My wire fox terrier and I were walking along a favorite path on a cool autumn morning. Stopping to investigate a patch of tall plants clinging to the steep sides of a ridge that used to carry Virginia Anthracite Coal and Rail Company trains, I leaned in to get a closer look at the bright purple blooms. Dash dove headfirst into the leafy labyrinth at ground-level, probably hoping to surprise a still-sleepy rodent or a lethargic lizard. Once our curiosities were satisfied, we stepped back onto the path… then realized those greater burdock (Arctium lappa) plants were not going to let us go quite so easily.
Dash’s beard, belly, and legs, as well as my shoes, socks, leggings, and sweater sleeves were peppered with the prickly seed-heads of this Old World migrant. We’d planned to continue moving on down the line for at least another half-hour or so but after many long minutes prying the most irritating hangers-on from his fur and my clothes, we beat a hasty retreat back home, resigned to a morning of tedious, uncomfortable grooming.
Lucky for us, I have fingers and opposable thumbs, not to mention a variety of combs, brushes, and adhesive tape at my disposal; wild creatures, like the red fox (Vulpes vulpes) pictured above, can have a much harder time ridding themselves of hitchhikers.
Anyone who has stepped out of a field or vacant lot covered in American lopseed (Phryma leptostachya), beggar’s lice (Hackelia virginiana), enchanter’s nightshade (Circaea lutetiana), sweet cicely (Osmorhiza claytonii), or other barbed, sticky seeds knows ridesharing wasn’t invented by Über and Lyft. Those companies simply came up with a catchier name for zoochory [Latin: zoo- (animal) + -chory (dispersal )], a transportation strategy that caught on long before the arrival of smartphones apps.
Plants aren’t the only lifeforms that foist their progeny onto passersby as a way of launching them into the wider world. It’s also a common practice among some invertebrate species, given their general lack of interest in the parenting side of reproduction, to deposit their nascent descendants upon some unsuspecting traveler.
Seeds and eggs in need of a lift have two rideshare modes at their disposal: traveling on the outside of the vehicle (epizoochory) or traveling inside the vehicle (endozoochory). Ironically, the journey may be less challenging for those who hang from the sides of a biological trolley. True, there’s no protection from sun and rain but the outside generally has less organic detritus than the interior, and drivers are usually anxious to help their passengers disembark.
Still, there are real potential benefits to entering the literal belly of the beast, but only if the nomads are tough enough to make it out alive.
For one thing, they start life in their new location sitting on a stockpile of the resources they need to prosper. The advantages of being dropped off with a source of sustenance is easy to observe in plants—we can see seeds sprouting from piles of scat. Scientists assumed the eggs of at least some invertebrate creatures also stood to gain from this approach and a 2005 study out of Spain’s Estación Biológica de Doñana-CSIS provided the evidence to support that hypothesis. Researchers Andy Green and Marta Sánchez found that about 15% of midge larvae (Chironomus spp.) ingested did, in fact, survive a journey through the intestinal track of black-tailed godwits (Limosa limosa), a long-legged, long-billed shorebird, thereby increasing the distance over which the midges’ could disperse and expand their range.
Subsequent studies have reported on rotifers, copepods, bivalves, and others who successfully navigate the gut shuttle services of various vertebrate and invertebrate species. As recently as June of this year, The Scientific Naturalist published a study out of Brazil on the ability of killifish (Austrolebias spp.) eggs to disperse after being ingested by coscoroba swans (Coscoroga coscoroba). In fact, millions of years of selective pressure have resulted in a requirement for certain offspring to be tested by a torturous path before they can begin to grow and mature. Naturally, this increases the pressure to be devoured.
Rather than passively waiting for a hungry creature to happen along and accidentally swallow their children, progenitors do what they’ve always done… come up with ways to give their flesh and blood a competitive edge. One way plants do this is by surrounding or embedding their kids in tasty fruit to entice prospective diners.
Walking stick (Phasmatodea) kids, on the other hand, look like seeds… early on, anyway. Phasmida eggs may even feature a fatty capsule on one end that resembles the elaiosome, a food reward that’s a big hit in the ant world. Mimicking a dietary favorite increasing the chance that the egg will be picked up for a dinner date by a social insect from a good home. Imitation may be the sincerest form of flattery but in this case the similarity is less compliment than convergent evolution (the independent development of analogous characteristics in unrelated species).
Once the ants have had their fill of the nutrient rich capitulum treat, they’ll discard the rest of the phasmida egg, leaving it to gestate in the colony’s garbage dump, safe from predators. After hatching, the walking stick nymph will leave the nest and set about making a life for itself above ground.
One more thing… I’d be remiss not to mention that humans are not exempt from the ridesharing racket, and I don’t mean just your apparel.
If that thought gives you pause the next time you dine on fresh blackberries (and their seeds) for dessert or accidentally swallow a mouthful of river (and freshwater zooplankton) on a float trip… don’t overthink it. It’s not the first time you’ve given someone a ride, and it won’t be the last. You’re just another transportation worker in the gig economy!
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© 2019 Next-Door Nature. Reprints welcomed with written permission from the author. Thanks to the following photographers for mading their work available through the Creative Commons license: Tambako The Jaguar, Plant Image Library, Ken Slade, Max Wahrhaftig, Emilie Chen, Drägüs, Fritz Geller-Grimm and Felix Grimm, and Kathy.