Why are some animals hoarders?

Source Article: Zwolak R, Clement D, Sih A, Schreiber SJ. 2021. Mast seeding promotes evolution of scatter-hoarding. Phil. Trans. R. Soc. B 376: 20200375. https://doi.org/10.1098/rstb.2020.0375

Featured Image Caption: Chipmunks are scatterhoarders: animals that bury nuts and seeds to eat later. Image Source: Pixabay

Why do some animals, like mice and squirrels, bury their food? Storing seeds or acorns for later like this is risky; other animals frequently steal them, leaving the hoarding mouse or squirrel unprepared and taken advantage of. Scientists have struggled to explain how this behavior, called scatter-hoarding, could evolve when it seems like it would disadvantage the animals who do it. Through natural selection, the physical traits and behaviors of organisms who are better at surviving and producing offspring become more common from generation to generation. Animals who take on the high risk of burying their food seem like they would be less likely to survive and have kids. So, why has this behavior continued for many generations?

Squirrels are famed scatterhoarders, commonly seen burying acorns in the fall. Image Source: Pixabay

Several hypotheses have been considered by researchers in the past. Some suggested that perhaps scatter-hoarding individuals are much better at finding their own buried food than the buried food of others. However, evidence has suggested that individuals of some species may not be good enough at finding their own food for this hypothesis to be true. Others suggested that scatterhoarders all do enough stealing of each other’s buried food that they end up with enough to eat, even if their own buried food is stolen. This could help explain scatter-hoarding, but some researchers believe there’s another important explanation: tree masting.

Synchronized Seeding
During mast years, the ground can be covered with seeds– many more than the seed-eating animals can eat. Image Source: Wikipedia Creative Commons by Hans Hillewaert

Many tree species, including oaks and beeches, simultaneously produce huge quantities of seeds every few years, a phenomenon called masting. If you live somewhere with oak trees, this is why the ground is covered with so many acorns in certain years, and not in others. Trees can synchronize their seeding across whole continents in many complex ways, which is the subject of ongoing research. They may be taking cues from the weather, nutrient availability, pollen production, genetic controls, other environmental characteristics, or a combination of these mechanisms.

However they do it, by dropping so many seeds some years and so few other years, trees dramatically affect life for the animals that depend on those seeds for food. In non-mast years, these animals starve, and their population sizes decrease. Then when trees suddenly drop so many seeds at once, the small numbers of animals cannot eat them all, and at least some seeds are sure to sprout into saplings. Could this phenomenon have made scatter-hoarding an advantageous behavior for natural selection?

Modeling Masting

Recently, researchers have used mathematical modeling to test their hypothesis that tree-masting may explain the evolution of seemingly risky scatter-hoarding. Their model tracked the behavior and population size of a scatterhoarder species (modeled after the yellow-necked mouse) and the availability of tree seeds (modeled after the European beech) from season to season for several years. When the population of scatterhoarders changed, the tree seed availability would be affected, and vice versa. Their model carefully accounted for all the possible changes in these quantities, including rate of reproduction of scatterhoarders, rate of rediscovering buried food, rate of stealing buried food, and more. By running this model with a variety of yearly mast intensities, or quantities of available tree seeds, researchers could see when burying food was beneficial for scatterhoarders and why.

The mathematical model used by the researchers included variables for the density of scatterhoarders and various food sources. Changes to the amounts of each variable affect the amounts of the other variables to which they are connected. Based on the methods of Zwolak et al. 2021

The models found that after the fall of a mast year, buried seeds help the small and hungry populations of scatterhoarders to reproduce rapidly in the winter and spring. This temporarily brings the population size back up after a mast year, but then the scatterhoarders go hungry until the next mast year when the trees drop lots of seeds.

These dynamics affect how beneficial it is to hide seeds. The models revealed that scatterhoarders have more than they can eat during intense mast years, in part because there are fewer scatterhoarders after the years of starvation. This means that the chances of buried food being stolen are lower during mast years, and hiding food becomes less risky. Because buried food is less likely to be stolen and they can only eat so much at once, it makes sense for scatterhoarders to spend time burying food for later. This still holds true when the animals aren’t much better at finding their own buried food compared to that of another individual. The more dramatic the changes in seed availability are from year to year, the more helpful the behavior of scatter-hoarding becomes for survival and reproduction, possibly explaining why these animals have evolved to bury their food.

Forgetful Foragers Fuel Forests

The relationship between scatterhoarding and tree-masting demonstrates how important ecological relationships between species can be for the continued survival of biodiversity. The scatterhoarders depend on receiving a large quantity of seeds every few years to avoid dying off, and the trees similarly depend on the scatterhoarders. Scatterhoarders play a vital role in moving plant seeds around their environment and encouraging their growth by burying them. Some scatterhoarders, particularly rodents, forget about many of their buried seeds. In this way, they unintentionally plant trees, sustaining forests and providing themselves with their favorite foods for years to come. This relationship serves as a prime example of how species depend upon each other for survival. Accordingly, conservation of biodiversity is key not only for the sake of individual species but for the sake of their entire ecosystems.

Scatterhoarders cannot eat all of the seeds that fall in mast years, and forget about many of their caches, helping trees to reproduce. This acorn is sprouting into an oak sapling. Image Source: Pixabay
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Julia Bebout

Julia Bebout

I am a first year Master's student at the University of Calgary studying how competing species coexist. I graduated from Lehigh University in 2021 with degrees in Biological Sciences and Earth & Environmental Sciences. As an undergraduate, I studied paleoecology and the microbial ecology of alpine wetlands. I'm especially interested in community ecology, wetland and alpine ecosystems, and regenerative agriculture. I also love hiking, climbing, baking, and dancing! Twitter: @BeboutJulia

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