The sidewinder rattlesnake, one of many snakes that inadvertently transports seeds by swallowing small herbivores (Image Credit: Brian Gratwicke, CC BY 2.0, Image Cropped)

Seed ingestion and germination in rattlesnakes: overlooked agents of rescue and secondary dispersal (2018) Reiserer et al., Proceedings of the Royal Society B: Biological Sciences, DOI:10.1098/rspb.2017.2755

The Crux

Plants depend on outside forces to disperse their seeds away from the parent plant, and the most common way is via a process called zoochory, where animals spread the seeds. This can be due to seeds being stuck onto the fur of an animal, animals taking and storing the seeds in a different location, or when an animal eats the fruit and later defecates the seeds.

One indirect way in which seeds are dispersed is when a predator, such as a coyote, raptor, or bobcat, consumes an animal (like a mouse) that had seeds in its stomach or cheek pouches. Rattlesnakes commonly consume small rodents that carry seeds in cheek pouches, and though these snakes are known to eat these seed-carrying animals, their own role in seed dispersal remains largely unknown. In order to learn more, the researchers in this study dissected museum specimens to search for secondarily-consumed seeds.

Did You Know: Seed Dispersal

Because terrestrial plants are not able to move themselves, they need an outside force to move and disperse their seeds, otherwise plants would be surrounded by close relatives and this would lead to some serious inbreeding problems.

Some plants have lighter-than-air seeds that can be blown on the wind, some drop into passing streams or rivers. What we’re focusing on here, seed consumption by animals, is termed endozoochory, as opposed to epizoochory, whereby seeds stick to the fur of passing animals.

How it Works

The authors analyzed the gastrointestinal (GI) tracts of wild-caught rattlesnakes that had been preserved and stored at the Museum of Vertebrate Zoology in Berkeley, California. The authors examined 42 sidewinders (Crotalus cerastes), 4 Mojave rattlesnakes (C. scutulatus), and 4 southern speckled rattlesnakes (C. Pyrrhus) for a total of 50 snakes. They examined the stomach, large intestine, small intestine, and colon of all of the snakes.

Prey species were identified to genus and species where possible, but if this was impossible they were classified at higher taxonomic levels (lizard, rodent, etc.). Seeds were photographed and then scored as germinated if the beginnings of a root (known as a radicle) could be seen.

What They Found Out

Of the 50 snakes examined, 22 of them had intact prey remains in their stomachs, while 45 had rodent remains like hair or teeth, and nine had lizard remains. 971 total seeds were found in the GI tracts of the snakes, though only 19 of them were germinated.

Problems?

This study was limited in two ways: small sample size and localized sampling area. With vertebrates (animals with a backbone), it can be very difficult to collect large amounts of specimens. The second problem may have had a larger impact on the results. The specimens used in this study were all from Arizona and California in the United States, and though it is unlikely the results seen here may only apply to snakes from that exact region. To better determine how prevalent this phenomenon is, more dissections should be carried out with specimens from a wider variety of locations.

Dissection of dead specimens can be a valuable source of ecological information (Image Credit: Matt Hoffman, CC BY 2.0)

So What?

What this study showed was that rattlesnakes (all snakes, really) could play very important roles in a terrestrial plant ecosystem. The authors sum this up with four points:

1. All snakes are strictly carnivorous and do not eat the seeds themselves. They are actually unable to digest the materials that seed casings are made of, so the seeds will not be damaged inside of the snake.
2. Snakes swallow their prey whole, without chewing. This means that the seeds stand even less chance of incurring damage.
3. Because it takes snakes a while to digest prey, the seeds remain inside the snakes GI tract for a prolonged period of time. This increases the chance that the seeds will germinate in the GI tract. In addition, snake feces may provide a rich and suitable environment for the seeds once they are back outside of the snake.
4. Last but not least, snakes tend to have large home ranges. This, combined with the prolonged digestion period, means that snakes could possibly disperse seeds to a relatively large distance away from the parent plant.

So is snake seed dispersal a necessity for plant species? Again, more research is needed. But if so, any reduction in snake populations may mean range constrictions, and lower population viability for certain plants.

It is also important to consider, that while not an experiment, this kind of observational study is important for learning what kind of patterns we can see in the wild, later to be analyzed with experiments to determine the cause.