Comparative Cognition: Insights from Miniature Brains
Abstract
Arthropods and other small-brained animals, despite long being touted as simple, instinct-driven creatures, may provide us with some of the most important insights in the field of comparative cognition. Size constraints may be less severe than what many people might expect, and using Portia as an example, we can see instances of behavior rivaling that of much bigger animals. Such insights can help us better understand how widely particular cognitive capacities are expressed throughout the animal kingdom.
Keywords: brain size, representation, planning, numerical cognition, jumping spiders
Back in 1987, Robert Jackson and Stim Wilcox were in a rain forest in Queensland, Australia, where they had planned to make field observations of Portia fimbriata, a special spider in many ways. The genus Portia, which currently comprises 21 species (World Spider Catalog, 2024), belongs to the family Salticidae, a family of spiders renowned for having unique, complex eyes and an exceptional ability to see detail in visual objects (Harland et al., 2012). Many jumping spiders typically prey on insects (Jackson & Pollard, 1996), but Portia are notorious for preferring other spiders, an especially dangerous type of prey to prefer because other spiders are potentially other predators. These Portia must rely on a range of strategies to successfully kill and eat their preferred prey without getting attacked in the process (Cross et al., 2020).
One of these strategies was not immediately obvious to Robert and Stim after they had released their Portia fimbriata on a tree trunk underneath a spiderweb. They expected her to start stepping toward the other spider’s web, but instead they watched as she turned one way and then another before she wandered away from the web and seemingly into the void of the rain forest, lost forever. It was not until after Robert and Stim had slowly packed up their field gear and discussed what to do next when they looked again at the tree and discovered that their Portia was not lost after all but had instead found her way to a vine above the other spider’s web. From there, they watched as she attached a line of silk to this vine before she slowly lowered herself down on the silk until she became level with the other spider. She then swung in and killed the other spider, making no contact with that spider’s web until the moment of attack (Jackson & Cross, 2011).
Through serendipity, this was an extraordinary observation to make. This Portia had taken a detour by walking around and up the tree trunk before finding herself at a vantage point for reaching the spider. This observation had suggested a reliance on cognition, but this was also 1987. Back then, no one discussed the topic of spider cognition unless they meant it as a joke, and even now there are people who would find this topic more than a little frightening. However, further field observations (Jackson & Wilcox, 1993) and many experiments in the laboratory over many years (Cross & Jackson, 2016; Tarsitano & Andrew, 1999; Tarsitano & Jackson, 1992, 1994, 1997) have consistently shown that Portia takes circuitous routes to reach a vantage point for attacking another spider. Without Robert and Stim’s initial observation in the field, these later discoveries would not have been made so readily.
Over the years, spider cognition has increasingly become a respectable field of study, helping us better understand what minute brains can do as well as further enriching the field of animal cognition as a whole. A prime example has come from tasks requiring a detour, which many bigger animals may often solve by relying on insight (Kabadayi et al., 2018). The findings with Portia have suggested a different kind of explanation, with the evidence instead suggesting that Portia will consistently make a slow, deliberate plan before taking a detour, even though it had never taken the detour path in question before and even though it had received no prior training to solve such a task. The types of deliberate plans made by Portia might normally be expected from animals with much bigger brains (Shettleworth, 2010), but Portia routinely addresses life-or-death situations for solving problems while using a brain that would comfortably fit on a pinhead (Jackson & Cross, 2011), a remarkable feat considering that the number of neurons that can be housed inside such a tiny brain is drastically reduced compared with birds and mammals. Unlike humans and other large mammals, which have neurons numbering in the millions or even the billions (Eberhard & Wcislo, 2011), the brains of larger spiders have been estimated to contain neurons numbering only in the tens of thousands (Babu & Barth, 1984) and yet, despite this discrepancy, they can use their neurons in amazing ways.
In detouring experiments, the prey is typically hidden from Portia’s line of sight for much of the detour path, suggesting that, to successfully reach the prey, Portia needed to hold a representation of that prey in working memory. Much of our enthusiasm for spider cognition research has been drawn from this and other instances of apparent representation use by a spider because it suggests extraordinary proficiency by a tiny brain. Our intuitions might normally expect spiders and other arthropods to behave in rigid and inflexible ways, but instead we have seen many examples of flexible cognitive strategies by Portia (Cross et al., 2020) that we might normally expect from a much larger animal. Evidence with honeybees (Srinivasan, 2010) has also suggested that size constraints may be less severe than what many people might expect.
In more recent years, embodied cognition and extended cognition have been suggested as alternative explanations for how spiders can solve tasks (Barrett, 2011; Japyassú & Laland, 2017); these are ways that might compensate for small brain size and that steer away from conclusions about representation use. However, further evidence of apparent representation use by spiders and by other small-brained animals, especially bees (Howard et al., 2019), continues to surface. Portia, for instance, adopts search images for finding a hidden spider (Jackson & Li, 2004) and expresses expectancy violation when encountering an unexpected type of prey spider (Cross & Jackson, 2014) or an unexpected number of prey (Cross & Jackson, 2017). These tasks are all clearly within the realm of “cognition” and require an animal to rely on its brain to “think” instead of requiring a reliance on other means, such as the use of silk in an extended cognition scenario.
The future of comparative cognition should never fail to overlook the wonder that can come from admiring the intricacies of miniature brains, as well as from admiring the capability of these brains. It is encouraging that findings relating to arthropod cognition have gained so much scientific respectability over the years, but this has also brought the unwelcome tendency to suggest that feelings of surprise and intrigue are now irrelevant when describing what tiny brains can do. For instance, I have routinely seen reviewer comments that blandly state, “We already know that small brains can do a lot,” but future studies should never lose sight of how extraordinary this is.
Some of the most important insights of all in the field of comparative cognition may be derived from learning what small brains can do. With arthropods, for instance, we can better understand how widely particular cognitive capacities are expressed throughout the animal kingdom. One example is numerical cognition. Vertebrates are the usual subjects for this research, but other findings have revealed that bees, ants, spiders, and mealworm beetles are also members of the “numerical competence” club (Butterworth, 2022). Learning why number matters to small animals can help us better understand why number matters to much bigger animals, as well as provide insights into the cognitive precursors from which human numerical capacities evolved. Intriguingly, capacity limits for the number of objects that can be attended at a given time are similar for Portia and for human infants (Cross et al., 2020), suggesting that there are apparent limitations regardless of brain size and that the limitations of a miniature brain are not as critical as what many have traditionally assumed.
The innumerable ways that small animals can help us learn more about comparative cognition are astounding. Rather than pushing small animals into the shadows, we should push them further into the limelight because there is still so much to learn. Gone are the days of treating spider cognition as a joke, and here are the days of using further discoveries and advances in technology to delve deeper into what these tiny brains can do.
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