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At the first opportunity, we visited Adrian and his team and helped to design geometric experiments to test the theory. It was some years before the experiments were done, but the results were well worth the wait. In all cases, we found that nutrient balance is the strongest determinant of domestic pets’ food choice and eating behavior. We also found some fascinating differences between these species that relate to their evolutionary history.
Cats selected a diet with 52 percent of energy from protein, which is a typical value for wild predators, including the ancestors of domesticated cats and wolves. Dogs—all five breeds we studied—selected a diet of only 25 to 35 percent protein, much lower than the diets of the wolves from which dogs were domesticated and more like that of omnivores. This suggests that dogs have been altered in the process of domestication to a greater extent than have cats.
Why? Several years later, David witnessed the most likely reason.
He was at Tuanan research station, in the swamp forests of Borneo, studying wild orangutans. At the station were both cats and dogs. Tuanan is no holiday destination (as we will see in Chapter 9), and like everybody else based there, both species had their job to do. Cats were there to catch mice, which otherwise would threaten the precious food supplies, and dogs to alert the researchers when wild animals like leopards approached.
David noticed two important things about these working animals. The first, if he didn’t know better, might have seemed an injustice: only the dogs were fed. The cats were left to fend for themselves, which improved their performance in pest control.
The second is what the dogs were fed. The research station is extremely remote—to get there, the team drove for several hours over rough roads, then traveled for several more on a large motorized wooden canoe up a river through the forest, like a scene from Joseph Conrad’s Heart of Darkness. Space on the boat was at a premium. The human passengers were tightly packed, in physical contact with each other, and all remaining space was filled with valuable supplies and research equipment.
There were, therefore, no cans or bags of fancy-flavored dog meals aboard, and none made their way to the station. The dogs relied on a diet like that of their domesticated ancestors since before the invention of “dog foods,” even before the invention of agriculture: our table scraps.
And this is the likely reason why domestic cats and dogs prefer different macronutrient mixtures. Being small and often valued for their ability to control rodent populations, cats have continued to hunt and eat prey throughout their evolution and domestication. As larger animals, an important (and probably early) priority in the domestication of dogs was to breed out the hunting tendencies for which wolves are renowned, in the interests of human and livestock safety. Instead, dogs were forced to rely on our table scraps, which are much higher in carbs and fat than usual carnivore fare, and therefore became more similar in their nutrient selection to us omnivores, their owners.
Another result of the dietary switch of dogs is that they have developed an ability to digest starch more efficiently than other carnivores through evolving an increased number of genes for producing amylase (starch-digesting) enzymes. Over time, as we’ll see in Chapter 10, humans underwent a similar evolutionary change in response to agricultural production of starchy crops, such as grains. This shows how a shared environment, in this case the carb-rich world occasioned by farming, can produce similar changes in different species, a process called convergent evolution. Our dogs have become more human.
Still, even though they got the proportions of macronutrients in their diet spot on, some dog breeds we tested overshot the mark when it came to the amount they ate. In fact, they ingested far more calories than our calculations suggested they needed. It won’t come as a surprise to anyone who owns one that Labradors ate nearly twice what was necessary. A likely evolutionary reason is that their wolf ancestors are adapted to a “feast and famine” lifestyle in which they compete with others in the pack to gorge on occasional kills, then go long periods without eating. In the context of our story, though, it holds an important message: even gluttons must balance their nutrient intake.
It seemed we had learned something important from the impressive nutrient balancing we observed in predators, but it also got us thinking. Prevailing foraging theory had predicted that predators would have no need to balance their nutrient intake because animal tissues are already balanced to meet the needs of carnivores. But, why then do predators feed selectively to balance their nutrient intake? In this, they behaved much the same way we had observed for herbivores and omnivores.
Then we realized—the initial premise was wrong.
Foraging theory erred in assuming that the body composition of animals is constant. In fact, we realized, there’s a great deal of variation depending on diet, season, health, and many other factors. We saw an example above: by changing the diets of flies, we were able to create fat and thin prey for the spiders in David Mayntz’s experiment. Think also about the fat content of our own species—it can range from a mere 2 percent of body weight in Olympic athletes to over 50 percent among the obese, equivalent to the difference between dry lentils and creamy ranch dressing. In a single species!
Even more important, no single dietary composition would be optimal throughout a predator’s entire lifetime. This is because, like any animal, the nutrient needs of predators change depending on whether they are still growing or fully grown and reproducing; healthy or diseased; young or old; active or sedentary; and so forth. Therefore, like herbivores and omnivores, predators feed selectively to optimize their diets for specific circumstances, and the wide variety of prey available provides plenty of opportunity to do so.
This was illustrated by another study that we did with David Mayntz. It involved the same species of ground beetle from the initial experiment together with spiders, but with a twist. This time, David went out into the field and collected beetles immediately after they emerged from a long hibernation through the cold Danish winter, then brought them into the lab to study.
During hibernation, they eat nothing, living off the fat stores they have accumulated in advance. We knew, therefore, that they would be very lean when collected and urgently in need of some fattening up. Our question, then, was whether this influenced their food choices.
At first, they selected a fat-rich diet. Then, as their own fat stores grew, they gradually toned down that nutrient’s intake and ate more protein. This, too, was no coincidence: at that stage they were preparing to reproduce, which in insects is a protein-intensive process.
The message, of course, is that there is no such thing as a single balanced diet for these beetles—their needs change across the life cycle. Nutrient selection even changes with an animal’s activity level. Another student working with us, Louise Firth, made locusts fly for differing periods and then found that the ones who had flown the longest selected a diet higher in carbohydrate than protein, since carbs provide the fuel used in flight.
For (almost) all animals, then, predators included, feeding is a process of a wobbly gun barrel aimed at a moving target. Specialized stabilizing mechanisms—in the form of interacting appetites—are needed to collaborate if there is to be any hope of success. Exceptions are likely to be few, restricted to very special cases.
One exception is the food designed specifically to meet all the nutrient needs of an animal: mammalian milk. Particularly fascinating is the Australian tammar wallaby. In this species, the young live inside a pouch on the mother’s belly and therefore have no opportunity to eat anything other than milk. But this is a single food only in name: the wallaby mother’s milk undergoes complex changes in composition over time, modifying the mixes of nutrients needed for the specific stage of the baby’s development. For example, differing mixes of amino acids are produced to enable the growth of the brain, lungs, nails, and fur. More than that, females may simultaneously be carrying around two young of different ages. When that happens, each has a dedicated nipple producing the cocktail of nutrients it needs for its specific age. We predict, though, that when the young leave their privileged position in the pouch to fend for themselves, feeding mechanisms would be no different from other species. They, too, will have to develop nutrient-specific appetites.
We had answered our original question: nutrient balancing is widespread across species and not an exception. Our experiments had shown that it occurs in herbivores, omnivores, and carnivores, both domesticated and undomesticated, and we had rethought foraging theory to explain why.
But as biologists with a habit of watching animals in the wild, we realized that only in certain circumstances was nature obliging enough to provide the abundance and diversity of foods that could always ensure a nutritionally balanced diet. Often, in the real world, it is impossible for an animal to eat the correct amount of all nutrients. Such imbalance was so common, we suspected, that animals would have to have a Plan B—meaning that their appetite systems would need a way to respond when getting the desired nutrients was impossible. They would need a response that could compromise and help them balance eating too much of one thing against too little of others.
This was exactly the question we had set about to answer in the locust experiment: What is the nature of Plan B for a locust? The answer was that they ultimately prioritized protein over other nutrients and would even extend their development time and become obese if those were what it took to get to a target intake of protein. What, we wondered, would Plan B be for humans? As far as we knew, the question had never been asked, let alone answered. We decided to find out, and what happened next set the direction for the rest of our careers.
CHAPTER 5 AT A GLANCE
1 Even the most unlikely animal species—from cockroaches to cats—can use their multiple appetites to mix a balanced diet, like nutrient-seeking missiles.
2 However, as we saw in Chapter 3, these appetites compete when the diet is imbalanced—and in locusts, protein wins the competition.
3 What about humans?
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