To Detect a Predator: A Multimodal Approach

This is another guest post from undergraduate scientist, Jenny Schefski. She previously wrote a post on this blog last summer when she was conducting research for her independent project (read the post here). Now, she elaborates more on the concept behind her research and why it is important. 

As humans, we rarely need to rely on all of our senses to find food, shelter, or mates. Indeed, these necessities are often advertised and/or delivered to us with little effort. With virtually no predators, we aren’t even likely to need our senses to avoid becoming prey. 

Unlike wild animals, most people have little trouble finding food, shelter, or even mates.

(http://www.brennersigns.com/wp-content/uploads/2012/12/hotel-led.jpg, http://listabuzz.com/wp-content/uploads/2013/12/match.jpg)

Unless you’re unlucky enough to encounter Hannibal, you probably aren’t too concerned about becoming a predator’s next meal.

(http://ic.pics.livejournal.com/cleolinda/1427760/456345/456345_600.png)

However, most animals heavily rely on their senses to avoid predators, and to find food, shelter, and mates. By utilizing multiple senses, animals increase their likelihood of success in a variety of conditions. Sharks, for example, are capable of using a wide array of senses to locate prey.¹ This is especially important in the ocean since prey can be sparse and visibility can be poor. By utilizing multiple senses to find their prey, sharks maximize their likelihood of foraging success.

Sharks maximize their hunting efficiency by using multiple senses to track down prey.

(http://www.nature.com/scientificamerican/journal/v297/n2/images/scientificamerican0807-74-I4.jpg)

The use of multiple senses to gather information about one’s environment is called multimodality.² Multimodality is important to the study of animal interactions because multiple sensory inputs can lead to complex behaviors. In simpler terms, a noise from Animal 1 might result in a specific behavior from Animal 2. However, the scent of Animal 1 might lead to a different behavior from Animal 2. 

Let’s say a= a snake’s pattern, and b= a snake’s scent. The shapes that follow the arrows represent all of the possible response behaviors from a hypothetical squirrel. (Partan & Marler 2005).

Predator-prey interactions often involve complex behaviors. In order to avoid becoming a predator’s next meal, prey must be able to identify and detect their predators.  Over millions of years, prey have fine-tuned many of their senses to the detection of specific predators. For example, wolf spiders can detect the specific frequency of vibrations from their bird-predator’s pecking on a tree.³  Spiders that were experimentally exposed to the pecking frequency stopped all courtship behaviors and movement. Even more, wolf spiders also respond to the shadow of a bird predator; except, in this case, they increased locomotion and escape behaviors. This study is not only a prime example of how prey can detect sneaky predators, but also how multimodal interactions can lead to complex outcomes.

Wolf spiders can identify and respond to the pecking, calling, and even shadow of a bird-predator. Of course, there are always prey that miss the memo(s). This unfortunate wolf-spider probably should have paid more attention to his senses.  

Not only is multimodality important for predator detection, but it is also key for species discrimination. For example, brown anoles can detect their bird-predator, the grackle, by sight and by sound.⁴  Throughout the day, brown anoles see and hear multiple birds. So how can they know when to hide or when it’s safe to do important things like forage or look for a mate?  Since the anole can identify the grackle’s specific appearance and call, it can discriminate between the grackle and other non-threatening birds. By having the ability to cue-in on the grackle using multiple senses, the anole can maximize its time to forage and search for a mate, and minimize its likelihood of becoming a grackle’s next meal.

Brown anoles can identify the appearance and call of their predator, the great tailed grackle. This ability allows anoles to distinguish the grackle from non-threatening birds so they don’t have to hide all day.

(https://c1.staticflickr.com/5/4086/5066322444_44a14302a2_z.jpg, http://www.planetofbirds.com/wp-content/uploads/2011/07/Great-tailed-Grackle.jpg)

Another predator-prey system that is ideal for the study of multimodal predator detection and discrimination is that of the California ground squirrel. California ground squirrels have two snake-predators: the Pacific gopher snake and the northern Pacific rattlesnake. Gopher snakes are non-venomous and rely on their stealth to invade squirrel burrows in search of pups. Conversely, rattlesnakes are venomous and can quickly kill both pup and adult squirrels. Since each snake poses a different level of immediate risk, it would behoove ground squirrels to not only identify a snake predator, but also discriminate between a venomous and non-venomous one. 

California ground squirrels have two snake-predators: the northern Pacific rattlesnake (top) and the Pacific gopher snake (bottom). Rattlesnakes are more threatening to squirrels because of their ability to quickly inject squirrels with deadly venom. (Photos: Joseph Chase)

Previous studies show that ground squirrels do indeed discriminate between gopher snakes and rattlesnakes.⁵ This is evidenced by squirrels’ behavior toward each snake species. Ground squirrels tend to be more aggressive toward gopher snakes and will approach them more closely. When presented with a rattlesnake, ground squirrels are likely to monitor it more often, but maintain more distance from it.

Exactly how California ground squirrels discriminate between each snake-predator remains unclear. We do know that squirrels can identify each snake by its visual appearance.⁵ However, there are a variety of reasons why vision is not always a reliable mode of detection for ground squirrels. Not only do squirrels often encounter snakes in their very own, dimly lit burrows, but they also encounter them often in dense vegetation. Furthermore, both rattlesnakes and gopher snakes blend into their surroundings very well. All of these factors suggest that squirrels might use another sense to detect each snake and discriminate between the two species.

Can you find the snakes in these photos? California ground squirrels often encounter snakes moving through the grasses that they feed on. Spoiler: a gopher snake (left) and a rattlesnake (right). (Photos: Joseph Chase)

My research focuses on how California ground squirrels use multiple senses to detect and discriminate between gopher snakes and rattlesnakes. One previous lab study suggested that California ground squirrels can identify the scent of each snake-predator.⁶ My work takes this study to the field, a more realistic setting. By manipulating the scent of rattlesnake and gopher snake models, I can tease apart the role of each cue in squirrel response behavior. My ongoing analysis has led me to many questions: Does the smell of a snake make squirrels more wary of their environment or does it elicit anti-snake behavior? What will squirrels do when presented with a rattlesnake model that smells like a gopher snake? Do squirrels trust visual or olfactory input more when deciding how to react to a snake predator?

I am still in the process of analyzing seemingly endless field footage, but I hope to have some answers soon!

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References:

1.  Hueter, R.E., D.A. Mann, K.P. Maruska, J.A. Sisneros, and L.S. Demski. 2004. Sensory Biology of Elasmobranchs. Biology of Sharks and Their Relatives 1: 326-358.

2. Partan, S.R. and P. Marler. 2005. Issues in the classification of multimodal communication signals. The American Naturalist 166:231-245.

3.  Lohrey, A.K., D.L. Clark, S.D. Gordon, and G.W. Uetz. 2009. Antipredator responses of wolf spiders (Araneae: Lycosidae) to sensory cues representing an avian predator. Animal Behaviour 77:813-821.

4.  Elmasri, O.L., M.S. Moreno, C.A. Neumann, and D.T. Blumstein. 2012. Response of brown anoles Anolis sagrei to multimodal signals from a native and novel predator. Current Zoology 58:791-796.

5.  Towers, S.R. and R.G. Coss. 1990. Confronting snakes in the burrow: snake-species discrimination and antisnake tactics of two California ground squirrel populations. Ethology. 84:177-192. 

6.  Hennessy, D.F. and D.H. Owings. 1977. Snake species discrimination and the role of olfactory cues in the snake-directed behavior of the California ground squirrel. Behaviour. 65:115-123.