Abstract
Species diversity in experimental neuroscience research provides a vital resource. Addressing contemporary questions using nontraditional model systems (i.e., studies of species other than rats or mice) have regularly led to serendipitous breakthroughs in this discipline. The “comparative” approach to neuroscience and neuroendocrinology harnesses the diversity of organisms—and their nervous systems—that have been refined and differentiated over evolutionary timescales. Here, we review some recent examples of unexpected and impactful outcomes resulting from research on nontraditional study species. This work shows that maintaining broad diversity in study species will continue to provide the best path forward for extraordinary advances and insights into the neural mechanisms of behavior.
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Of mice and men . . . and other animals: studying diverse species often leads to major scientific and clinical discoveries.
Key Points
Most current neuroscience research focuses on a few select species.
Selecting study species based on their biology and behavior in addition to methodological convenience often results in scientific advances and can lead to novel clinical discoveries.
We highlight several examples of curiosity-driven research that led to unexpected and impactful findings in neuroscience.
Introduction
Unfettered exploration and curiosity are at the heart of scientific progress. The history of research on the brain and behavior clearly illustrates this point. Exploration of the brain in a staggering variety of species has led to tangible improvements in societal outcomes such as education, health, and innovation. Several excellent prior reviews and essays have illustrated these points (Brennan, Irschick, Johnson, & Albertson, 2014; Carlson, 2012; Manger et al., 2008; Marder, 2002), and two key perspectives can be distilled from this literature. First, the particular species chosen for a study usually has unique experimental advantages for the particular research question at hand. Second, curiosity-driven, serendipitous discoveries that arise from studying diverse species in neuroscience are often several steps removed from the original “intent” of the initial experiments. For example, original studies of the large, accessible photoreceptors (i.e., cells that specialize in converting light to electrical impulses in the eye) in horseshoe crabs eventually led to the discovery of both visual receptive fields (i.e., dedicated regions of the visual environment to which individual cells respond) and the concept of lateral inhibition (i.e., neighboring sensory cells that inhibit each other to enhance sensory acuity and perception) (Hartline, Wagner, & Ratliff, 1956), while studying the mechanisms of vocal learning in songbirds eventually led to the discovery of adult neurogenesis (i.e., the formation of new cells in the adult brain; see below). Each of these seminal advances owes its success to careful selection of unconventional model systems for traits that were especially suited to the original question, but also to thoughtful insights about the research surprises that cropped up along the way.
Despite a common view among neuroscientists that diversity in study species can accelerate advances, systemic hurdles persist. The current decade of belt-tightening in scientific research funding coincides with a critical era in neurobiological research. We have entered a bottleneck period in which the vast majority of studies are carried out on humans and laboratory rodents. Despite the recognition that this bottleneck has significant drawbacks (Brenowitz & Zakon, 2015), some have argued that, given current constraints, “purely curiosity-driven university research—with no particular societal benefit in mind—has largely run its course” (Aarssen, 2013; p. 417). Here, the visionary perspective of Abraham Flexner (1939), the founding director of the Princeton Institute for Advanced Studies (which in Flexner’s day encouraged so-called useless pursuits that directly led to revolutionary benefits such as atomic energy and the personal computer) is as informative and relevant as ever
. . . throughout the whole history of science most of the really great discoveries which had ultimately proved to be beneficial to mankind had been made by men and women who were driven not by the desire to be useful but merely the desire to satisfy their curiosity. (p. 56)
Like all areas of science, curiosity about a problem—in this case, a neural mechanism, or a behavior—leads neuroscientists down a circuitous path of exploration, and this path can often lead to extraordinary and novel insights. We highlight a handful of examples of this phenomenon below to emphasize the power of harnessing diversity in study species. We find that when we look at the recent history of neuroscience and neuroendocrinology, we clearly see a central role of diversity in model systems in research, and marvel at the many serendipitous discoveries that have arisen from original investigations of neural mechanisms in unconventional animal species (Figure 1). Although our highlights below are by no means an exhaustive list, our intention here is to describe some of the clearest examples.
Nontraditional model species provide many examples of serendipitous scientific and medical advancements.
Basic, Curiosity-Driven Science Can Lead to Unexpected, Impactful Outcomes
From Imprinting Behavior in Geese to Critical Period Neurobiology
Critical periods are stringent windows of opportunity in development, during which experience is necessary to shape neural circuits and essential adaptive behavior. One of the earliest scientists to describe a critical period phenomenon was the Austrian scientist Konrad Lorenz. In his 1935 monograph, Lorenz wrote detailed observations about filial imprinting in hand-reared greylag geese. Lorenz noticed that minutes after hatching, goslings will form a lasting bond with the first animal they see, and will follow that animal presuming it to be their parent, until adulthood.
Lorenz’s seminal work provided a conceptual framework to explore the neurobiology of critical periods for other visual experience-dependent learning. In the 1960s, David Hubel and Torsten Wiesel published their milestone studies on vision in domestic cats (Wiesel & Hubel, 1963). Cats reared from birth with one eye closed shut (monocular deprivation) had a similar imbalance reflected in their neural real estate: The nondeprived eye claimed more neural property in the visual cortex; this was established during an early critical period in life of both cats and, later, monkeys (Hubel, Wiesel, & LeVay, 1976). Hubel and Wiesel’s work galvanized the field, leading to the discovery of additional sensory-based critical periods in other nontraditional animal models (Knudsen, 2004), and inspired new research on the molecular mechanisms gating critical periods.
Recent studies have built on this foundational work, and have focused on the molecular triggers and brakes that regulate behavioral and circuit plasticity (Hensch, 2004). Groundwork on critical periods in neural development has subsequently led to several discoveries in humans, including the way that prenatal exposure to antidepressants can shift language development in newborns, as well as the ability to reopen perfect pitch perception in adults (Gervain et al., 2013; Weikum, Oberlander, Hensch, & Werker, 2012). Most notably, molecular work on critical periods for vision in animals has led to promising new potential medications, 1 and paved the way for treating amblyopia (sometimes called “lazy eye”) in children (Hensch & Bilimoria, 2012). Together, basic science research on critical periods has led to numerous insights on brain plasticity, and has provided new targets for future treatment outcomes for neurological and psychiatric disorders.
Fascination With Songbirds: Toward New Understanding of Brain Sex Differences and Adult Neurogenesis
The existence of sex differences in the size and function of brain areas is firmly established today, but scientists were resistant to this idea until the pioneering work of Fernando Nottebohm and Art Arnold in the 1970s (Nottebohm & Arnold, 1976). They began searching for brain regions that might explain the observation that male canaries sing more elaborate songs than females. The team soon discovered one of the largest sexually dimorphic brain regions in the canary song circuit. In canaries as well as other songbird species, song production brain regions were nearly 6 times larger in males than females—differences so large that they could be seen with the naked eye. Although this was not the first report of sex differences in the brain, prior observations had faced great skepticism (Raisman & Field, 1971). It was not until findings in songbirds that other researchers were convinced to revisit the mammalian nervous system, leading to new descriptions of morphological sex differences in the rat hypothalamus (Gorski, Gordon, Shryne, & Southam, 1978) and the current proliferation in studies of human brain sex differences.
The idea that brain regions differed in size between male and female songbirds led to a second fundamental insight about the brain and behavior. It was long an accepted fact that brain structures were complete at birth and no new cells could be added in the adult brain. Once again, initial suggestions counter to this dogma were met with great resistance (Altman & Das, 1965). However, this rigid view was challenged by a series of landmark reports, again from songbirds in the 1980s. Bruce Goldman and Fernando Nottebohm observed that the song production brain regions in females could grow to match the size of males if they were treated with the steroid hormone testosterone, and that testosterone-treated females also began to sing elaborate songs similar to males. They were curious whether this neural enlargement was due to the addition of entirely new neurons to the song circuit, and in testing this idea they showed that new neurons could be born in the adult brain (Goldman & Nottebohm, 1983). Moreover, these new neurons were incorporated into functional neural circuits that support key behaviors such as singing, song learning, and spatial memory (Barnea & Nottebohm, 1994). These findings then directed the field to examine the capacity for new neuron growth in adulthood in other species, and neurogenesis has since been reported in a host of mammalian and nonmammalian species, including humans (Gould, 2007). Because of this work, neuroscientists now view the human brain as a site of plasticity, self-repair, and learning, each associated with neurogenesis.
Stress in Newts and a New World of Steroid Signaling
Steroid hormones (such as sexual or stress hormones) were once believed to act solely on receptors in the interior of cells by changing gene expression. This mechanism accounts for the powerful effects of steroids throughout the body that can persist for hours to days. However, several physiological observations showed that these same steroid hormones can also act on a faster time scale (seconds to minutes; too fast to act via the regulation of gene expression: (Kelly, Moss, & Dudley, 1976; Szego & Davis, 1967), and that steroids could bind to the surface of cell membranes (Pietras & Szego, 1977). However, it was by studying the rough-skinned newt (Taricha granulosa) that the field gained fundamental insight into how steroid hormones can act rapidly on membrane receptors in the brain to directly affect behavior.
Male newts exhibit the classic amphibian mating behavior, called amplexus—where they clamp the female in a hug to stimulate egg deposition. F. L. Moore and Miller (1984) discovered that stressed males performed less amplexus toward females. Injecting newts with corticosterone (akin to the stress hormone cortisol) also rapidly reduced this behavior. Careful follow-up work identified receptors on cell membranes that had high affinity for corticosterone in amphibian brain preparations, and further showed that corticosterone inhibited behavior within 8 min of injection (Orchinik, Murray, & Moore, 1991). This work provided the first thorough description of steroid activation of neuronal membrane receptors that resulted in behavioral changes.
These studies inspired and intensified the search not only for membrane stress hormone receptors in mammalian neurons (Liu, Wang, & Chen, 1995) but also for membrane receptors for other steroid hormones. Estrogens, for example, were already known to act on intracellular receptors, which could be found in cellular locations away from the nucleus (Blaustein, 1992), but no conclusive data about membrane estrogen effects came to light until the early 2000s. The membrane estrogen receptors for rapid brain effects were revealed to be a mixture of modified versions of the classical intracellular receptors as well as novel membrane-associated estrogen receptors (Filardo, Quinn, Bland, & Frackelton, 2000; Owman, Blay, Nilsson, & Lolait, 1996; Revankar, Cimino, Sklar, Arterburn, & Prossnitz, 2005; Toran-Allerand et al., 2002).
Subsequent work derived from this perspective of rapid, membrane-initiated steroid signaling has provided a wealth of knowledge about their potential for cognitive enhancements and clinical applications. Estrogen treatments can enhance spatial memory (Packard, Kohlmaier, & Alexander, 1996; Packard & Teather, 1997) and synaptic plasticity in rodents (C. S. Woolley & McEwen, 1994). An example of how brain estrogens can be clinically important came from Sato and Woolley’s (2016) recent work on a rat model of epilepsy: Seizures induce estrogen production in the hippocampus and blocking this production alleviates seizures, suggesting a novel therapeutic approach for epilepsy management. Thus, the curiosity-driven examination of how stress influences the sexual behavior of newts opened up a whole new aspect understanding of steroid signaling in the brain.
Melanopsin, From Frog Skin Into the Brain
Melanopsin was first isolated from frog skin cells by Ignacio Provencio as a molecule involved in pigmentation changes (Provencio, Jiang, De Grip, Hayes, & Rollag, 1998). This discovery allowed Provencio and colleagues to compare the melanopsin DNA sequence with other known gene sequences in mammals to search for matches. Melanopsin was successfully characterized in a specialized subpopulation of cells in the retina of mice, monkeys, and human beings. In mammals, melanopsin cells turned out to be the primary entry point for light to directly signal a part of the hypothalamus that specializes in regulating daily rhythms in the body, where they are crucial for relaying external light/dark information directly to the brain, to keep the body’s clock on precise time according to the daily sleep/wake cycle.
Melanopsin cells exhibited special properties as compared with the other light-sensitive cells in the mammalian eye. Specifically, melanopsin cells are uniquely excitable: Unlike the conventional rod and cone cells of the retina that respond to light by paradoxically becoming less active (the so-called “dark current”), melanopsin cells are directly responsive and activated by light. This property is key to their role in regulating daily rhythms and the reflexive closing of the pupil in response to sudden illumination. Following the discovery of melanopsin and characterization in the brain of many vertebrates, further exploration soon explained previous observations that some human patients with hereditary blindness could still exhibit daily rhythms in response to light cues from the environment. Furthermore, one particular mutation in the melanopsin gene accounts for a portion of people’s susceptibility to seasonal affective disorder (Roecklein et al., 2009). Most recent analyses have proposed that melanopsin dysfunction may be involved in neurological maladies including migraine and sleep dysregulation (Ksendzovsky, Pomeraniec, Zaghloul, Provencio, & Provencio, 2017). Therefore, melanopsin has become a promising target for remedying many human conditions that depend on sleep and light cycle entrainment, all with humble origins in the skin of a frog.
GnIH, a Missing Link Neurohormone for the Control of Reproduction
The hormonal regulation of the pituitary by the brain is critical for reproduction. Until 2000, this control mechanism was thought to be exclusively the domain of the master hormone GnRH (gonadotropin-releasing hormone). The hypothalamus releases GnRH to then trigger the release of reproductive hormones from the pituitary to control important functions such as the ovulatory cycle and puberty. In the late 1990s, Kazuo Tsutsui and colleagues were exploring novel hormones in the hypothalamus of Japanese quail, a ground-dwelling bird species. In the course of this work, Tsutsui and colleagues (2000) stumbled upon one protein that potently inhibited the pituitary release of reproductive hormones. The newly discovered GnIH (for inhibitory hormone) provided a missing link for the coordinated feedback regulation of reproduction in the brain and gonads. The discovery of GnIH itself depended on a longer prior history of similar studies of neuropeptide hormones in invertebrate species (Tsutsui et al., 2010). Subsequently, Tsutsui and others went on to show that this fundamental mechanism was important for reproduction in many other bird species, as well as frogs, monkeys, humans, rats, sheep, and fish.
The long-range practical applications of this relatively new discovery of GnIH are yet to be determined. Even still, the recent discovery of a fundamental mechanism of how the brain controls reproduction illustrates the value of pursuing curiosity-driven questions in nontraditional species, because they often reveal basic principles that are applicable across most animal species.
Voles and the Role of Oxytocin and Vasopressin in Social Behavior
Much of what is known about the role of oxytocin and vasopressin in social behavior is due to groundbreaking observations from microtine voles. Closely related vole species exhibit wildly different mating systems. Prairie voles are monogamous, while meadow voles are solitary and exhibit a promiscuous mating system (Gruder-Adams & Getz, 1985). These behavioral observations initiated the search for related species-level differences in brain structure, and culminated in the discovery of stark differences in the expression of oxytocin and vasopressin receptors (Insel & Shapiro, 1992; Insel, Wang, & Ferris, 1994). The monogamous prairie vole had much higher expression of both neurohormone receptors in regions of the brain associated with sexual behavior and reward. These differences in receptor densities therefore led to causal tests of their role in monogamous social behavior. For example, blocking these receptors decreases partner preferences (Lim & Young, 2004, 2006; Young, Lim, Gingrich, & Insel, 2001), while artificially elevating vasopressin receptors in reward and social brain regions increases partner (mate) preferences in the ordinarily promiscuous vole species (Lim et al., 2004; Lim & Young, 2006). Understanding the mechanisms underlying monogamy in prairie voles provided a path forward to causally test the role of neurohormone receptor expression in monogamy. The Young group conclusively showed that sociality and monogamy-like behaviors could be induced in promiscuous species like meadow voles and the traditional laboratory mouse (Young, 1999; Young, Nilsen, Waymire, MacGregor, & Insel, 1999).
Despite the relative scarcity of monogamy in mammals, understanding the neural mechanisms underlying these strong social bonds in voles can inform our understanding sociality of all sorts, including social bonds in humans (Anacker & Beery, 2013; Beery & Zucker, 2010). Studies in voles therefore laid the groundwork for future translational research. Mutations in the oxytocin receptor gene have been associated with a variety of neuropsychiatric disorders in people, including depression, bipolar disorder, and partner attachment/separation anxiety (Costa et al., 2009). Most promising of all, dysfunctions in human oxytocin and vasopressin receptors appear to have major applications for understanding and treating autism spectrum disorder (Insel, O’Brien, & Leckman, 1999). Intranasal oxytocin treatment given to children with autism has shown therapeutic potential for enhancing social interactions (Anagnostou et al., 2014). However, other work has shown mixed results, with some studies reporting no effect (Dadds et al., 2014), and others reporting moderate positive effects (Yatawara, Einfeld, Hickie, Davenport, & Guastella, 2016). The mixed results of ongoing clinical trials reinforces the need for a better understanding of the oxytocin system and its involvement in social behavior, and the continued fruitful exploration of these now key questions in animal models.
Sexual Diversity in Animals Mirrors Diversity in Humans
Sometimes, studying nontraditional species can yield independently convergent findings and thus provide better insights into broader phenomena. An example of this is the flexibility and adaptability of reproductive biology, which has been observed across the animal kingdom. Some bighorn mountain rams have male-male preferences, first reported from behavioral observations in the wild that accounted for 4% of the population (Geist, 1974). Four distinct sexualities have been observed in rams: female-oriented, male-oriented, bisexual, and asexual (Roselli, Reddy, & Kaufman, 2011). This variety can be explained in part by the volume of the sexually dimorphic nucleus in the hypothalamus (Roselli, Larkin, Resko, Stellflug, & Stormshak, 2004). Further work has indicated that neural circuitry of several brain regions participating in the reproductive and social circuits are also involved (Mirto, Austin, Uthlaut, Roselli, & Alexander, 2017).
Along these same lines, biologists have long been captivated by an all-female lizard species (Cnemidophorus uniparens), which reproduces via parthenogenesis. Two females will mount each other producing both male-like and female-like reproductive behaviors that are regulated by increases in estradiol and progesterone throughout the reproductive cycle (Godwin & Crews, 1999; M. C. Moore & Crews, 1986; M. C. Moore, Whittier, & Crews, 1985; S. C. Woolley, Sakata, & Crews, 2004). Same-sex sexual behaviors have also been documented in several species, notably in monogamous pair-bonding birds such as finches (Elie, Mathevon, & Vignal, 2011; Tomaszycki & Zatirka, 2014) and parrots (Abbassi & Burley, 2012) in which partnering serves an essential role for survival because the social benefits can outweigh the necessity to reproduce sexually. Several tropical fish species exhibit complete sex-change (sequential hermaphrodites), based on the size, dominance hierarchy, and sex ratio of other fish within their social group (Munday, Buston, & Warner, 2006).
These examples demonstrate the importance of science for the sake of curiosity, rather than strict utility. Together, these independent explorations of sexual diversity have led to the conclusions that there is considerable plasticity in reproductive mechanisms that were once considered to be fairly fixed. Humans also exhibit a wide range of romantic and sexual partnering, and the diverse examples of nonheterosexual behaviors across animals provide insight into the biological underpinnings of sexual diversity, including the hormonal, neuroanatomical, and genetic mechanisms that may underlie some these behaviors.
Rich Diversity Among Human Subjects for Research
In line with the importance of diversity in animal species used in scientific research, the same perspective can be applied to studying humans. Despite being a single species, human beings vary in significant ways. Outside of clinical applications, human research has continued to inform basic scientific questions, particularly via broadening the selection of research participants. This has been done for decades in endocrinology by studying persons with disorders of sexual development; but more recently scientists have expanded to other underrepresented groups. Imaging studies with transgender people have demonstrated that cross-sex hormones given in adulthood (i.e., estradiol and testosterone blockers for transgender women) can change brain volume and connectivity (Zubiaurre-Elorza, Junque, Gomez-Gil, & Guillamon, 2014). Also, sex differences reveal clear masculinization of transgender men and feminization of transgender women in cortical thickness in otherwise untreated adults (i.e., in the absence of cross-hormone treatment; Zubiaurre-Elorza et al., 2013). One study in transgender youths also demonstrated that while sex differences related to genetic sex are maintained in these individuals, differences in brain volume relate to gender identity (Hoekzema et al., 2015), suggesting neural correlates of gender self-identification.
Similarly, research examining participants of varying sexual orientations, relationship status, and romantic partnering (i.e., monogamy vs. polyamory) have shown how hormones, such as testosterone, fluctuate dependent on relationship status, satisfaction, and number of partners (monogamous vs. multipartnered) (Edelstein, van Anders, Chopik, Goldey, & Wardecker, 2014; van Anders & Goldey, 2010; van Anders, Hamilton, & Watson, 2007; van Anders & Watson, 2007).
Together, these lines of research demonstrate fundamental aspects of how hormones impact the brain and how social and relational constructs impact hormonal states in healthy human subjects. By selecting the research participants around the question, rather than limiting samples to traditional groups of people (or animal models), researchers can broaden their understanding of the basic science of hormonal and neurobiological mechanisms.
Conclusion
Here, we have highlighted a few exploratory neuroscience endeavors that have led to unexpected, valuable, and, in some cases, exceptionally broad insights about the interaction between hormones, brain, and behavior. The advent of new genome-editing technologies as well as molecular strategies using viruses that can deliver genes to specific neuron types now expands the application of molecular tools to model organisms beyond the traditional genetic models such as mice and fruit flies. These new approaches mean that the pace of advances that originate from serendipitous observations in nontraditional study species can be expected to accelerate in the near future. We imagine, given the history presented in this essay, that current science that has no particular societal benefit in present view will in some cases lead to innovative future outcomes in health, education, and beyond.
The unpredictable outcomes of basic research are part of the everyday process of science. Any scientist can routinely expect surprises arising in carefully controlled experiments. This unpredictability can also manifest in new insights and applications to problems entirely unrelated to the initial aims of the academic research pursuit, per se. As such, this unpredictability requires patience and the steady investment of time, resources, and intellectual freedom to pursue a wide range of research topics, some of which may seem frivolous or useless at the time, but which can often lead to transformative insights.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.