We Need Friendlier Pawpaws
If monkeys are your only friends, you’d better have a plan to be nice to monkeys
Charles Darwin noticed that, compared to their wild ancestors, many types of domesticated animals share a range of distinctive physical traits. A classic comparison is wolves versus dogs - where the latter tend to have shorter muzzles, floppy ears, curly tails, mottled coats, and less sexual dimorphism (i.e., males and females are similar in size). Although Darwin originally referred to the suite of altered traits as “domestication syndrome,” the domestication part has turned out to be a bit of a misnomer because it has increasingly been documented in free-living wild animals, such as foxes.
I grew up playing in the woods of suburban Maryland, and I only ever caught a couple brief glimpses of foxes. Nowadays, I see foxes all the time. There’s one particularly friendly fox I see about once a month in a tiny scrap of suburban woodland I walk through on the way to work. The other day, a fox traipsed through my parents’ suburban Maryland back yard. My sense is that its muzzle is less pointy than the foxes I remember growing up.
If the “domestication” part of domestication syndrome is tricky, the “syndrome” part may be even trickier. Friendly bonobos have slightly flatter faces and less sexual dimorphism than their more feral chimpanzee cousins, but the two species have similarly floppy ears and neither of them has a mottled coat. Instead, bonobos have other distinctive traits such as pink lips and long slender limbs. In other words, the suite of morphological traits that come along with friendliness may be different in different animal groups (or in different individual members of a given species). Observable elements of the syndrome don’t necessarily march in lock-step.
Whatever you want to call the phenomenon Darwin dubbed domestication syndrome, I’m beginning to suspect that the rapid emergence of ensembles of seemingly unrelated traits might be a common trick among multicellular organisms. To illustrate the broader hypothesis, let’s look at the strange story of a beloved North American wild fruit tree called a pawpaw (Asimina triloba). Pawpaw trees produce a mango-like fruit with creamy flesh reminiscent of a banana. Every autumn, my family hikes out to a favorite pawpaw grove on the banks of the Monocacy River to collect a little bit of the wild bounty.
But only a little bit. I hate to be the bearer of bad news, but the pawpaw story has a dreadful dark side. The flesh of the fruit is loaded with a nasty neurotoxin called annonacin. Rats injected with a few milligrams of annonacin every day for a month develop brain lesions reminiscent of Parkinson’s disease. A single pawpaw fruit can have as much as a gram of annonacin (i.e., hundreds of rat doses). You might entertain the naturopathic hope that eating annonacin in the form of a whole food would mitigate its toxicity, but the opposite seems to be true - other compounds in pawpaw fruits appear to increase the neurotoxic potency of annonacin. People who routinely eat annonacin-rich fruits show high rates of parkinsonism. Neal Peterson, the Johnny Appleseed of pawpaws, appears to have tremors. In a published case study, a man in the habit of eating around 75 pawpaws per year died from an unusual variant of Parkinson’s disease. These aren’t randomized controlled clinical trials we’re talking about, but if I’m a betting man I’d say pigging out on pawpaws is very risky behavior. Sometimes plants don’t really want you to eat them.
Before humans reached North America, it’s likely that pawpaw seeds were distributed far and wide in the dung of mastodons. Maybe mastodons didn’t absorb annonacin? Because annonacin is destroyed during hindgut fermentation? Elephants are colorblind, which could help explain the fact that pawpaw fruit is an inconspicuous dull green, as opposed to the bright yellow or red colors that typically advertise ripe fruits to primates.
In 1916, The American Genetic Association held a contest to find the best pawpaws. Superior-quality fruits were sent in from Ohio, Maryland, Kansas, Missouri, and Indiana. In 1975, when Peterson found the abandoned fields of some of the old contest winners, the woods still contained a few pawpaw descendants with superior-quality fruits. Superior pawpaws generally share an ensemble of traits - smaller seeds, sweeter flesh, and thin yellow skin. And here’s the kicker: it turns out that fruits with a clean sweep of traits preferred by primates also have ten times less annonacin1! It appears that, in many distant locations, a tiny percentage of trees activated a coordinated “be nice to monkeys” program that’s latently pre-embedded in the pawpaw genome.
The common thread that ties together the fox and pawpaw stories is the idea that there are germline genetic sliders that can stably activate or suppress cooperative behavior2. The idea could produce a seismic shift in the way we think about ecosystems. I’m currently reading Fred Pearce’s brilliant 2015 book The New Wild3, which makes a compelling case for the provocative idea that ecosystems aren’t delicately balanced equilibria that take eons to evolve and can be collapsed simply by pulling out a single thread. It didn’t take millions of years for pawpaws to figure out how to cooperate with primates instead of mastodons, it only took a few decades! The idea that most organisms have genetic sliders that can enable rapid adaptation - including rapid development of mutual cooperation - could explain the surprising success of places like Ascension Island, where ecosystems comprised almost entirely of bottlenecked inbred plants and animals humans brought in from the four corners of the Earth rapidly established intricate cooperative networks. This new way of looking at things upends the received wisdom of Rachel Carson’s seminal book, Silent Spring. It also disturbingly undermines some of the comforting starting assumptions in Jared Diamond’s seminal book Guns, Germs, and Steel.
With that revolutionary assertion, I’ll remind you that I’m engaging in scientific speculation that offers testable predictions. I’d love to hear thoughts about why the idea sounds implausible, and I’d especially love to hear thoughts about how we could experimentally resolve such debates.
It’s infuriating that Springer is still extorting $40 to read this important publicly funded health research four years after it was published. I have access via the tithe my employer pays the oligopolists, so I’ll briefly relay the key data from Table 3. The pawpaw cultivar with the lowest annonacin content is named Mango. Other cultivars with relatively low annonacin content are called Overleese, Shenandoah, Sunflower (one of two samples), Wells (one of three samples), and Wabash. Rappahannock, NC-1, 10-35, and Susquehanna have nearly thirty-fold more annonacin than Mango.
Song of the day: “That’s Cooperation!” by Sesame Street
I’m a ridiculously slow reader. Can we please get an audio version of this thing?