When bees are sick, their reflexes suffer. Presented with a drop of liquid, the infirm insects don’t extend their proboscises to inspect it as quickly as they do when they’re not ill. Similarly, sick rats take longer to navigate an underwater maze than healthy ones, songbirds don’t learn as many tunes, and crows are less inclined to solve puzzles when they’re under the weather.
It’s a trend that holds true throughout the animal kingdom: Across taxa, wild animals seem to lose cognitive capacity when suffering from infections and in the aftermaths of some illness, according to a new review study published in the journal Trends in Ecology & Evolution.
Scientists took a closer look at existing research in the field of animal cognition and found that disease could be playing a larger role in animals’ ability to navigate the world than previously thought, and in lots of different ways. They also found large gaps in our knowledge of how getting sick impairs animals’ functioning.
“There’s actually very little information out there about how disease affects cognition in wild animals. The goal of our review was to bring all of these studies together and look for patterns,” said lead researcher, Andrea Townsend, in an email to Gizmodo. Townsend is a behavioral ecologist at Hamilton College in Clinton, New York. “We’re seeing an accelerated emergence of all of these infectious diseases, and yet we know very little about how disease might affect cognition and the implications of this for wild animals,” she added.
The impetus for the review was two-fold. Townsend and her team had recently wrapped up a study of American crows, which found that sick birds were seemingly worse at problem-solving. She had wanted to compare her research to other findings out there, but she found that similar studies were scant. Then there was, of course, covid-19.
Townsend pointed out that the issue of disease and cognition is very timely for humans amid the ongoing pandemic. “I think almost all of the co-authors or our friends or families got Covid at some point in the writing process. So we became personally interested in questions like ‘what is brain fog?’, ‘why do we get it?’, ‘how long will it last?’, ‘can other infections cause it?’, [and] ‘do other species get brain fog when they are sick?’” she said. “We learned a lot about the answers to these questions while writing the review.”
Current pandemic aside, understanding how disease hurts animals’ cognition matters more and more as climate change and other human environmental impacts worsen. We’re simultaneously driving up both the risk of certain infections and what’s at stake for species struggling to adapt. As the review says, “if disease and other factors associated with habitat degradation combine to suppress cognitive abilities of wild animals, their capacity for appropriate behavioral responses to environmental change could decline.”
Just about every multicellular animal, even down to the smallest invertebrate, takes in information about its environment, stores that info, and acts based on it. In other words: “all animals are cognitive,” explained Alex Thornton, a biologist studying cognitive evolution at University of Exeter in the U.K. who was uninvolved in the new research, in a video call with Gizmodo.
He brought up the example of nematodes, which only have around 300 neurons but are “capable of habituation and associative learning.” Even sponges, which have no formal nervous system, still seem to engage with their surroundings, regulating their filter feeding and avoiding infection via cellular communication.
One of the primary things Townsend and her co-researchers noted in their research was that there are lots of different ways disease can impair cognition in both humans and wild animals.
Some diseases directly infect the nervous system and cause damage. For instance, meningitis in people, West Nile virus in birds, or Ebola in both humans and non-human primates. There’s even a subcategory of pathogens that specifically target and change their host’s behavior for the benefit of boosting spread, like the nightmare that is Toxoplasmosis.
But then, there are also the numerous, indirect ways disease can stymie smarts. If a stomach bug causes diarrhea, a side effect is that the sick host—animal or human—doesn’t absorb as much nutrition from food. Fewer nutrients means less energy for the brain and body to run on. Malnutrition has short-term impacts but also long-term consequences across species. One study of wasps found that just a single day of eating sub-par food led to a lifetime of worse memory.
A host’s own immune system can further cause cognitive problems by triggering inflammation in the nervous system. Or, through the series of cascading behavioral effects that prompt the desire for rest above all else. Those sick crows that Townsend studied were less likely to successfully pull a string for a food reward than their healthy counterparts. If they had all of their usual brain capacity, the ill birds would theoretically understand that completing a simple task to get food would be a net gain. Instead, the sick crows often didn’t even try—disease seemed to make them unmotivated.
As climate change and other human-caused environmental shifts continue on, the question of how animals are adapting and adjusting to the world becomes more important. “We know that humans are causing substantial changes to the environment, and cognition is what allows animals to track and respond to change,” said Thornton. “And so, if parasites and pathogens are affecting animals ability to do that, then that could potentially have major consequences for animal populations.”
Plus, climate change and habitat destruction are bringing new diseases into new places. Humans and animals will face pathogens they’ve never encountered before. And, as with covid-19, the lack of initial immunity will make those illnesses particularly dangerous.
Townsend further noted that stressed-out animals are more likely to get sick. “So, here you might have a snowball effect where animals in stressed environments are more likely to get sick and their cognitive abilities are impaired. Then, they are less able to deal with these stressful, changing environments because of their impaired cognitive abilities. It could increase the costs of environmental change for some wild animals.”
But to understand that true cost, more research is needed. Townsend sees the review study as more of a starting point than a definitive statement, and she has lots of lingering questions she’d like to explore. For example: How does disease-impaired-cognition impact animals’ ability to reproduce? How are entire communities of animals affected? What are the long-term consequences of lost brain power? And are animals evolving in response?
One small silver lining though, is that covid-19 has brought these questions to the forefront. “The fact that we’ve been experiencing these things personally, makes them more salient,” said Thornton. Now, with brain fog on our minds, “it’s more likely that people are going to start to think about and acknowledge these issues.”