Wednesday, September 12, 2007
Bwah-ha-ha-ha . . . I tricked you with that title, didn't I!
And then you saw another blasted skunk picture. Well, I know you're tired of seeing skunks on this blog, and I'd much rather be writing about pictures of California quail, red tree mice, or kit foxes. But these seedy skunks delivered a story.
Skunks tank up at the water trough nightly, so the camera trap gives me a pretty good idea of who's coming and going. Seedy skunks started to show up back in July. A week later they were gone, and only well-groomed skunks were making appearances.
I can't tell you what kind of seeds they were, but the skunks, I suspect, dispersed the seeds far and wide as they grubbed about on their nocturnal beat. I imagine a lot of the seeds just dropped off, but carnivores are pretty persistent at grooming seeds out of their fur, too.
Epizoochory is an old story. Grass parts were recently discovered in dinosaur dung, so Juarrasic park might have been seeded by giant reptiles, but that's endozoochory.
We're talking about the first plants to hitch-hike "on" rather than "inside of" mammals (epizoochory). The earliest evidence for seed dispersal by hairy entanglement comes from the Dominican Republic in the late Eocene. A grass spikelet attached itself to some mammalian hair, a blob of tree pitch snared the entanglement, and the pitch was transformed to amber. The spikelet had hooked hairs on the lemma, which means that hairy entanglement had been going on for some time.
When the middle Miocene (c.20-10Ma) rolled around, prairies and savannas were widespread, and it is likely that mammals, especially migratory ungulates contributed to their spread. In seasonal climates sheep, bison, antelope, camels, and horses cover large distances moving between summer and winter ranges, and bushy manes and hair capes can carry a lot of seed. In domestic sheep the area between the shoulders is particularly prone to collect seeds and plant rubbish, and sheep growers refer to it as the "bird's nest".
You have to marvel at the myriad and frustrating ways that plant seeds stick to your sneakers and bury themselves in your socks. It's not just a chance thing.
So when my daughters had to do a science fair project in middle school, I suggested "Furs and burrs"! Our working hypothesis was that mammalian fur differs in its ability to catch burrs. The experimental design was simple. After school, the girls suited up with fur leggings of cottontail, woodchuck and deer skin (patches held on with rubber bands), and then traipsed through a measured track of weeds in the field next to the house. Afterwards, they combed the furs, and sorted the seeds for counting.
I just called them on the telephone to learn the results, and guess what? They didn't remember! They said it was great fun wearing the leggings and combing the furs, but they couldn't remember the results! Oh well, it was a fun if not elegant experiment.
I was pleased to find that just a few years ago some researchers did the experiment more scientifically. They tested seeds of 66 species on seven mammals to develop a "seed adhesivity score", but before I go any farther, let me state that I had to be satisfied with the limited information contained in the abstract of their publication -- the codger wasn't willing to pay $30 for a pdf file of the article.
As reported in the abstract: "Deep furs with long, rough, undulated hairs implanted at a large angle were most suited for seed adhesion, while seeds adhered less well to shallow furs with short, smooth, straight hairs implanted at small angles." Clearly we are talking about all kinds of bushy tails, and skunk tails fit the billing to a T.
The authors go on to say, however, that there was "an interaction effect between certain seed and fur types", which I take to mean that some furs and seeds were meant for each other. They also learned that though seed structure is a good predictor of hairy entanglement, less specialized seed types can still get a good grip on fur. In other words, all the species they tested seem able to disperse as hairy entanglements.
As frustrating as seeds may be in boots, pants, and mittens, they offered society a wonderful invention. It didn't take George de Mestral much time to figure out the hook and fastener principle that plants evolved millenia ago. That's how we got Velcro.
Couvreur M, B. Vandenberghe, K. Verheyen, and M. Hermy. 2004. An experimental assessment of seed adhesivity on animal furs. Seed Science Research, 14(2):147-159.
Manzano, P. and J.E. Malo. 2006. Extreme long-distance seed dispersal via sheep. Frontiers in Ecology and the Environment, 4(5):244–248.
Milton, S.J., W.R. Siegfried, and W.R.J.Dean. 1990. The distribution of epizoocoric plant species: a clue to the prehistoric use of arid karoo rangelands by large herbivores. Journal of Biogeography, 17(1)25-34.
Poinar Jr, G. O. and J. T. Columbus. 1992. Adhesive grass spikelet with mammalian hair in Dominican amber: First fossil evidence of epizoochory. Journal of Cellular and Molecular Life Sciences, 48(9):906-908.