Sarah holding her study organism, Nucella lapillus. Photo credit: Chris York.
Sarah holding her study organism, Nucella lapillus. Photo credit: Chris York.

Dr. Sarah Donelan, a Postdoctoral Fellow at the Smithsonian Environmental Research Center, discusses her paper, “Sex-specific differences in the response of prey to predation risk”, as well as her current research interests and how she became involved in ecological research.

What’s your paper about?

There are two ways that predators can affect their prey: by eating them (of course!), but also by scaring them. Prey exhibit some strikingly different behaviours when they are “scared” i.e., when they can sense that a predator is on the hunt: they are often more vigilant or seek out safe habitats where predators can’t eat them. These behavioural changes have other consequences for prey such as reduced access to food, which means that prey often grow less when they’re around predators.

This concept of the “fear of being eaten” occurs in many different prey species. But even within a species, prey individuals can exhibit different responses to predators based on their specific characteristics, like their age or their sex. In this paper, we ran an experiment to explore whether male and female prey respond differently to predators during the reproductive season.

Nucella lapillus snails lay egg capsules in rocky crevices. Male and female snails look the same from the outside. Photo Credit: Sarah Donelson.
Nucella lapillus snails lay egg capsules in rocky crevices. Male and female snails look the same from the outside. Photo Credit: Sarah Donelson.

We did this using carnivorous snails that live on rocky intertidal shores that are preyed upon by green crabs. We exposed half the male and half the female snails to predators while the other half remained unexposed. We hypothesized that because females need more energy for reproduction than males, females would need to keep foraging even in the presence of a predator, while males would forage and therefore grow less when exposed to predators. This is indeed what we found, but not for the reasons we thought! Females grew the same amount, ate the same amount, and were as efficient with the energy they ate in both the presence and absence of crabs. Males, though, grew less when they were exposed to predators, but not because they ate less. Instead, males were less efficient at converting the energy they consumed into body mass in the presence versus absence of predators, so males had less energy available for growth. This unused energy was likely “lost” because it was expended on the stress response (like how your heart rate rises when you’re nervous). In contrast, females didn’t show this elevated stress response.

We also looked at whether female snails laid as many eggs when they were exposed to predators compared to females that weren’t exposed to predators. We hypothesized that in order to conserve energy, females would lay fewer eggs if they were exposed to predators, and also that larger females would lay more eggs than smaller females. Our results were surprising: females actually laid similar numbers of egg capsules regardless of their exposure to predators, and being large only increased females’ reproductive output among female snails that weren’t exposed to predators. For example, being exposed to predators reduced egg production by over 50% among the largest females in our experiment.

What are the key messages of your article?

The vast majority of species (~60%) are in the middle of food chains, so they are eaten by something! It’s important to understand the different ways that prey respond to predators because the fear of being eaten can alter prey population sizes and biodiversity. Our work helps further clarify these effects by showing that male and female prey can respond differently to predators even in prey species where males and females look similar.

How is your paper new or different from other work in this area?

One key difference between our work and other work in this area is that male and female snail prey look and act the same. Most of the work looking at sex-specific responses of prey to predators use species where males and females look very different (think a male versus female peacock), and so may be more or less vulnerable to predators. Our results show that male and female prey can respond differently to predators based solely on the energy required for reproduction. Females need more energy to lay egg capsules than males need to make sperm, so they can’t afford to stop eating even if they’re scared by a predator. Our results are novel because they suggest that these energetic differences alone can affect how prey respond to predators.

What is the broader impact of your paper (outside of your specific species/study system)

Green crabs eat and scare Nucella, but the paper by Donelson et al. shows that male and female Nucella respond differently to green crabs. Photo Credit: Sarah Donelson.
Green crabs eat and scare Nucella, but the paper by Donelson et al. shows that male and female Nucella respond differently to green crabs. Photo Credit: Sarah Donelson.

The concept of sex-specific responses of prey to predators could be important to look at in species of conservation concern. Using a hypothetical example from another marine system, male and female oysters look the same from the outside, but our work suggests they might respond differently to predators. If males are “more scared” of predators than females, they may have less energy available for growth and reproduction. This may result in reduced sperm production, which could affect the number of oyster larvae produced and affect population sizes and the restoration of these critical species. This type of information would be useful for managers to know.

How did you get involved in ecology?

I grew up in New England and loved exploring tidepools when we went to the beach. This turned into a fascination with marine invertebrates, which are the species that I work on now! I worked in a stream ecology lab as an undergrad, and spent my summers doing research programs in marine science. I then decided to get my PhD in ecology after I finished undergrad.

What are you currently working on?

I’m now working with eastern oysters to look at how exposure to environmental stress early in life affects how oysters respond to stress later in life. The stressors I’m using are hypoxia (low dissolved oxygen) and temperature, which are two intensifying climate change stressors in coastal marine systems.

What’s your current position?

I’m currently a postdoctoral fellow at the Smithsonian Environmental Research Center in Maryland.

What project/article are you most proud of?

A lot of my other research looks at how stress experienced by one generation (e.g., parents) affects the responses of the next generation (e.g., offspring) to environmental stress. I recently co-lead a conceptual paper that explores how these “transgenerational effects” may make it difficult for species to acclimate and adapt to human-induced anthropogenic changes. Check it out here or in Trends in Ecology and Evolution!

What is the best thing about being an ecologist?

My job is to watch and quantify how animals behave and grow, so it’s always interesting! I also get to spend a lot of time on the ocean for work, which is fantastic.

Read “Sex-specific differences in the response of prey to predation risk” here!