Onoda; the war’s been over for a while, hungry mastodons are no longer a threat, but the plant isn’t laying down its arms.

Anachronistic situations have been proposed outside the plant kingdom as well. Consider the pronghorn (Antilocapra americana), native to the open areas of the American West. While commonly referred to as the “pronghorn antelope”, it is not at all an antelope, and is the last remaining species in the family Antilocapridae, a group of mammals closely related to giraffes that once enjoyed great diversity in Pleistocene North America. Pronghorns also have the distinction of being the fastest land animal this side of the Atlantic. I mean, these things can move. A spooked pronghorn can easily break the 55 mph speed limit on the rural roads that criss-cross its habitat, and hold that pace for half a mile. Such speed is an obvious adaptation for predator evasion, but in modern North America, it far exceeds the requirement for outrunning comparatively sluggish layabouts like wolves and cougars. Like the mammoth-puncturing thorns of the honey locust, perhaps the reason comes in the form of a dearly-departed influence.

John Byers, a zoologist at the University of Idaho, thought so, and suggested that the living Ferrari that is the pronghorn owes its unreal get-up-and-go to ancestral predation by the American cheetah (Miracinonyx). Yes, a goddamn cheetah. North America was once home to a pants-shittingly diverse collection of big cats, including American lions, saber-toothed cats, and Miracinonyx, which wasn’t a Yankee version of the African animal, but a relative of the cougar that converged on the same “savanna rocket” hunting strategy. Miracinonyx was the lithest cat in town, meaning that if there was ever a selective pressure favoring blazing speed in antilocaprids, this guy was the whirlwind of muscle and claws behind it. Nothing gets the lead out of your species’ ass like the culling power of teeth with turbo boosters. Now, thousands of years later, Miracinonyx is no more, and the pronghorn hasn’t yet caught on, evolutionarily. Its accelerator is stuck to the floor, and will remain that way until the “need for speed” is sufficiently challenged by the “need to breed.”

If your ancestors recently put up with cheetahs, you’d be pretty fucking alert too

Additional suggested anachronisms abound, ranging from familiar fruits like papaya and prickly pear, to the idea that unique plant growth forms in Madagascar and New Zealand evolved in response to pruning by extinct flightless birds like moas and elephant birds.

Evolutionary anachronisms are likely tenuous, passing phenomena. Energy-sapping waste tends to be efficiently cropped out by natural selection, and those species that cannot transition quick enough into the new ecological context, will inevitably follow their extinct partners into the permanence of extinction. Indeed, proposed anachronistic plants are often clinging to existence, tending to be rare with patchy distributions (many times associated with unfortunately competitive river floodplains, since their uneaten fruit rolls or floats into these lowlands). Imagining great beasts munching on gooey osage-oranges, or crunching dried honey locust pods, is a wonderful thought-experiment. But, in addition to this, perhaps we should consider, if evolutionary anachronisms are a common side-effect of extinctions, how many temporal upsets we are generating as a result of the on-going, human-driven mass extinction.

Image credits: Sliced avocado, hanging avocado, Eremotherium, osage-orange, honey locust thorns, pronghorn.

© Jacob Buehler and “Shit You Didn’t Know About Biology”, 2012-2014. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Jacob Buehler and “Shit You Didn’t Know About Biology” with appropriate and specific direction to the original content.