Agoutis: saviours of the forest.

I discovered a really neat paper this morning, published in the latest issue of the Proceedings of the National Academy of Sciences (PNAS). Mr IncreasingDisorder had left the PDF open on the computer, which was very helpful in me discovering it! I really liked the paper – it was a simple hypothesis tested in a very clever way. And it involved rodents. Here’s a short summary of the paper.

Jansen et al. (2012) Thieving rodents as substitute dispersers or megafaunal seeds. PNAS 109, 12610-12615. 

In the rainforest of Panama, many palms and trees have fruits bearing very large seeds. Too large, in fact, to be ingested whole by any of the extant fauna, leading researchers to suggest that these palms adapted to dispersal by megafauna during the Pleistocene, now extinct. Very large animals would have been able to eat the fruit and swallow the seeds intact, allowing them to be dispersed as the animals moved around. Yet the palms still survive, and disperse. The simple, yet elegant aim of the research was to find out how good agoutis were at dispersing the seeds.

Dasyprocta punctata (via Wikipedia)

Agoutis, Dasyprocta punctata, are caviomorph rodents who like to cache their food – called ‘scatter-hoarding’ in the paper. While rodents are typically not known for their seed-dispersal ability, the agoutis are large enough to carry the seeds around. In the absence of any other larger herbivorous mammals, the agoutis have gone from partial seed disperser to the only seed disperser for the target palm species, Astrocaryum stanleyanum.

Astrocaryum sp. (via Wikipedia)

Using radio-tracking, of both individual palm seeds and agoutis, the researchers were able to plot the fate of the tagged seeds. Known agouti caches were monitored using remote cameras, which gave insight into the frequency of caches being dug up, as well as information on who it was doing the digging. They found that the agoutis did indeed serve as the principal dispersers of the seeds (over 80% of tagged seeds were taken by agoutis), but it was not via single agoutis moving single seeds to new areas. A complex network of caching, re-caching and cache-raiding was going on. Agoutis did not tend to eat their seeds immediately, but had a tendency to dig up and re-cache their seeds. This re-caching created a step-wise movement away from the point of collection of the seed, increasing the potential dispersal area for the seeds.

But what about the raiding? Via the remote cameras, it was seen that most seeds (over 80%) were dug up by other agoutis, not by the owner of the cache. To quote the article: “Theft was strongly reciprocal: individuals that were robbed also stole cached seeds from others. Thus, the stepwise dispersal of seeds across home range boundaries was driven by reciprocal theft.” (I love the phrase ‘reciprocal theft’.)

What all this means is that the agoutis are far better at dispersing seeds than previously thought. The researchers were surprised by the distance travelled by the seeds, which was facilitated by the stealing and re-burying of seeds elsewhere. The agoutis do not eat all the seeds though, and it was estimated that approx. 14% of palm seeds taken by agoutis would survive to germination. Despite the agoutis being far smaller in size than the megafauna that previously dispersed large tropical seeds, they appear to be doing a rather nice job of filling the vacant niche left by extinction.



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