Avocados, Giant Ground Sloths, and the Machine of Popular Science

You may have heard that avocados evolved to be eaten by giant ground sloths. After all, most nutrient-rich fruits exist to get seeds deposited in a pile of poop, and what animal could be swallowing and pooping out the huge seeds of avocados? The idea that this might have been giant ground sloths goes back to a biologist named Dan Jansen in a paper published in 1982, but Jansen only offered it as a suggestion in a study mainly about other things, and he did not defend or analyze it. Then a popular science author named Connie Barlow found that obscure paper and put the notion in a book called The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms (2002).  From there it spread across the the whole landscape of popular science: Atlas Obscura, Smithsonian, etc.

But is it true? According to this this video from people who call themselves Sci Sho (transcript here), it is not. These people claim that 1) we have a lot of data about what ground sloths ate, from analysis of their bones and vast quantities of fossilized poop – some giant ground sloths slept in caves, and they pooped on the megafaunal scale – and it mostly points to leaves and grass, not fruit; and 2) avocados seeds used to be much smaller, 1-2 centimeters instead of the 5-6 centimeters of modern varieties, and that the seeds grew large thanks to domestication by humans. But they cite no sources and don't really radiate reliability.

So I started searching to see what I could find out. One thing I learned very quickly was that the ideas in that video are spreading, so you can get a lot of results if you search for the Myth of Ground Sloths and Avocados. 

But once something gets dubbed a "Myth" in this kind of discourse, you should start to suspect that it might have some kind of truth in it. So we must look deeper.

Somebody on Reddit who calls himself 7LeagueBoots says this:

Just regarding the avocado/giant sloth connection, there is a lot of misinformation embedded in that. There are a lot of different kinds of wild avocados, some with very large seeds, some with seeds about the size of an olive. They're reproducing just fine in the wild right now without any giant sloths running about. The fruit portion of these wild avocados is eaten by a wide range of animals, including bears in South America, birds all across their range, squirrels, and many more. The fact that these wild avocados, including those with large seeds, still persist places a very large question to the oft cited presumption that the were depended on giant sloths that have been extinct for around 11,000 years at this point. The research papers I've read on this subject say that it's a possibility that the trees were dependent on the giant sloths, but they all fail to address the elephant in the room. I worked in Ecuador for a while tracking Andean Spectacled Bears and one of their favorite foods was these wild avocados.

So he basically agrees with Sci Sho, although he says wild avocados have a diversity of seed sizes. But one of the responses to that post, from somebody called Bromelia_and_Bismuth, still comes down hard for ground sloths.

So I decided to look into avocado domestication. This very techincal paper on the avocado genome says that modern avocados are hybrids of three distinct wild "races" that were domesticated separately, and have been selectively bred so they bring out the desirable characteristics of each wild ancestor. And this:

The avocado is heterodichogamous, with 2 flowering types: A and B. Type A trees are female (receptive to pollen) in the morning of the first day and shed pollen as males in the afternoon of the following day. In contrast, type B trees are female in the afternoon of the first day and male in the morning of the next day.

This is presumably to limit self-fertilization, but, wow. Genderfluid trees.

Anyway.

This 2009 article on domestication says, "The large size of the avocado fruit appears to have developed before humans arrived in Mesoamerica, and then changed little in size or shape under human influence." Which seems to be a direct refutation of the claims made by Sci Sho. And on the subject of the three landraces, a term of art new to me, "Fruit size does not help differentiate between domestic and wild avocados due to the variation in fruit size caused by environmental factors and individual tree traits." This is important because the oldest archaeological evidence of avocado consumption by humans comes from mountain caves, so the small size of the seeds in that environment may tell us nothing about the size of ancient avocados in the tropical lowlands.

The data on ground sloth diet has a similar problem, because the surviving coprolites are found in mountainous areas with caves, and they only tell us what ground sloths ate in that environment. We don't have any data on what ground sloths ate in the tropical lowlands where most  avocado trees grew. Tropical forests are just a terrible environment for the preservation of anything.

So far as I can tell, this remains an open question. Avocado fruit are nutrient rich, so just about any herbivore could have eaten them. The fact that wild avocados of recent times survive without ground sloths seems to show that the dependence was not absolute, but it does not rule out ground sloths as key transmission vectors or co-evolution between them. After all there is no real "wild" place left in Central America, and all the forests have been shaped to one degree or another by humans; just as acai palms are a wild species that in practice mainly grows around old human settlements, wild, large-seeded avocados may in practice grow mainly where humans have encouraged them.

There is neither fact nor myth here, just an ongoing struggle to understand.

Which brings me to my real point, the perilous state of popular science writing. To get attention, the science press is constantly proclaiming that things are true when they are at best iffy, revolutionary when they may be decades old, "myths" when they still might be true. Sometimes much of the fault resides with the scientists themselves, who exaggerate their findings. But in this case the scientists are blameless. All Dan Jansen did was wonder what used to eat tropical fruit and suggest that somebody should investigate the possibility of ground sloths, which was a great idea. It might be true. But we don't know, and I hurl foul oaths and imprecations at anyone who says otherwise, and no I don't care what you have to do to get clicks. Tell the truth as best you can or shut up.