Growing up, I had a friend who was extremely outgoing, had boundless energy, and liked to take more risks than most other people I knew. I remember asking myself, “Where does all that energy come from? What gives this person such a confident personality?” Bold, confident personalities in people can make them seem like ‘go-getters,’ and it turns out that many animals have ‘personalities’ as well. For example, animals can have more or less ‘bold’ behavior in relation to other animals of the same species. But what does this mean? Are these ‘bold’ animals more entertaining at parties? More likely to speak their minds perhaps? The way scientists that study animal behavior think about ‘boldness’ in wild animals is to ask how likely they are to take risks. It turns out there are some interesting biological reasons for differences in boldness between individual animals.
Animals can gain a lot from risky behavior. Bolder animals may be able to access more food by venturing into dangerous areas, or they may be able to find an excellent new territory by crossing through a risky area that other animals aren’t willing to travel through. But, of course, since these areas are risky there is always real danger present. Animals can fall victim to predators or environmental dangers if they are too bold.
So, being bold can get animals more energy, but there is a chance that they could face injury or death. Is there something about the biology of bold individuals that causes them to take more risks? One of the biological factors that scientists think may explain differences in boldness is metabolic rate, or how quickly animals are using energy. Scientists can measure metabolic rate by putting an animal inside a chamber and measuring how much oxygen it is using while it rests. Just like differences in behavior, there can be some big differences in metabolic rate between animals, even within the same species! But, the connection between boldness and metabolic rate can get complicated very quickly. For instance, environmental conditions can change metabolic rate. If there are predators around, some animals will lower their metabolic rate. In other scenarios, their metabolic rate will rise if there are pollutants in the environment.
I study whether these environmental factors can change behavior by way of affecting metabolic rate in Pacific tree frog (Pseudacris regilla) tadpoles. Essentially, this means I look at how closely their behavior is connected to their metabolic rate. The idea is that animals that are more energetic – or have a higher metabolic rate – are more likely to take risks, especially if those risks let get them more energy! Unfortunately, many animals are now experiencing multiple types of environmental stress due to climate change, which may affect the connection between metabolism and behavior. By studying how environmental stress changes the biology and behavior of animals, we can make better predictions about how animals will deal with rapidly changing environments. This is important in amphibians because they are especially sensitive to environmental change, and their populations have been declining for decades as a result.
To get this information, we first have to measure their metabolic rate using a process called intermittent flow respirometry, where we put them inside a little jar and measure how quickly they use up oxygen, pumping in fresh water as they need it. Measuring oxygen lets us know roughly how much energy they are using. This is because, like other animals, tadpoles need oxygen to make ATP (or biological energy) efficiently. The respirometry takes about 90 minutes, and afterwards we quickly get the weight of each tadpole. This step is important since a bigger animal will tend to need more oxygen, and we want to take this into account for our results so we aren’t just measuring how big each tadpole is!
After we get respirometry and weight data for each tadpole, we wait a day to let them get a little rest and relaxation, then measure how bold they are. We do this by first putting them into an ‘arena,’ or an experimental setup that is specialized for the behavioral test we use. In this case, we have a small container of fresh water that we transfer each tadpole into. The test is pretty simple: we gently tap them on the tail with a small paintbrush! This might seem a bit silly, but the tadpoles feel this tap and think that it could be a predator. When that happens, they swim away as fast as possible (a burst swim), and then freeze. The part we measure here is how long they burst swim for and how long they freeze for before deciding its safe enough to start moving around again. Some tadpoles start swimming again after only a few seconds (the ‘bolder’ tadpoles), but others can stay still for so long that we have to limit the time we wait! After this first round of experiments, we divide the tadpoles into groups and let them hang out in water that either has some salt in it, predator scent, both salt and predator scent, or normal water for a couple of weeks before measuring their metabolic rate and behavior again. This lets us see if environmental stressors can change their behavior and how quickly they use energy.
In the wild, the time a tadpole freezes can have big effects on growth and survival. Not everything that touches a tadpole’s tail will be a predator; if a tadpole freezes for half an hour every time a leaf drifts past it, then it may not spend enough time foraging and get enough food. On the other hand, if the tap was an actual threat and the tadpole only freezes for a few seconds, then that tadpole might turn into someone else’s lunch. This is why looking at how salt pollution, the presence of a predator, or both will affect metabolism and behavior could be important information for helping our amphibian buddies deal with changing environments.
Richard Szeligowski is a Ph.D. student in the Ecology Graduate Group at UC Davis in Dr. Andy Sih’s lab. He is studying the relationship between environmental change, physiology, and behavior in a variety of different critters.
[Edited by Cassidy Cooper]