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  Behavioral Education for Human, Animal,
Vegetation & Ecosystem Management

Stories of Applied Animal Behavior
Created by members of a graduate Foraging Ecology Class
     at the University of Idaho and Washington State University
     under the direction of Drs. Karen Launchbaugh and Lisa Shipley

Seed Dispersal by Animals: Behavior Matters

By Suann Yang, Ph.D. Candidate in Botany, School of Biological
Sciences, Washington State University


   Plants, being stationary, require a mobile mode for seed dispersal.  Modes for seed dispersal include self-projectile mechanisms, wind, water, and animals.  Dispersal by animals falls into the category of plant-animal interactions, a subject of interest because of the reciprocal adaptations that can be observed.  These reciprocal adaptations lead to coevolution, or change in organisms as a result of their interactions with each other.

Animals as Dispersal Agents

     Plants that depend on animals for dispersal have seeds that are adapted to traveling on the outside or the inside of the animal.  Seeds with burrs or hooks can attach to an animal’s fur.  For seeds that are transported internally, plants provide an attractive fruit pulp reward in return for the ride.

     The process of internal seed dispersal begins with the plant producing fruits.  Next, an animal is attracted to these fruits based on their characteristics (such as odor), and begins to feed.  After these fruit-eating animals (also known as frugivores) ingest fruit, they process them in their digestive tracts, and then regurgitate seeds or deposit seeds in their feces.  

    Behavior, both innate and learned, is involved in every step of the dispersal process.  First, the stimulus of being hungry makes the animal begin searching for food.  While in this state of searching, the animal is receptive to stimuli (e.g., color, odor) that may signal the presence of food items.  If the fruit encountered has an acceptable taste, the animal will probably choose to eat it.  Where seeds are deposited depends on how long the animal stays at the plant feeding, and where the animal goes afterwards.    

      Not all animals that eat fruit are appropriate dispersers.  The effectiveness of an animal as a seed dispersal agent depends on how many seeds it disperses, and how it treats these seeds.  Animals that visit a plant for fruit more frequently may be more reliable than those animals that eat fruit more rarely.  Some fruit-eating animals are considered seed "predators" if they kill seeds by digesting the seed along with the fruit pulp.  On the other hand, seeds can sprout better after passing through the digestive tract of certain frugivores.  In addition to enhancing sprouting, an effective dispersal agent deposits seeds in appropriate habitats for their survival to reproductive adulthood.  The survival of a seed greatly depends on where it lands.  Seeds that move farther away from other seeds have greater success because they can better escape resource competition, interbreeding with parents, and post-dispersal mortality. Because not all animals that eat fruit are equally good at dispersing seeds, plants should change fruit characteristics to discourage frugivores that are seed predators, and encourage frugivores that are effective dispersers.  

Fruit Characteristics and Forager Choice

     Fruit characteristics (or fruit traits) that foragers use to choose fruit as food items include toxin content, fruit appearance, and nutrient content.  Together, the presentation of these traits is called the fruiting display.  Foragers learn to recognize these traits as cues to trigger selection or avoidance of certain fruits as food items.  The reliability of the behavioral response depends on the consequences of eating the fruit selected.  Animals will prefer fruits that fulfill nutritional requirements, but avoid those that are toxic.  Reciprocal change (coevolution) between plants and frugivores can occur when behaviors of foragers select fruit traits that in turn select for foraging behavior. 

Toxins.  Some fruits contain toxins to deter seed pathogens or predators.  Animals can learn which fruits are toxic, and thus learn to avoid them.  Toxins can also limit the amount of time a more toxin-tolerant frugivore spends feeding at the plant.  Because animals are limited to the amount of toxins they can handle over a period of time, a forager must stop eating a food item when maximum toxin load is reached.  The foraging animal may then leave to find an alternative source of food or to seek antidotes.  By forcing an animal to leave early, the plant may be able to ensure that its seeds will be deposited far enough away to avoid inbreeding, competition, and pathogens as mentioned above.

Fruit appearance.  Although toxin content limits ingestion of fruit, appearance can both limit and encourage it.  Fruit appearance provides salient cues for recognition of fruit as a food source.  Pigments such as carotenoids, flavonoids, and betalains give fruits their colors, and volatile compounds provide odors.  In general, bird-dispersed fruit are red or black, and mammal-dispersed fruit have distinctive odors.  The morphology (size and shape) of a fruit may also influence forager choice.  Animals are limited to food items small enough to fit in their mouths and throats, especially birds and reptiles that tend to swallow fruits whole.  In addition, birds may also consider the bulkiness of seeds when choosing fruit.  Not only does seed bulk take up space in the digestive tract, it also may need to be jettisoned before flight.

Nutrient content.  Finally, fruit nutrients may be important in influencing forager choice.  For most temperate fruits, energy in the form of sugars is the most abundant nutrient.  Other vitamins and minerals may also be influential.  For example, rose hips are known to be high in vitamin C.  Nutrients may be more indirect in effect than appearance.  Fruit appearance provides the cue for fruit choice, but nutrients provide the positive feedback that maintains an animal’s preference for the fruit.


     By understanding what influences an animal’s decision to choose fruit, we can attempt to predict whether or not a plant’s fruiting display will lead to successful dispersal.  The fruiting display must balance maximizing gains from attracting effective dispersers while minimizing losses from also attracting ineffective dispersers.  Thus, we can determine the effectiveness of a fruiting display by the number of seeds successfully dispersed per effort for the entire fruit crop.  Effort can be determined by identifying the factor that influences an animal’s decision to choose the fruit.  For example, if the animal selects fruit on the basis of energy content, then effort is the amount of kilojoules of energy of the entire fruit crop.  Effectiveness is then measured in units of numbers of seeds dispersed successfully relative to the amount of energy of the fruit crop.

     In conclusion, foraging behavior is an important component of the plant-seed disperser interaction.  Foraging behavior determines how fruits are chosen, and where the seeds are deposited.  Toxin content, fruit appearance, and nutrient content may influence how fruits are selected and where seeds are deposited.  By examining the behavioral components of the seed dispersal process, we can better understand how coevolution between plants and their dispersers may take place.

Additional Reading:

Herrera, C.M. 2002.  Seed dispersal by vertebrates.  Pages 185-208 in C.M. Herrera and O. Pellmyr, editors.  Plant-animal interactions:  an evolutionary approach.  Blackwell Science Ltd.  Malden, Massachusetts, USA.

Schupp, E.W.  1993.  Quantity, quality, and the effectiveness of seed dispersal by animals.  Pages 15-29 in T.H. Fleming and A. Estrada, editors.  Frugivory and seed dispersal:  ecological and evolutionary aspects.  Kluwer Academic Publishers, New York, USA.

Stiles, E.W. 1989.  Fruits, seeds, and dispersal agents.  Pages 87-122 in W.G. Abrahamson, editor.  Plant-animal interactions.  McGraw Hill, New York, New York, USA.


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Learn more about the Foraging Ecology Class by visiting http://www.cnr.uidaho.edu/range556/