Spiders choose their most nutritious next meal

We’ve all been stood in a supermarket aisle peering at the packaging and wondering whether the food contained within includes the nutrients needed to sustain us. Well, it turns out that humans might not be alone in this. Some species of spider have been seen indulging in what has been described as ‘nutrient specific foraging’ - which means they know what kind of meal to go after in order to fulfil their dietary needs. Newcastle University’s Jordan Cuff explains more…

Jordan - Animals much like us, have to balance their nutrient intake. So they have to eat a nice balance of carbohydrates, lipids, and proteins to ensure that they live nice, long, happy lives. So there's a lot of evidence for this phenomenon. Invertebrates and even in invertebrates as well, but mostly from, from lab studies. So my aim with this work was to try and show that nutrients are important in these foraging choices, particularly in spiders in the field as well.

Will - Were the nutrients differing depending on the age or sex of the spiders involved?

Jordan - Yeah, absolutely. So one of the main things we wanted to look at was how the different prey and the nutrients within those prey differed between different groups of spiders. So we looked at different spider species and we found that some of them, the money spiders particularly, which sit on these beautiful sheet webs and just wait for prey to come to them, they ate particularly carbohydrate rich prey compared to the wolf spiders, which are much more active and run around on the ground. And they tended to eat much more lipid and protein rich prey. And then when we compare between life stages, we start to see some really interesting patterns. So the adult spiders are eating more carbohydrate-rich prey. So this is likely because they're a lot more mobile, running around trying to find a mate and doing quite a bit of hunting as well. Whereas the juvenile spiders tended to prefer more protein-rich prey, which aligns with the fact that they're developing and they're still growing. And similarly, we noticed differences between sexes as well. So male spiders, similar to a lot of the adults, preferred that that carbohydrate rich prey, perhaps again because they're seeking the mates, whereas the female spiders that tend to wait for the males to come to them, but instead have to invest more resources in developing little eggs to form the next generation of spider lings. They were investing more of their time in finding protein and lipid-rich prey.

Will - I mean, that all makes perfect sense in terms of why they would want to eat those sorts of things. But I struggle to understand my necessary nutrient intake when food is spelled out for me on the back of a packet. How on Earth do the spiders know what to go for?

Jordan - Sadly their prey doesn't come in convenient packaging. So instead they have to try and perceive the nutrients within the prey. And there's not much known about how they might do that yet. It'll vary quite markedly between different predators and different animals. Generally a lot of animals depend quite heavily on a sense of smell, for example. And it might be that some nutrients give off a particular scent. And some of our preliminary analyses in that direction seem to indicate that that's likely to be the case. But spiders have a particularly bad sense of smell most of the time. And instead they rely on things like vibrational cues to identify their prey. So in that case, there's likely to be an aspect of learning. So much like some of the recent work on bumblebee showing that they can learn to problem solve. I think these spiders are likely to be learning which nutrients are associated with which prey groups, and that's likely to be an imperfect system, but at least one way to start to identify which nutrients they need next.

Will - Could we therefore introduce spiders of a certain age or sex or species into say a field to take care of certain crop pests?

Jordan - Essentially the main rationale behind this research was to identify ways that we could manipulate this nutrient specific foraging behaviour to get spiders to eat more crop pests. And what we found is many of the pests are really rich in lipids and carbohydrates, particularly because they're sucking all the juices out of these crop plants. But essentially with that richness in lipids, it's likely to be really beneficial to the spiders because as you go up these trophic levels, there's evidence that shows that there's less and less lipid. And carbohydrates as well are mostly prominent in plants, and spiders don't tend to have much access to plants as a resource. So again, carbohydrates are likely to be a really important part of their diet. So we could, using research like this, begin to identify spiders that preferentially target prey rich in those lipids and carbohydrates and promote those spiders by managing our farms and our habitats in a particular way to try and get more of those spiders eating crop pests.