Photo from Kluane trail cams courtesy of Darcy Doran-Myers
Photo from Kluane trail cams courtesy of Darcy Doran-Myers
Plants can’t move. That may seem like an obvious statement, but it has a lot of consequences for how plants live their lives and the kinds of adaptations that they have. Not being able to move is particularly problematic when they have offspring. For example, if a plant’s offspring grew next to the parent plant, the parent and offspring would probably end up competing with each other for space, water, and nutrients for the rest of their lives. So parent plants have to get creative in transporting their offspring elsewhere, even though the parents themselves cannot move. This movement away from the parent plant is called dispersal. Some plants have solved this problem by manipulating animals to transport their seeds. For example, their seeds could be contained within a fleshy, edible fruit that would then be eaten by a foraging animal. After the edible fruit was digested, the seeds could be deposited some distance away, thus solving the parent plant’s problem. Having seeds transported in this way comes with the added benefit that animal poop can be highly nutritious, which is great for a seedling just starting to grow. However, it’s pretty common that seeds don’t just germinate wherever they’re deposited: seeds are often transported by multiple animals or other means such as wind or water. For example, a cherry eaten by a bird could have its seed first deposited by the bird and then transported by ants where it then grows into a cherry tree.
We were interested in seed dispersal because this process can get quite bizarre. Many animals that eat seeds or fruit fall prey to predators. If the prey had recently eaten, they could still have seeds or fruit in their gut when they were killed by the predator. This means that the seeds that started out being eaten and then dispersed by one animal, ended up in the gut of a predator instead. In our cherry tree scenario, this could happen if the bird ate the cherry only to be consumed by a fox afterward. The cherry seed could then hitch a ride with the fox instead of the bird. The process of a seed being transported in the gut of multiple animals, such as first by a prey animal that was then eaten by a predator, is called diploendozoochory.
Our paper was recently published in Ecosphere and is Open Access:
We wanted to know how widespread this phenomenon was and how important it was for plant populations. After reviewing scientific literature, we found that this kind of predator-assisted seed dispersal was first described by Charles Darwin in 1859. Since then, there have been other sporadic observations and we found that there is potential for this phenomenon to occur in many habitats and species. These studies showed that seeds consumed by prey that were eaten by predators may be moved greater distances than seeds deposited by the prey alone. Predators and prey may travel through different kinds of habitats, which means that seeds can end up in different places depending on who deposits them. Some seeds have particularly thick shells, which must be cracked open for the seedlings to grow. These plants can benefit from the wear and tear of passing through the guts of two animals, making them better able to germinate than if they had passed through the gut of the prey alone. It’s even possible that some plants have evolved specifically to take advantage of these predator-specific behaviours, in other words their seeds have evolved counting on the prey being eaten by a predator. However, these different factors are like pieces in a puzzle: to fully understand the big picture of how they affect plant populations, we need to know how all of these pieces fit together. So far, studies have only looked at small parts of the puzzle, and no study has put all of the pieces together to see the overall importance of this phenomenon for plant populations or its role in seed evolution.
Because predators may transport seeds somewhere different than prey, diploendozoochory has broader impacts than just affecting plant populations. For example, predators are often larger than their prey and can thus cover larger distances. As humans continue to fragment and alter wilderness, such as by cutting down forests or building roads, predators may be the only animals large enough to navigate across these areas and enable plants to recolonize them. Climate change will alter where some plants can find suitable places to grow, and seed-carrying predators could have a role in helping plants cover a larger area and hence move with the changing climate. On the other hand, plants that have been introduced to new countries and continents by humans, called introduced or invasive species, may invade new areas faster thanks to predators giving them a hand. Our work has highlighted how interesting and important this phenomenon is, and we hope that it will help and encourage others to fill some of these gaps in our understanding.
More info can be found here:
A little while back, I did a Master’s degree at the University of Guelph with Prof. Andrew McAdam. I worked on the Kluane Red Squirrel Project, a collaborative project between several universities in Canada and the U.S. This long-term project was started nearly 30 years ago by Prof. Stan Boutin at the University of Alberta. The project has involved many undergraduate, graduates, and post-docs over the years studying a variety of ecological and evolutionary questions on a population of red squirrels in Kluane, Yukon.
For my Master’s project, I was interested in red squirrel territorial behavior and the vocalizations, known as rattles, used to defend their territories. Red squirrel rattles are individually unique and have been shown to be used to discriminate kin, though the mechanism underlying this ability is unknown. In a recently published paper in Behavioral Ecology, I sought to distinguish between the mechanisms of ‘prior association’, where animals learn the phenotypes of kin they associate with early in life, and ‘phenotype matching’, where animals use a template to match phenotypes, thereby allowing them to recognize kin without an association early in life. I recorded rattles from squirrels in the field, and used those recordings in playback trials to measure the behavioural responses of squirrels to rattles from familiar kin, unfamiliar kin, and non-kin. One of the major benefits of the Kluane Red Squirrel project is that there is pedigree information for each squirrel, which means that we know who their mother and father is and who their siblings are. Without this information, this project would not have been possible, and full pedigree information is difficult to obtain for wild populations of animals.
For red squirrels, familiar kin consisted of pair of squirrels that shared a natal nest (e.g. mother-offspring pairs and siblings from the same litter), and unfamiliar kin consisted of pairs of squirrels that did not share a natal nest (e.g. father-offspring pairs, siblings from different litters). Initial analyses revealed that red squirrels did not discriminate between familiar and unfamiliar kin, but also did not discriminate between kin and non-kin, despite previous evidence indicating this capability. Post-hoc analyses showed that a squirrel’s propensity to rattle in response to playback depended on an interaction between relatedness and how the playback stimuli had been recorded. Rattles used as the playback stimuli were either recorded from squirrels as they moved freely around their territories (unsolicited), or from squirrels as they were released from a trap or in response to a broadcast rattle (provoked). Red squirrels discriminated between rattles from close kin (relatedness coefficient of at least 0.5) and rattles from less related kin or non-kin (relatedness coefficient of less than 0.5) when the rattles were recorded from provoked squirrels. Squirrels did not exhibit kin discrimination in response to rattles that had been recorded from unprovoked squirrels.
This figure show the probability of a rattle response from the subject squirrel during the playback period by relatedness coefficient calculated from the pedigree and the collection method of obtaining the rattle stimulus. Unsolicited rattles were recorded from squirrels moving freely around their territories (n = 67 trials), and provoked rattles (n = 38 trials) were recorded from squirrels as they emerged from a live-trap or from squirrels responding to a rattle playback
This is potentially quite interesting, but it is important to note that this relationship was identified through exploratory post hoc analyses and needs to be tested more rigorously. If these results are robust, however, they would suggest that a squirrel’s physiological state might influence the structure of its rattles, including those individually distinctive structural features that are presumably used in discrimination. This raises interesting questions about what kind of information may be contained in the rattles and suggests that rattles may reflect the current state of stress or aggressiveness of the squirrel.
Photos and post by Julia Shonfield
Julia Shonfield, Jamieson C. Gorrell, David W. Coltman, Stan Boutin, Murray M. Humphries, David R. Wilson, Andrew G. McAdam. 2016. Using playback of territorial calls to investigate mechanisms of kin discrimination in red squirrels. Behavioral Ecology arw165. doi: 10.1093/beheco/arw165.
The abstract and a link to the full text can be found here:
If you are unable to access the article but are interested in reading it, you can email me at firstname.lastname@example.org and I can provide you with a copy.