Illness is a powerful force in evolution, pushing animals to evolve a suite of behaviors - "sickness behaviors" - to help them get over those illnesses more efficiently. For a long time, doctors thought that they were just side effects of being sick, but in recent decades researchers have increasingly come to recognize the valuable, adaptive nature of sickness behaviors.
In the late 1980s, veterinary physiologist Benjamin L. Hart proposed that sickness behaviors are an "organized, evolved behavioral strategy to facilitate the role of fever in combating viral and bacterial infections." After all, animals have been evolving for millions of years alongside pathogens and parasites, so it makes sense that animals (humans included) would have evolved not just immunologic defensive strategies for fighting off illness, but behavioral defenses as well, which "might serve as a first line of defense before the…immunologic systems are activated."
Lethargy and sleepiness, for example, serve to minimize the sick animal's activity, allowing the organism to devote more energy to the immune system. And a reduced appetite could be a clever way of depriving the infectious bacteria of the nutrients they need to survive. These and other sickness behaviors have been observed throughout the animal kingdom: in insects, amphibians, reptiles, birds, and mammals.
Disentangling Sickness and Sickness Behavior
After Hart's initial proposal, other researchers began to take seriously the idea that sickness behaviors were adaptive rather than an unfortunate side effect of illness. And they could study it in the lab by de-coupling actual illness from the "feeling" of being ill. That's because sickness behaviors – lethargy, sleepiness, reduced appetite, and more – aren't the direct result of infection, but byproducts. They're driven by the hosts rather than by the pathogens. They're activated by the release of proteins called cytokines, like interleukin-1 (IL-1) and tumor necrosis factor alfa (TNFa). It turns out that if you administer a dose of those cytokines to a healthy animal (like a rat), it induces sickness behaviors even though the animal won't actually be sick. The animal simply acts as if it was sick.
That, in turn, has allowed researchers to determine that, sometimes, it's actually in the animals' best interest to pretend as if it's not sick at all. An obvious situation in which it's best to appear healthy is when there's a predator on the prowl. It's easiest for predators to take down the weakest individuals, so if there's a hungry lion and you're a sick zebra, it's understandably in your best interest to appear as energetic as possible.
But there are other, more subtle reasons to pretend as if you're feeling fine, and they're entirely social. University of Zurich evolutionary biologist Patricia C. Lopes explains in a paper published this week in the journal Proceedings of the Royal Society B.
Social Context Matters
Strictly speaking, the main motivations driving animal behavior are survival and reproduction, and for some species, parental care (which is, effectively, an extension of reproduction). But there are times, for sick animals, when the drive to survive comes into conflict with the drive to reproduce, such as during the breeding season.
How do potential mates impact sickness behaviors? Despite their overall reduced activity level, male rats retain elevated sexual behaviors, while male zebra finches' activity levels resume to normal levels. Similarly, in normal cases, female mice have reduced aggression when they're sick, even in the presence of an intruder. But when their newborn offspring are around, their maternal aggression remains high.
For some species, the ability to hold a territory has a direct relationship with their ability to breed. That's either because those who hold territories have better access to resources, which makes them more attractive as mates, or because those who hold territories dominate the social hierarchy, which likewise makes them more attractive as mates.
Song sparrows are territorial, but sick song sparrows tend to defend those territories less actively. The chance of dying in a fight is probably worth giving up a territory today, if it means that you can be strong enough to try to steal back that territory after you've recovered. But during breeding season, sick male sparrows defend their territories as if they weren't sick at all. In that case, the risk of missing out on the chance to breed and sire offspring outweighs the risk of dying while defending your territory.
Even more interesting, the breeding season's influence on sickness behaviors changes depending on how long the breeding season lasts. Song sparrow populations at lower latitudes (nearer to the equator) have a longer breeding season than populations who live at higher latitudes. That means that males form lower latitudes have more time to successfully breed than those from higher latitudes. If the pattern holds, then that, in turn, means that sick males from lower latitudes can invest more energy in recovering from illness compared to those from higher latitudes. Indeed, that's what researchers found. Wild birds from lower latitude populations had both more intense and longer lasting sickness behaviors after being injected with cytokines than those from a higher latitude population. The effect even held up when males from either population were briefly kept in a captive environment.
Rhesus macaques, which are also highly territorial, show a similar pattern. When it comes to breeding time, sickness behaviors are reduced in favor of mating behaviors.
Hidden Illness in a Zoonotic World
That the social context in which an animal finds itself modulates its behavioral response to illness or infection reveals a fascinating aspect of evolutionary biology, but it also has important implications for veterinary medicine and global health.
From an animal welfare perspective, understanding that sick animals may sometimes appear perfectly healthy is important knowledge for pet owners, for zookeepers, and for those who work with live animals in research laboratories. In part, zookeepers and curators have long known that animals are good at hiding their illnesses. Part of the reason that keepers maintain such meticulous records of the daily behaviors of the animals in their care is so that they can identify even the smallest aberrations, which might reveal hidden illnesses. As knowledge improves about the specific social factors that modulate sickness behaviors for individual species, veterinarians and others involved in animal care will be able to more effectively care for their patients, both wild and domestic.
In addition, more and more infections are capable of entering the human population from non-human animals. Or, at least, we're becoming better able to recognizing the cross-species nature of those diseases, which are called zoonoses. According a 2001 assessment, of 1415 organisms (viruses, bacteria, fungi, protozoans, and more) known to infect humans, 61% were zoonotic, meaning that they can be transferred between humans and other animals. "Early detection of diseased animals," writes Lopes, "should help prevent disease spread and prove relevant in a world where infectious diseases cause a major burden in terms of both lives lost and economic damage… The increased connectivity of animal populations (including humans) has led to an unprecedented potential for disease pandemics."
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