Have you ever thought about how your preference for food changes with the weather? In winter, we often fancy something warm and highly calorific, like roasts or cheese-loaded pasta, while we might prefer something fresh and crunchy, like a salad, in summer. This decision is not just influenced by our desire to fit into our swimwear. On days when the sun is burning and the air is dry, it is important to keep our body hydrated, and this is often reflected in our decisions of what to eat. So, why should this only be the case for humans?
Israel Leinbach, Kevin McCluney and John Sabo from Arizona State University, USA, wondered whether the feeding decisions of predators might also vary in response to water availability and how these changes might influence the lives of their prey. The team went to the area surrounding the San Pedro River in Southeast Arizona during the dry season and conducted feeding experiments in which they varied the amount of water available to dining spiders. After capturing large wolf spiders (Hogna antelucana), the team offered the arachnids a choice of meals: small spiders (Pardosa sp.) or crickets. The tiny spiders are low in water content, but rich in energy, whereas the crickets provide a low-energy snack that is relatively moist. The scientists then supplemented the diets of some of the spiders with additional water, and left other spiders to cope with the water that is naturally available, which is usually quite low during the dry season.
Not surprisingly, the researchers found that the spiders' decision on whether to feed on moist prey with a high water content or a delicacy with high energy content depended on the amount of water that was available to them. When their water supply was supplemented, the spider predators fed primarily on smaller arachnids and less on crickets. Moreover, the availability of additional water not only reduced the wolf spider's appetite for crickets, but the crickets' survival rates were even higher than when the wolf spiders were completely absent. While the researchers are not entirely sure why the crickets' survival chances are better when well-hydrated wolf spiders are around, they assume that the wolf spiders’ appetite for smaller spiders reduces the overall spider population, which is a good thing for cricket wellbeing. However, the team also found that male crickets were more likely to be preserved by water supplementation. Looking into the differences between the water content of male and female crickets, this makes sense as Leinbach and colleagues showed that male crickets have a higher water content than females.
Importantly, the decisions made by thirsty spiders influence the surrounding animal and plant communities. Animal food webs can be described as a cascade; large predators at the top of the cascade feed on smaller predators or plant eaters beneath, while plants are right at the bottom. If wolf spiders feed more on plant-eating crickets, the plants that are beneath them in the food chain thrive. However, if wolf spiders are not thirsty and fill up on smaller spiders instead of crickets, this leads to an increase in the numbers of plant eaters and, therefore, fewer plants. Hence, a thirsty wolf spider can have far-reaching implications for their neighbours.