The Early Mouse Gets the Cheese
Maximising survival by shifting the daily timing of activity (2019) van der Vinne et al., Ecology Letters, https://doi.org/10.1111/ele.13404
All animals need to eat food to survive and maintain their energy balance, but unlike us they can’t just order a pizza and have the food brought to them. They must always forage for food themselves, and every time that they do they expose themselves to predators. Small mammals like mice balance this trade-off by foraging for food at night, when their risk of predation is lowest.
One interesting strategy that mice can employ is to switch their foraging from the nighttime to the day, if they cannot get enough resources during the night or if their nighttime predation risk increases. The authors of today’s paper wanted to develop a model to predict under what conditions these temporal switches would occur, a model which they then tested with mice in the field.
What They Did
For the modeling half of this study the authors quantified the energy expenditure during different time periods. These differ not only due to different activity rates at different times, but also because the nighttime tends to be cooler, and small mammals will use more energy to stay warm than they would during the day. The authors modeled these energy expenditure scenarios across all combinations of foraging yield (how much food the mice would get) and predation risk (how much danger the mice were in while foraging).
For the second, practical half of the experiment, the authors placed tags on mice that would alert a sensor whenever the mice foraged for food. To reduce perceived predation risk an opaque covering was placed in the field, providing a safe path from the mice’s nest to the food. The experiment started with the mice receiving a reduced amount of food that would put them at an energy deficit, such that they would need to forage more than normal to make up the difference and get back to their energy balance. After a period of time, the researchers increased the amount of food available to the mice by either removing half of the mice in the enclosure or giving them twice the normal amount of food. This was done in order to test the prediction that an increased amount of food could result in a shift in the timing of mice activity now that the mice were no longer at an energy deficit.
Did You Know: Periods of Activity
There are a variety of reasons why the mice in this study tend to be active at night during normal situations, and the authors attribute most of it to the reduced risk of predation. Many animals maintain stable periods of activity and, like the mice in this study, escaping predators seems to be the main reason. Small rodents tend to be nocturnal, while we are diurnal creatures (active during the day). Other animals are what’s known as crepuscular, meaning that they are active at dawn and dusk. One interesting thing is that an animal being active at night to escape predators only provides a new niche for a predator to fill: that of a nocturnal predator.
What They Found
For the model, the authors found that being active during the day was always less energetically costly, and as such the model mice were less active during the daytime, no matter the amount of resources available for forage. Not surprisingly, daytime foraging was favored when nighttime predation risk was higher than daytime risk, and nightime foraging was favored when daytime predation risk was much higher. Interestingly, in situations when predation risk was moderately higher during the day, daytime foraging was still favored due to the reduced activity needed to maintain an energetic balance.
The models predicted that mice would maximize their survival by switching from nocturnal activity to daytime activity if their predation risk was sufficiently reduced and more food was available, and that’s exactly what happened! When provided with cover (reducing predation risk) the mice foraged more during the daytime when they needed the energy, but getting a bigger helping of food stopped this shift and the mice stayed active at night.
Although they included both male and female mice in the field experiment, the female mice were tested from August to November (Autumn) and the males were tested from March to June (Spring). The seasons are different because the amount of sunlight changes throughout the year, and this has cascading effects throughout ecosystems. Not only are the temperatures different, but the amount of light also varies, which could have an impact on activity rates. The female mice would have gone from more light to less light in November, while the male mice started with less light and ended with more light in June. Despite these differences, both the average temperature and amount of light available were about the same for the male and female mice, and the authors are confident that any difference between the two testing periods would have been minimal.
This study confirms the importance of how animals time their activity for maximizing survival by demonstrating this shift empirically. The ability to be flexible and change the timing of activity under different environmental conditions is a strategy that will be useful in the future where rapid environmental change can result in high variability in predation risk and energy limits.