Saturday, November 3, 2012

Through the belly of the tortoise, passes the wise seed

Hitching a ride with a tortoise is a speedy way to travel- at least for Galapagos plants living rooted to the spot. According to research published in the Journal of Biogeography's special issue on how island plants spread their seeds, forty-five different plant species on Santa Cruz Island may take advantage of Galapagos tortoise mobility and undiscriminating appetite to move their offspring from place to place.

How does one go about discovering such a thing? You've got to be willing to wade through a bunch of crap... literally. In a series of cleverly planned experiments, mostly involving tortoise poo, the researchers were able to piece together which plants were being transported, how far they might be traveling, and how well seeds survived the harrowing journey through a digestive tract without the aid of Ms. Frizzle's magic school bus.

First, the scientists tramped around the island, picking up 120 piles of tortoise feces, which were later carefully dissected to find all of the tiny seeds (sometimes more than 1000!) that might be hiding inside. In order to find out how far these seeds may have traveled, they attached GPS tags to 14 tortoises and recorded how far they wandered over a period of a year. Of course, a seed probably doesn't sit in a tortoises' stomach for a whole year, so the researchers needed a way to calculate how far on average a tortoise moves from the time a seed is eaten until it pops out the other end. By feeding over 15,000 little plastic seed-sized beads to 19 captive tortoises, the researchers were able to monitor, to the hour, how long a seed takes to pass from food to feces (usually about 12 days- quite some time compared to a human's speedy 1-3 days). Combining these digestion times with the GPS records of where the tortoises walked showed that about half of seeds eaten by a tortoise are likely to travel more than 1/2 a kilometer.

Finally, the scientists compared whether seeds that had been eaten by tortoises were more or less likely to sprout than seeds taken directly from their fruits. Some plants with especially hard seeds might actually sprout better after being digested because the acidity and abrasion in the stomach wear away outer protective layers that prevent a seed from starting to grow. Not to mention that seeds ending up in a mound of dung are surrounded by lots of fresh fertilizer. But, for Galapagos plants at least, tortoise digestion doesn't seem to help or hinder a seed's ability to sprout, nor does landing in a warm pile of poo.

Studying Galapagos tortoise feces may seem esoteric, to put it kindly, but life on earth might look quite different if it weren't for animals spreading seeds from the fruits they eat. This is especially true for islands, whose stationary flora required an improbable leap across miles of ocean.

You can find this article at:

ResearchBlogging.orgStephen Blake, Martin Wikelski, Fredy Canbrera, Anne Guezou, Miriam Silva, E. Sadeghayobi, Charles B. Yackulic, & Patricia Jaramillo (2012). Seed dispersal by Galapagos tortoises Journal of Biogeography, 39, 1961-1972 DOI: 10.1111/j.1365-2699.2011.02672.x