By Jessica Daly, SRC intern
Sometimes, predators have the ability to choose what size prey to consume when feeding, but little is known about how this decision is made. Several previous research experiments have examined the relationship between prey size and “predator gape size,” or how long it takes to chase, capture, and consume the food. It has been hypothesized that predators may select their prey in part because of possible effects of prey size on digestion and metabolism, but evidence is required to support this.
Metabolism is the rate at which food is broken down into energy. Aerobic scope (AS) is the difference between an animal’s resting and maximum aerobic metabolism, and tells how well the animal can take in oxygen and provide energy to its cells. The specific dynamic action (SDA) measures how much energy is needed to digest and break down food, and is part of the AS. While eating bigger prey will provide more energy to a predator, bigger meals take more energy to metabolize and absorb than smaller meals. If the animal uses a lot of energy breaking down food, that means that it will have less energy to do things like fight or escape from a predator. Because of this, it was hypothesized that it might actually benefit the predator to intentionally choose smaller prey.This idea was tested with the predator fish barramundi, which usually eats large amounts of food at once. Twenty-four juvenile fish were fed different amounts of food, somewhere between 0.6 and 3.4% of their body mass, for two minutes. They were then transferred to individual respirometry chambers for 42 hours. Information gathered from the chambers allowed the scientists to calculate the metabolic rate and SDA for each fish. They were then transferred to a tank for a three-minute “chase” exercise, to simulate a predator attack. After the three minutes, the fish were returned to the respirometers to calculate the aerobic metabolic rate, SDA, and SA after exercise. The growth rates of the fish were also calculated over a period of seven weeks. The study found that the fish who were fed larger meals had less excess energy, and higher growth rates came at the cost of a decreased AS. This means that after eating a larger meal, the fish spent so much energy metabolizing it that there was less energy left over afterwards. Less energy stores means that a fish can’t swim as fast or as long, and is more likely to be caught and eaten by a predator. The evidence from this experiment suggests that the increased risk of predation outweighs the benefits of the larger size that come with eating large meals. It would likely be beneficial for the barramundi to choose its prey size based on its environment. For example, if the fish were in a relatively safe and isolated area, it might be better to consume more food, whereas in a high-traffic area with high predator abundance, it would be more beneficial to eat less.
Norin T, Clark TD. 2017. Fish face a trade-off between ‘eating big’ for growth efficiency and ‘eating small’ to retain aerobic capacity. Biol. Lett. 13: 20170298. http://dx.doi.org/10.1098/rsbl.2017.0298