With our kitchens and restaurants it’s been easy for us humans to settle into a comfortable eating routine of a morning breakfast, mid-day lunch, and evening dinner.  Animals, on the other hand, have to hunt down their meals out in the wild.  For agoutis, and other ground-dwelling frugivores, this means walking around waiting for fruit to fall out of trees and land on the ground.  That leads to a simple question – when do fruits fall out of trees?

As far as I can tell, this question has never been studied – who wants to sit around and watch fruits slowly fall down all day, and night?  Yet this is a basic bit of natural history information that could be quite important when considering the survival strategies of agoutis, in particular, and daily rhythms in the forest, in general.

Camera traps are a perfect tool for this question, and our student Vivian Mass set some aimed at the fruits of Astrocaryum palm trees.  For this to work she had to find another tree at just the right distance away from the targeted palm-fruits and then get a camera trap up there, which she did with the help of ladders and some canopy access climbing by Daniel and Alejandro.  This was a bit of a side project from Vivian’s main thesis, and the data remained unanalyzed until this spring when high school student Tessa (Taz) Holliday joined our team as an intern from the Emma Willard School.

Taz looked through each picture and noted if any fruits had fallen off since the last picture.  In some cases it was obvious – a big howler monkey was on the tree picking fruits, eating the fleshy part, and then dropping the nut down to the ground, where agoutis and other terrestrial critters could find them.  In other cases fruits would fall off without any animal intervention, just because they were ripe.  In total Taz noted 450 seed fall events from 8 different Astrocaryum trees, with slightly less than half being dropped by animals.

Looking at the fruit fall over the entire fruiting period (figure 1) it was obvious that arboreal animals had a huge effect on when a fruit would fall.  A few trees that had no arboreal animals visit would drop a few fruits a day for 2-3 weeks.  Once an animal visited they would drop all, or nearly all, of the fruits in one sitting.

Fig 1. Timing of fruit-fall over four weeks for 6 Asrocaryum trees.

These arboreal animals also had an impact on what time of day the fruits hit the ground. The monkey- and parrot-fed fruits fell in the day while the kinkajou-fed fruits fell at night (figure 2).  Overall, this led to more fruits falling during the afternoon than you might otherwise expect (figure 3b), although with only 4 trees in this analysis (the time lapse photos didn’t work for 4 trees), we should be careful in what we conclude from it.

Fig 2. Time of day that fruits fell out of 4 Astrocaryum trees

Even more surprising was considering what time the fruits fell when no animals knocked them off. This is tree-behavior, when do they ‘want’ their fruits to fall to the ground.  Surprisingly, this showed a strong trend to more fruits in the late night (midnight-5am, figure 3a).  We know from our agouti tracking that this is the most dangerous time for agoutis to be out foraging, we have found quite a few agoutis killed by ocelots in this time period.  Are the trees trying to temp the agoutis out for an early breakfast?

Fig 3. Expected and observed times of day that fruits fell out of trees, with and without arboreal animals (significantly different than random p<0.0001)

Lets call this the “Machiavellian plant behavior hypothesis”.  Long-lived agoutis might be bad for trees because they remember where they buried all the seeds and come back and eat them year after year.  However, if dropping seeds more at night led to a higher turnover of agoutis nearby due to ocelot predation, it might also lead to a better chance that their cached seeds survived, since the “new agouti on the block” wouldn’t know where the “recently deceased” buried the seeds. This Machiavellian plant behavior seems a long shot, but this preliminary result that Taz teased out of data collected earlier by Vivian suggests it might be worth following up on.

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