How long time does a T. Rex take to digest a seed?Submitted by editor on 19 February 2019.Get the paper!
When an animal ingests a seed, how long does it take for the seed to pass through its gut? This is an important question for plant ecologists, because seed retention time (SRT) can determine the distance the seed can travel. A number of studies have measured SRT for a variety of animals. Since the late 1980s, researchers have used SRT data to estimate the spatial pattern of seed dispersal by combining them with animal movement data. However, no explicit model has been developed to predict the SRT for specific taxa.
A brown-eared bulbul Hypsipetes amaurotis consuming fleshy fruits of figs (left), and a Bonin white-eye Apalopteron familiare excreting a seed (right). (Photos by Kazuto Kawakami)
To fill this gap, we collected measured SRT data and developed predictive models for mean SRT for the main taxa of seed dispersers. We collected studies on seed dispersal by animals, and collated the mean SRT data of 112 animals, including birds, mammals, reptiles, fish, and slugs. Using this data, we explored allometric relationships between body size and mean SRT, as well as effects of other animal traits on SRT, for each of the taxa studied.
There was a large variation in the allometry among the major taxa. Reptiles had a much higher intercept and smaller slope than did birds and mammals. This means that small-sized reptiles have much larger mean SRT than birds and mammals of similar sizes, but this difference reduced as the body size increased. Birds had a larger slope and smaller intercept than mammals. For birds, food habits were also associated with SRT, with frugivory linked to short SRT.
Allometric relationships between body size and mean SRT for taxa of seed dispersers.
Using the allometry for birds, we estimated mean SRT of theropod dinosaurs, direct ancestors of extant birds. Accumulated evidence suggests dinosaurs could act as seed dispersers. Together with herbivorous species, we focused on carnivorous theropods, such as Tyrannosaurus, because they could become secondary seed dispersers by consuming seed-ingesting prey animals. The possibility that predatory animals could act as seed dispersers was originally raised by Charles Darwin in his On the Origin of Species, and has been supported by contemporary ecologists. The estimated mean SRT of Tyrannosaurus based on our model was 4-5 days, which suggests their potential as long-distance seed dispersers.
We propose our model as a first step to approximate SRT for wide range of seed dispersers, and to contribute to trait-based modeling of seed dispersal functions at the community level. However, we recognize that this model requires further development. Some studies indicate that gut retention times of herbivorous mammals are independent of body size and suggest the importance of considering other digestive traits for each guild. Thus, future modeling of SRT should incorporate the complicated processes of animal digestion for each guild. We hope our results can facilitate future development of trait-based modeling of the seed dispersal services of different taxa and promote recognition of the ecosystem functions that ancient animals might have played.
Tetsuro Yoshikawa on behalf of all co-authors