Reconsidering the Subject and Object of Comparative Cognition
Abstract
This article examines interrelated issues of the subject and object of comparative cognition. First, we discuss field’s focus on a limited selection of subjects; rodents and primates are common, whereas taxa outside of mammals and birds are rare. We suggest new directions for a more comparative approach. Second, we discuss an epistemological view of cognition that helps discern behavioral observations from cognitive inferences and promotes a clearer, less anthropocentric approach vital for studying underrepresented taxa and enhancing species diversity.
Keywords: comparative cognition, comparative psychology, model organism, epistemology, Morgan’s canon
Comparative cognition is a rich, multidisciplinary field that investigates psychological abilities across species, thereby facilitating an understanding of the principles governing cognitive processes. Like any field, it faces challenges that require reflection. As comparative cognition continues to evolve, it is necessary to reevaluate both the subject and object of study. First, does the literature cover a diverse range of species sufficient to warrant the label “comparative”? Second, is the object of study “cognition,” or are there alternative explanations that should be considered? The answers to both questions are influenced by an interplay of historical, practical, and epistemological factors. In this commentary, we first examine species diversity and make suggestions for more inclusion of underrepresented taxa. We then consider cognition itself. We aim to probe its definition and propose a more nuanced perspective. We hope these discussions will have a beneficial impact for the future of comparative cognition and related fields.
Let’s begin by reflecting on the comparative goal of comparative cognition. For more than a century, the comparative nature of the field has been questioned (Beach, 1950; Dewsbury, 1992; Gottlieb, 1976). As early as the 1920s, rats became the preferred subjects (Logan, 1999). This trend held until the 1980s, only for nonhuman primates to emerge as the preferred taxa (Burghardt, 2006; Gallup, 1989). Other groups remain underrepresented to this day. Recent reviews indicate it is not just species, genre, or families that are underrepresented, but entire orders and classes have been neglected for decades (Beran et al., 2014; Burghardt, 2021; Miller & Hill, 2014; Varnon et al., 2018; Vonk, 2021). Mammalian and avian taxa are the most studied; however, even among these groups there are notable discrepancies. Most research is still conducted with rats and primates, but pigeons are not uncommon (Beran et al., 2014; Varnon et al., 2018). The situation is more dire for reptiles, amphibians, fish, and invertebrates. Research with these taxa is still relatively rare (Varnon et al., 2018; Vonk, 2021). Of the thousands of species combined across these taxa, only zebra fish and honey bees make regular appearances in comparative literature (Varnon et al., 2018).
This issue is so poignant that recent editors of the Journal of Comparative Psychology have drawn attention to this problem (Dingfelder, 2004; Price, 2010), suggesting that without investigating a range of taxa, our understanding of the evolution of cognition, and even our own species, will remain woefully incomplete. This glaring lack of variety is a missed opportunity. It is possible that within these lesser studied taxa, unique abilities have evolved, offering invaluable insights into the animal mind. Neglect of such vast biodiversity not only results in a loss of potential findings but also inadvertently strengthens a model organism-centric perspective on cognition, which may be fundamentally flawed.
Although there is a deficit in taxa diversity, recent research offers promising avenues for exploration. The rise in canine cognition research exemplifies how the field can expand (e.g., N. J. Hall et al., 2023). An important factor may be the reversal of traditional lab dynamics; rather than bringing the animal to the laboratory, research occurs in the animal’s environment (Brown et al., 2020; Feuerbacher & Wynne, 2012, 2015). This approach holds potential beyond canines and may be especially useful for research with other companion animals, work animals, and livestock. It also emphasizes the applied aspects of comparative cognition (Moran, 1987), which may lead to continual improvement in care of captive animals (Dettmer & Bennett, 2021).
Comparative researchers may also benefit from considering model organisms from other fields. The adoption of zebrafish, a developmental biology model, into psychological studies offers a precedent (Lee & Freeman, 2014; Naderi et al., 2014; Potrich et al., 2015; Santacà et al., 2021; Stewart et al., 2014). Other candidates include clawed frogs and axolotls. Both are established laboratory biological models with well-understood husbandry, making them practical and accessible (Chum et al., 2013; I. C. Hall et al., 2016; Tonge & Leclere, 2000). Pioneering psychological methods for these organisms could significantly improve the gap in amphibian research.
Exploring species that have pronounced interactions with humans, especially those posing potential threats (e.g., venomous snakes: Krochmal et al., 2018; Place & Abramson, 2008; Place et al., 2017) or invasive species with significant ecological consequences (e.g., Burmese pythons in the Florida Everglades: Emer et al., 2014, 2022; Asian carp in the Mississippi river system: Vetter et al., 2015; Wilson et al., 2021), can offer valuable insights. Focus on human and ecological relevance may also help enhance dissemination and funding potential. Moreover, for invasive species such as cane toads and lionfish, which are routinely culled (Shine et al., 2018; Smith et al., 2017), wild collection of subjects will not negatively impact the ecosystem.
Finally, although invertebrates are often overlooked, they offer excellent opportunities, particularly for new faculty. The pragmatic advantages of many invertebrate taxa are manifold: They can be inexpensive, be easy to maintain in large numbers, and require less stringent ethical protocols than vertebrate counterparts. The reduced ethical concerns not only can expedite research timelines but also may be the only possibility for animal research in some facilities. Although honey bees and other Hymenoptera have long been staples of invertebrate research, other species are starting to receive more consideration. Notably, several laboratories in the United States have begun using cockroaches specifically for their practical nature (Dixon et al., 2015; Proctor & Jones, 2021; Varnon & Adams, 2021; Varnon et al., 2022), and excellent research from Japan has elaborated on their potential in behavioral neuroscience (Hosono et al., 2016; Watanabe et al., 2017). Other promising invertebrates include crayfish (Kubec et al., 2019; van Staaden & Huber, 2018), jumping spiders (Aguilar-Arguello & Nelson, 2021; Jakob & Long, 2016), and tardigrades (Wincheski et al., 2020; Zhou et al., 2019).
The integration of open-source technology offers another promising frontier for research with atypical species that may be especially beneficial for new faculty or seasoned researchers exploring new areas. Numerous laboratories have published free software and equipment designs that can be used with nontraditional species, including operant chambers for spiders (De Agrò, 2020), flexible video tracking systems (Conklin et al., 2015; Franco-Restrepo et al., 2019), infrasound detectors for elephants (Bergren et al., 2019), and multispecies experiment control systems (Craig et al., 2015; Dinges et al., 2017; Varnon & Abramson, 2013, 2018). Harnessing 3D printing also facilitates crafting and disseminating specialized equipment (Vora & Abramson, 2020). A crucial benefit of this technology is cost-effectiveness relative to commercial equipment, which often caters only to common laboratory species. For further discussion, including solutions for traditional species, see Varnon et al. (2018).
Let’s now address the second issue, which we believe is vital when considering new species. Is the object of comparative cognition solely cognition, or is behavior often the primary measure? The distinction between behavior and cognition extends beyond semantics. Behavior can be defined as directly observable actions. Cognition, in contrast, does not have one agreed-upon definition (Abramson, 2013; Franklin & Ferkin, 2006; Smith-Ferguson & Beekman, 2020). We suggest an epistemological definition. Cognition cannot be directly observed. Instead, it must be rationally inferred from observable stimuli and behaviors. For example, an animal revisiting a location to find resources produces behavior, in an environment, across time, all of which are observable. Although the animal’s memory is not directly observable, we can infer memory as a cognitive process by analyzing behavior, environment, and time. Manipulating these factors can reveal distinct forms of memory, suggesting that a cognitive explanation is useful. Information processing models of memory, such as the multi-store model (Atkinson & Shiffrin, 1968), follow this general approach and have support from neurophysiology (e.g., Rutar Gorišek et al., 2015; Squire, 1992). Although this case indicates a successful example of research into cognition (for more examples, see Zentall, 2002), not all behavioral processes require a cognitive explanation.
Consider the fascinating case of the slime mold Physarum. Despite lacking a nervous system, Physarum has several interesting behaviors that are often labeled cognition (Smith-Ferguson & Beekman, 2020) but may be explained in other terms. For example, Physarum can solve navigation tasks using “external spatial memory” (Reid et al., 2012). However, the memory metaphor is unnecessary. The movement of Physarum can be described in two rules: (a) If food is detected, move toward food. (b) If food is not detected, move away from extracellular slime. In a behavioral sense, we might call this chemotaxis, a term also used by Reid et al. (2012). The fact that movement leaves behind slime that inhibits future movement does not require a cognitive explanation. Indeed, complex behaviors often emerge from simple rules (Shettleworth, 2012). Proponents of slime mold cognition appear to recognize this is not the same “true cognition” as our memory and instead use terms like “minimal cognition” (Smith-Ferguson & Beekman, 2020), or “basal cognition” (Reid, 2023). But if slime mold cognition is not “true cognition,” then where is the distinction? We believe our epistemological definition provides a solution.
Cognitive explanations may also lead to incorrect assumptions or overshadow behavioral findings. Consider the research on pessimism in bees (Bateson et al., 2011). Bees were trained to associate one odor (CS+) with a sweet solution and another (CS−) with an aversive solution using a proboscis conditioning procedure (Bitterman et al., 1983). Both odors were composites of the same elements: the CS+ was a 1:9 mixture and the CS− was a 9:1 mixture. After training, one group of bees rested while another was shaken. Then responses to CS+, CS−, and novel intermediates odors were tested. Responses of both groups were high to CS+, low to CS−, and intermediate to novel mixtures. However, the shaken group responded much less to the CS− and novel odors. The authors suggested that the bees displayed a pessimistic cognitive bias after being shaken, as they classified ambiguous stimuli as aversive. The results are fascinating; however, the cognitive explanation may be premature. A behavioral description of observable events might be that shaken bees show improved discrimination. This description can be used to summarize the findings and relate them to existing literature without speculating on an unknown mechanism. The issue with cognitive explanations, such as pessimism, is that they may be interchangeable without further evidence. For example, an alternative cognitive interpretation might be that the trauma of shaking causes hypervigilance of stimuli, and thus facilitates discrimination. These cognitive explanations may also overshadow important behavioral results. The behavioral description alone should be considered sufficient for such a novel finding.
An established solution to these dilemmas is Morgan’s canon, a guiding principle of comparative parsimony that suggests avoiding interpreting an action as caused by a higher psychological process if it can be interpreted in terms of a lower process (Morgan, 1894, p. 59). Although there are efforts to classify psychological phenomena in terms of complexity (e.g., Shah et al., 2020; Toates, 2006), it is better to think of Morgan’s canon as a tool to help avoid unnecessary assumptions than a specific hierarchy of complexity that should be applied rigidly. In the case of the shaken bees, inferring an unobservable cognitive mechanism, like pessimism, is less parsimonious than simply using a behavioral description of events, like discrimination. Conservative explanations need not be viewed as “killjoy explanations” (Shettleworth, 2010), as there is value in understanding both simple processes that may underlie complex behaviors and validated cognitive mechanisms. See Zentall (2018) for an excellent review on the application of Morgan’s canon.
Focusing on humanlike cognition, without a clear epistemological framework, may overshadow important behaviors. Employing a conservative bottom-up approach will yield a richer initial understanding of behavior (de Wall & Ferrari, 2010; Eaton, 2018) that may be a prerequisite to later understanding cognition. This is especially relevant for underrepresented taxa without an extensive literature base. Indeed, the subject and object issues we discussed may be inexorably intertwined. By emphasizing the importance of studying a broad range of taxa within a parsimonious framework, we hope that research on these topics will inspire both current and future generations of comparative scientists, addressing a crucial need in the field (Abramson, 2015, 2018).
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