We all know the stories of guppies that mysteriously disappeared from the fishbowl. But anyone who thinks that such mysterious disappearances also occur frequently in the wild, is wrong, new research shows.

Guppies in the wild are extremely unlikely to eat their congeners, scientists have to conclude in the paper Ecology and Evolution† And that’s a bit surprising. Not only for people who have kept guppies for years and who regularly witness the small fish eating their conspecifics, but also for the researchers themselves. “Cannibalism among these fish is a problem that biologists regularly deal with in the lab, so it was believed to be common in nature,” said study researcher Brian Langerhans. “But we are now showing that that is really not the case.”

Eaten

Anyone who has ever had guppies has probably had to conclude that on a day there were far fewer specimens in the bowl than in the days before. It is very common that guppies – but also, for example, the mosquito fish frequently kept in fishbowls and aquariums – eat each other in captivity. And usually the young are the victims. It happens so often that researchers who work with guppies and mosquito fish are forced to take measures, says researcher Rüdiger Riesch. “It is so common for mosquitofish and guppies to engage in cannibalism that research labs have protocols aimed at separating offspring from the larger fish as quickly as possible.” It got Riesch thinking. Because: how would that go in the wild?

New research

That question led to a whole new study showing that captive guppies unfairly portray their wild counterparts in a bad light. Because: in the wild there are hardly any guppies and mosquito fish with cannibalistic traits. “If you look at the diet of fish in the wild, there’s very little evidence for that,” Riesch said.

Findings

He reached that conclusion after working with colleagues to scrutinize the diet of nearly 12,000 wild fish. Some fish were dissected for this, others underwent their X-ray scan to clarify what had been on their menu. The findings are striking; In their extensive study, the researchers found 35 cases of cannibalism among just three species of mosquitofish. This made cannibalism among wild fish a rarity.

It’s all about…food

What struck me was that the few populations in which cannibalism was practiced had one thing in common. Within all these populations there was fierce competition for food. These populations had few enemies, so the population density was very high; too big actually for the amount of food available.

Experiment

To make sure that population density and resulting food scarcity was the driving force behind the cannibalism, the researchers set up an experiment. They placed 720 fish in large containers in which the natural habitat of the fish was simulated in detail, but in which the researchers were able to determine certain factors at the same time, such as population density, the presence of predators and food. The researchers created different conditions in the containers; sometimes the population density was high, sometimes not, sometimes there was enough food available, sometimes not. And each time, they watched that scenario play out over a week. The experiment also hints that food scarcity encourages fish to cannibalism. The absence of predators also indirectly plays a role in this. “Without predators, the population density can increase rapidly, reducing the amount of food,” Langerhans said.

Alternative explanation

Other explanations for the cannibalism, such as the frequency with which larger fish encounter smaller conspecifics, can be ruled out by the researchers based on their experiments. “Cannibalism doesn’t happen more often when big fish very often meet smaller fish,” Langerhans says. Nor was it simply body size that could predict which individuals engaged in cannibalism. Females – who are larger than males – cannibalized much more often than males, but that seems to be related more to the fact that they need more energy to give birth than to their size.”

Implications for the fishbowl?
That population density and resulting food scarcity can encourage fish cannibalism in the wild naturally raises the question of what that tells us about our aquariums. Are we overfilling them? Or are we not feeding our guppies enough? However, such conclusions cannot be drawn directly on the basis of this study. What the study does show is that wild fish are less likely to turn to cannibalism. Only if there is really intense competition for food is there a chance that they will eat each other. “Cannibalism is rare among wild mosquitofish and guppies,” the researchers reiterate in their statement research article† “Probably partly because hunting conspecifics costs a lot of energy (because those conspecifics are much larger and show more evasive maneuvers than the typical prey of these fish).” It is therefore a trade-off between costs and benefits. And in the wild, the costs are simply very high. “The hunt for conspecifics is only worthwhile if the competition for food is very intense.” But in captivity it’s different. “Although quite rare in the wild, cannibalism in captivity may be much more common because it takes much less energy there to grab smaller conspecifics in a smaller and defined space.” In short: cannibalism pays off much faster in captivity than in the wild; it doesn’t take intense competition for food to make cannibalism worthwhile.

The conclusion that guppies and mosquito fish in the wild hardly practice cannibalism is not just a funny fact that can score you some points at the coffee machine. The findings of this research may also have far-reaching implications for research in evolutionary biology in which mosquito fish are frequently used as model animals. “These fish are used as models for evolutionary research – to find out how certain traits evolve,” explains Langerhans. “Now that we know that cannibalism is not a common trait in the wild, we also know that the unnaturally high rate of cannibalism in a laboratory setting may alter the traits of these fish (which they acquire through evolution) in such a way that the research results may affected, especially in behavioral evolution studies.” Some caution about conclusions drawn from captive mosquito fishes may also be warranted, the researchers say in their research paper. Certainly if no or hardly any measures are taken to reduce the risk of cannibalism.