Spiders turn to cannibalism when food sources are limited

A recent study from the University of Nebraska-Lincoln has revealed that a limited menu of prey in an ecosystem may lead to wolf spiders of multiple species dining on each other and even resorting to cannibalism. This phenomenon could contribute to the maintenance of an ecological balance among these arachnid predators.

Ecologists have long understood that predators with similar diets can coexist by effectively sharing the food sources within a community. This distribution of resources reduces competition and ensures enough prey for all predators involved. 

Maintaining equilibrium by eating your neighbors

However, the researchers found that when the diversity of mutual prey is limited, wolf spiders in Nebraska may turn to eating one another in order to maintain equilibrium.

A decline in prey variety often spells trouble for weaker predators, as it puts them into direct competition with their stronger counterparts. 

Stella Uiterwaal, who led the study while earning her doctorate at Nebraska, explains that predators that occasionally kill and eat their more competitive peers could benefit in a couple of ways that act as an “equalizing mechanism.” 

“Some of your diet is now coming from that other predator, instead of the shared prey that you’re competing for,” said Uiterwaal, now a postdoctoral researcher at Washington University in St. Louis. “And you’re also reducing the population size of that better predator, so you have fewer of them to compete with.”

How the study was done

The study was inspired by Uiterwaal’s observations at Cedar Point Biological Station, a lake-adjacent field site in southwest Nebraska. She and her colleagues, including doctoral adviser John DeLong, noticed that multiple local wolf spider species seemed to be defying an ecological principle by occupying more or less the same niche within the same habitat. 

“We noticed that there are so many different wolf spider species that all seem to be doing the same thing. And there’s this classic ecological idea that species can’t be doing the exact same thing. If that happens, they won’t be able to persist in the environment for very long,” said Uiterwaal.

To investigate, the researchers spent two summers collecting specimens of eight wolf spider species and their potential prey. In order to accurately count the prey, Uiterwaal used a novel method of placing hollow wooden boxes onto unsuspecting plots of earth and then vacuuming up every ground-bound and flying insect within. 

Many of the captured insects and arachnids were part of the wolf spider diet, which included flies, grasshoppers, crickets, butterflies, moths, aphids, and other spiders. Uiterwaal commented, “You name it, and they’ll eat it. We’ve even seen spiders out there eating toads.”

Using DNA to determine exactly what the spiders ate

To analyze the actual diets of 605 wolf spiders, the researchers employed sophisticated techniques, such as analyzing the DNA of the spiders’ digested food to identify unique barcodes corresponding to each type of prey consumed. 

Uiterwaal also developed a mathematical method that helped the team determine how much of each prey a spider had consumed. Contrary to the team’s expectations, the diet of any one wolf spider species mostly resembled that of the others, shedding light on the complex dynamics of these predators’ food web.

“All these spiders are essentially eating the same things – which I wasn’t expecting, because you do find these spiders in slightly different places, and they look different, and they have different behaviors,” Uiterwaal said. “You would expect that to reflect in their diet somehow. But it turns out that they overlap a lot.”

Overcoming an unexpected challenge

The research team faced a challenge in capturing spider-on-spider predation due to difficulties in distinguishing among the DNA of wolf spider species. To overcome this obstacle, the researchers analyzed the ratio of lighter versus heavier nitrogen atoms, or isotopes, in the tissue samples of every wolf spider they collected.

Heavier nitrogen atoms persist and accumulate through the food web, meaning that predators tend to contain more of those isotopes than their prey. Researchers can use this information to estimate an animal’s ranking in a local food web. If wolf spiders were regularly eating each other, isotope analysis would likely rank them higher on the food web than the barcode-based method did.

This is exactly what Stella Uiterwaal and her colleagues found, with the average ranking far exceeding their expectations. In many food webs, plants rank as a 1, plant-eating animals as a 2, and the predators of those herbivores as a 3, with the predators of predators coming in at a 4. 

Often, according to John DeLong, associate professor of biological sciences at Nebraska, that’s about as high as a terrestrial food web seems to stretch. However, the average ranking of a wolf spider species at Cedar Point Biological Station was nearly a 6, with one particular spider rating an 8.5.

Uiterwaal remarked that this is an especially lofty perch for a predator that “is not exactly what anyone would call the top of the food chain.” 

“It implies this level of complexity and predation that is probably really important in determining how the whole system works,” said DeLong. “Instead of thinking about these short food chains where everything is very vertical, it’s really this recursive thing where everybody’s eating everybody, kind of compounding on itself. The implication for how the food web is structured is really, really different than what we would have imagined going into this.”

Looking beyond the typical food chain

The team also analyzed how factors such as the sex and size of a predator, the characteristics of its environment, and the abundance and diversity of its prey might influence the likelihood of one wolf spider preying on another. 

Although prior lab-based experiments had suggested that all of these variables might play a role, the researchers found that only the variety of prey, or the lack thereof, was associated with wolf spiders attacking their own.

Uiterwaal suggested that the disparities between lab and wild environments might help explain the divergence, highlighting the complexity of food webs in natural settings and the need for further research to better understand these intricate dynamics.

“My guess is that foraging in the field is just so different compared to foraging in a Petri dish in a lab, where you don’t have all these other things to worry about,” she said. “You’re worried about getting eaten by other spiders or other predators (in the wild). Maybe you have parasites that you’re dealing with. You’re also trying to find mates, find areas that are the right temperature for you. You have all these other things that are going on and that may be drowning out these effects that we see in the lab, when we’re just playing with one specific variable.

“The fact that these expectations we have from the laboratory are not (necessarily) going to translate well into complex, real-life situations – that’s not just about spiders. That’s going to be true for any system.”

John DeLong gave due credit to his former advisee, whom he called a “true champion,” for carrying out such an ambitious study and revealing several distinctions that lab work might miss.

“It did pull together just really different kinds of data,” he said, “to paint a different kind of story than anybody had told before.”

The study is published in the Journal of Animal Ecology. Uiterwaal and DeLong authored the study with Nebraska’s Amber Squires and Bennett Grappone, Cornell University’s Brian Dillard, and the University of California, Merced’s Sora Kim and Ariadne Castaneda. The researchers received support from the National Science Foundation.

More about wolf spiders

Wolf spiders (family Lycosidae) are a diverse group of arachnids found throughout the world. They are named for their wolf-like hunting behavior, as they actively pursue and ambush their prey rather than relying on webs to catch food. Here is a summary of some key facts and features of wolf spiders:

Appearance

Wolf spiders are generally robust and hairy, with a body size ranging from 0.4 to 1.2 inches (1 to 3 centimeters), although some species can be larger. They typically have a brown or gray coloration, often with camouflage patterns that help them blend into their environment.

Eyes

One of the most distinctive features of wolf spiders is their eye arrangement. They have eight eyes, with the four largest arranged in a row on the top front of the head and the other four smaller eyes below. This arrangement provides them with excellent vision, which is crucial for their hunting strategy.

Habitat

Wolf spiders can be found in a wide range of habitats, including forests, grasslands, wetlands, deserts, and even human settlements. They are most commonly found in terrestrial environments and usually stay close to the ground, hiding under rocks, leaf litter, or other debris during the day.

Hunting and Diet

As active hunters, wolf spiders primarily feed on insects and other arthropods. They rely on their keen eyesight, speed, and agility to stalk, chase, and pounce on their prey. Some larger species may also consume small vertebrates, such as frogs and lizards.

Reproduction

Female wolf spiders exhibit unique maternal behavior compared to many other spider species. After mating, the female produces an egg sac, which she carries around on her spinnerets (silk-producing organs) at the rear of her abdomen. When the spiderlings hatch, they climb onto their mother’s back and stay there until they are ready to disperse and fend for themselves. This behavior provides the spiderlings with protection during their early stages of development.

Venom

Wolf spiders possess venom, which they use to subdue their prey. While their venom is generally not harmful to humans, a bite can cause mild pain, swelling, and itching. In rare cases, some individuals may experience more severe allergic reactions.

Ecological Importance

Wolf spiders play an important role in ecosystems as both predators and prey. They help control insect populations and, in turn, provide a food source for larger predators such as birds, reptiles, and small mammals.

Overall, wolf spiders are a fascinating group of arachnids that contribute to the ecological balance in their habitats. Their unique features and behaviors make them an interesting subject for further study and observation.

Other instances of cannibalism in the animal kingdom

Cannibalism, or the act of consuming members of one’s own species, is not uncommon in the animal kingdom. It occurs for various reasons, including scarcity of resources, competition, and reproductive strategies. Here are some examples of cannibalism observed in different animal species:

Praying mantis

Female praying mantises are known to cannibalize their male partners during or after mating. This behavior provides the female with nutrients that can improve her reproductive success and the survival of her offspring.

Black widow spiders

Female black widow spiders are notorious for consuming their male counterparts after mating. This cannibalistic behavior provides the female with additional nutrition for producing more eggs.

Sand tiger sharks

Intrauterine cannibalism, also known as embryophagy, is observed in sand tiger sharks. The developing embryos feed on their smaller siblings while still in the womb, ensuring that only the strongest and largest embryos survive to birth.

Lions

Lions, especially males, have been observed to kill and consume the cubs of rival males when they take over a pride. This behavior increases the new male’s reproductive success by eliminating the offspring of previous males and causing the females to become receptive to mating more quickly.

Chimpanzees

These primates have been observed to engage in cannibalism, particularly during intergroup conflicts or when resources are scarce. They may kill and eat infants from rival groups or, in some cases, even from their own group.

Spadefoot toad tadpoles

Spadefoot toad tadpoles exhibit a fascinating form of cannibalism. Some tadpoles develop into “cannibal morphs” with specialized jaw structures that allow them to eat their smaller, conspecific siblings. This behavior is triggered by environmental factors, such as high population density or limited resources, and can improve survival chances in challenging conditions.

Rodents

Some rodent species, like mice and rats, have been known to engage in cannibalism, particularly when resources are scarce or when they are under stress. This behavior can involve eating their own offspring or other adult individuals.

These instances of cannibalism across various species demonstrate that this behavior can serve various functions, from enhancing reproductive success to improving survival chances in resource-limited environments.

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