Biology for biomimetics I: function as an interdisciplinary bridge in bio-inspired design

In bio-inspired design, the concept of ‘function’ allows engineers and designers to move between biological models and human applications. Abstracting a problem to general functions allows designers to look to traits that perform analogous functions in biological organisms. However, the idea of function can mean different things across fields, presenting challenges for interdisciplinary research. Here we review core ideas in biology that relate to the concept of ‘function,’ including adaptation, tradeoffs, and fitness, as a companion to bio-inspired design approaches. We align these ideas with a top-down approach in biomimetics, where engineers or designers start with a problem of interest and look to biology for ideas. We review how one can explore a range of biological analogies for a given function by considering function across different parts of an organism’s life, such as acquiring nutrients or avoiding disease. Engineers may also draw inspiration from biological traits or systems that exhibit a particular function, but did not necessarily evolve to do so. Such an evolutionary perspective is important to how biodesigners search biological space for ideas. A consideration of the evolution of trait function can also clarify potential trade-offs and biological models that may be more promising for an application. This core set of concepts from evolutionary and organismal biology can aid engineers and designers in their search for biological inspiration.


Introduction
From mini-drones inspired by insect flight [1,2] to natural product discovery [3,4] and the naked mole rat as a study organism in cancer biology [5,6], we have much to learn from the over 10 million species of organisms on earth. Bio-inspired design is a problem-solving approach that looks to how organisms tackle problems analogous to ours through evolutionary adaptations acquired over millions of years [7,8]. Bio-inspired approaches have become increasingly common over the last two decades in fields as diverse as engineering, chemistry, medicine and architecture [9][10][11][12]. Taking inspiration from biology greatly expands the generation of novel ideas and technologies [13][14][15], especially when engineers and designers are collaborating with biologists [16][17][18]. Bio-inspired approaches often improve design solutions [14,19] and can be more sustainable in terms of material and energy use [20].
While bio-inspired design can be a powerful problem-solving approach, it comes with challenges of being incredibly interdisciplinary [21,22]. In many cases, practitioners of bio-inspired design are limited by the siloed nature of human work, and biologists are often not involved in the design process-only 10%-40% of collaborations involve biologists [9,23]. Conducting bio-inspired design without an extensive biology background is possible, but difficult. Practitioners new to biology can be overwhelmed by biological diversity, trapped by the classic examples in biomimetics, limited by search terms, or misguided in their selection of biological models [24,25]. To overcome these challenges, we seek to bring more biological concepts, and biologists themselves, into the entire biomimetic process [9,17]. Here, we give an overview of the idea of function in biology and bioinspired design to help biodesigners generate better search terms, access a greater diversity of innovative biological models, and lay the groundwork for selecting the most relevant biological models. This review is as much for engineers interested in bioinspired design as it is for biologists who want to work with designers and engineers in this collaborative space.

An overview of 'function' as a bridge
For an engineer or designer looking to biology for creative ideas, the concept of 'function' provides a bridge from human applications to analogous biological traits [18,[26][27][28][29][30][31][32]. For instance, the online database AskNature categorizes the diversity of biological organisms based on the function of their traits [32]. These same functions describe human challenges that commonly come up in various engineering and design applications. As a result, an engineer interested in improving air filtration systems might search 'how does nature filter' to find dozens of biological mechanisms for filtration-from flamingoes to marine tunicates. A number of other databases and tools for bio-inspired design also use the concept of function as the bridge between biology and engineering [33][34][35]. The idea of function is not new to biology; it has long played a central role across biological disciplines [36][37][38]. For instance, in organismal biology, 'form and function' often speaks to how trait characteristics affect trait performance for the individual [39,40] and in ecosystem ecology, biologists often speak of traits that drive the 'functional roles' of different species [41,42].
The functional approach in bio-inspired design as allowed an explosion of biomimetic research over the last two decades ( [9,27]; note we use bioinspired design and biomimetics interchangeably [43]). However, distinct approaches to learning from nature and profession-specific jargon can get in the way of interdisciplinary work between biology and engineering [44][45][46]. Engineers and biologists often approach the idea of function in different ways [47,48], and functions of human products do not necessarily map easily onto biological functions [49]. Engineers and designers studying biology often focus on the 'immediate function' of a trait, and often assume that natural selection has molded traits to perform perfectly in their environment (e.g. [50]). Such misconceptions of natural selection predispose biomimetic approaches to overlook the limits of applying evolved, biological traits to human design [51][52][53]. Indeed, misunderstandings about biology, and separation from the biologists who could provide clarity, are common issues in bio-inspired design [54,55]. While there are many tools to aid designers in the biomimetic process, we are missing a more thorough integration of biology throughout the biomimetic process [9,17,21,48,56].
In this manuscript, we review core biology content relevant to the use of 'function' as a bridge between biology and engineering in bio-inspired design. We do not expect engineers to become biologists or biologists to become engineers. Rather, we want to integrate essential insights from evolutionary and ecological biology into the biomimetics process so that engineers and biologists can be more effective collaborators [48,56,57]. We structure this 'conceptual review' as a companion to a 'problem-based,' 'challenge-to-biology,' or 'technology-pull' approach in bio-inspired design (figure 1 [21,58]). Such an approach begins with an analysis of the human challenge and its translation into functions [32]. These functions are used to find relevant biological analogies, which inspire solutions to the original challenge. In this paper, we synthesize key concepts in evolutionary biology (e.g., adaptation, tradeoffs, function) that are relevant to the steps of this bio-inspired design approach. Throughout this text, we will use the action of 'crushing' as a guiding example with which we can apply and practice the key concepts in evolutionary biology. We focus in on this singular example for simplicity and continuity of communication, not because 'crushing,' or engineering more broadly, are the only relevant fields. We encourage the reader to bridge from this example to their domain of interest.

Articulate design function
In a top-down approach to bio-inspired design, where we move from challenge to biology, we often begin with a problem analysis to refine the initial problem statement [32,[59][60][61]. For example, say we were interested in improving the design of a jackhammer. We know that these machines can produce tremendous forces to crush materials like rock and concrete. However, these same forces risk harming the machine operator due to strong vibrations (e.g., Raynaud's syndrome [62]). Thus, a broad challenge of 'improving machines' becomes the more specific task of 'protecting the operators' hands from harmful vibrations' .
After narrowing the problem, we must articulate it in a way that we can then bridge to biology. Searching biology journals for 'jackhammers' or even 'protecting hands' will not yield relevant results. This is where the idea of 'function' helps by creating a bridge between the problem statement and an expanded biological solutions space; this is the first step in allowing biodesigners to move beyond literal analogies. In design and engineering, 'function' can speak to uses and activities, structural or mechanical systems, or any number of ways that a product or building works or operates, such as energy use or insulation. In order to tap into the wealth of biological models, we must describe an engineering or design  [58] (steps 1-8 on the right) for technology-pull or problem-driven approaches. In this manuscript, we use this process as a backdrop and layer on a more direct consideration of the concept of 'function,' shown in blue. After identifying and analyzing a problem (step 1), we first identify 'design functions' as part of abstracting a technical problem (step 2). In doing so, we can use the concept of function as a bridge to biology, and transpose our problem to organisms and biological traits (step 3). Next, we can broaden the range of potential biological models by exploring not only analogous functions, but also functions across fitness contexts, extremes across species, and indirect analogies (step 4). By considering tradeoffs across biological functions, we can help refine the most appropriate biological model (step 5). While this review focuses on finding biological models, subsequent steps in the bioinspired design process works to understand and abstract the biological strategies, then transpose these mechanisms to technology. [58] © 2017 IOP Publishing. Reproduced with permission. All rights reserved. challenge using language that is less connected to the particularities of human design and thus more applicable to searching biological knowledge [21,58]. In the case of improving jackhammer design, we might be interested in finding biological models that 'crush' , or 'apply force.' We will be especially interested in those biological models that perform these functions 'without causing damage to self.' With these search terms in hand, we might use various databases to start exploring ideas, starting with a list of synonyms for 'crush,' such as 'smash, break, shatter, or mash' . As in most design processes, the first step is to generate a list of ideas that is as broad as possible. We next review biological concepts related to function that allow us to expand the search of biological models in the biomimetic process (figure 1).

Explore analogous functions
Now that we have our design function, we can start to explore the biological world for possible solutions to our human challenges (figure 1). If we are trying to improve the design of machines that crush things or 'generate force,' we might first consider animals that are also crushing or smashing things in their environment, such as their food. The beak of the finch is an example of a biological trait that comes to mind for the function of 'crush,' and also a classic model to study how evolution works. We will consider this example to illustrate how biologists understand the idea of function as well as what other biological concepts that inform the bio-inspired design process.
Imagine a finch beak crushing a seed (figure 2), which allows the finch to access energy and nutrients from the seed. The amount of force generated depends on the depth and length of the finch beakshorter beaks generate more force (consistent with a basic lever system), and thicker beaks withstand this force, thereby preventing damage to the finch's skull (figure 2 [63,64]). In the Galapagos Islands, finches with shorter, thicker beaks are more likely to survive droughts because they can access the energy and nutrients in seeds that are hard to crack-in dry periods, food is hard to come by and these tough seeds become an important resource (figure 2 [65,66]). Finches with shorter and thicker beaks that survive the drought pass any genes related to beak shape to the offspring that form the next generation, resulting in a population shift in beak shape and underlying gene frequencies over time [65,66]. Such changes in gene frequencies in a population over a few generations are often termed 'microevolutionary' processes [65,67]. Over time, these gradual shifts can result in a biological trait adapted to an environmental challenge, such as cracking a seed, and divergence across species to match different environments ('macroevolution').
We can build on this initial finch example to search biological diversity for more biological models that 'crush' or 'smash.' What other species or traits crush or smash? While the finch beak is a classic example of 'crushing' in biology, it is far from the winner in a contest across species to generate greatest crushing force. For instance, smasher mantis shrimp can generate 1500 N of force as they attack their prey [68] and hyena jaws can crush bone to extract marrow [69,70] (figure 3). We can expand our list of organisms for inspiration to the extremes of crushing and smashing, in this case, all in the context of nutrition. Figure 2. The beak of the finch: form and function. Ground finches with shorter and thicker beaks produce stronger bite forces [63]. During droughts, thick, armored seeds increase in relative abundance, selecting for finches with thicker beaks. Following a severe drought in 1977, the relative frequency of finches with deep beaks increased [65,66]. Images by Lizzie Harper, graphs modified from [63] and [66]. Illustration by Lizzie Harper www.lizzieharper.co.uk ©2022. [

Explore function across fitness contexts
In our examples so far, we have focused on 'crushing' with respect to extracting nutrients or energy from food (figures 2 and 3). However, we know that organisms crush or smash things for many different reasons. While we may be interested in the immediate function of a biological trait, to generate a more complete list of biological models for consideration, we should explore across the many ways an immediate function of a trait (e.g. "beak crushing") may contribute to the 'fitness' of an organism. Fitness is a central concept in evolutionary biologyit is the reproductive contribution of an individual to the gene pool of subsequent generations [71,72]. However, it is challenging to quantify as fitness goes beyond just 'number of offspring'-it is affected by a range of traits that describe how an individual (and its subsequent relatives) survive and reproduce in an environment. These traits are sometimes termed 'life history traits' [73][74][75], and range from survival to reproductive age and lifespan to number of offspring in a given reproductive event and frequency of reproductive events [76]. Life history traits are affected by underlying traits related to the necessities of life-energy and nutrient gain, avoiding disease and predators, and buffering oneself against abiotic onslaughts. We refer to these as 'fitness contexts,' summarized in table 1.
Considering how a function relates to a range of fitness contexts can help us to further broaden our initial list of biological models in the biomimetic Figure 3. Finding a diversity of biological models that emulate the direct analogy of 'crushing.' Mantis shrimp, hyenas and crocodiles all crush or smash their prey. To consider extremes across species, we must control for variation across animals in body size. Modified from [68]. Images by Lizzie Harper (hyena), Daniel Yudi Miyahara Nakamura (mantis shrimp, Wikimedia commons), or in the public domain (crocodile, creative commons). Reproduced with permission from [67]. © 2019, Oxford University Press. Illustration by Lizzie Harper www.lizzieharper.co.uk ©2022. This Mantis Shrimp parental care image has been obtained by the author(s) from the Wikimedia website where it was made available by Daniel Yudi Miyahara Nakamura under a CC BY 4.0 licence. It is included within this article on that basis. It is attributed to Daniel Yudi Miyahara Nakamura. Reproduced from rawpixel.com. Image stated to be in the public domain. CC0 1.0.  4), and many other species use weapons to crush, pry, smash opponents in contests over mates [77]. What about in the realm of homeostasis and physiology? Here we might think of physical crushing or degradation as part of digestion, such as the gizzard of a bird, a muscular organ filled with grit that crushes food particles after swallowing (figure 4).

Use immediate and ultimate functions to identify tradeoffs
Considering the immediate function of a biological trait, and how it ultimately contributes to fitness, can point us to a diversity of potential biological models (figure 4), and can also give clues to possible tradeoffs which may be relevant in bio-inspired design [78]. To illustrate this, let us return to the beak of the finch (figure 2), which, it turns out, does more than just crush seeds (figure 5). While a shorter and thicker beak is more likely to break through a tough seed and result in survival through droughts [69,79], short, thick beaks also result in simpler songs that are preferred less by mates [80][81][82]. In addition, small overhanging structures at the end of the beak are particularly effective for parasite removal, but not necessarily crushing seeds [83,84]. Thus, we may be primarily interested in how the beak crushes seeds for food, but the fact that the beak is also singing to attract a mate and preening to remove predators, can result in tradeoffs. Evolution may not 'optimize' the trait with respect to the function an engineer cares the most about in their own design-evolution instead is acting on 'fitness.' These observations illustrate an inherent tension between how an engineer and a biologist might come at the idea of 'function' which can influence how we move between the disciplines [48,49]. For a human-built machine, 'function' speaks to what the machine does, which may be evaluated with specific performance metrics such as how fast the machine moves or how much force it generates. To a biologist, this is similar to what philosophers call the 'causal role' or 'proximate function' [38,85,86]. In other words, this is what a trait is doing in the here and now for the organism-what we have termed here as  the 'immediate function' (figure 6). However, when biologists discuss the function of an organism's trait, they also refer to the evolutionary forces that brought the trait into existence. This is often called the 'ultimate function' of a trait, or what philosophers call the 'etiological function' [87]. This distinction matters to biodesigners because delineating the immediate versus the ultimate functions of a trait can help in understanding tradeoffs across functions, and thus limitations to copying a particular trait in a human application.
The fact that biological traits do not serve one immediate function can result in tradeoffs. In the finch, the beak may have an immediate function of crushing a seed, but it is the range of ways the beak contributes to fitness which explains how the beak came into existence through evolution by natural selection [88]. In other words, the immediate function of a trait that an engineer may be interested in is not always the same as the ultimate function (figure 5 [85,89,90]). Engineers and designers do recognize the long-term causes of function in their own design, such as the iterative history of a product [91]. However, in general, human problem solvers tend to focus on 'function' in an immediate sense, while biologists often look to the natural processes that brought a current function into existence [87,92]. We can use this broader evolutionary view to overcome some of the trade-offs and limitations inherent in drawing inspiration from biological traits.

Overcoming trade-offs: different organisms, different tradeoffs
Biological traits are not always 'optimized' with respect to a design function of interest as they are often doing a range of things for the organism. In general, we can overcome this challenge by looking across species for ideas, as different species often come at the same function through different ways, and sometimes through innovations that allow them to overcome or reduce a tradeoff. First, we can look to organisms that have reached the extremes of a formfunction relationship that is relevant to biodesign applications (e.g., figure 3). Species at the extreme of a form-function relationship often have unique mechanisms that underlie that adaptation. For example, mantis shrimp generate forces by punching their prey with their front limbs; adaptations in this limb and supporting structures (the saddle) allow fast movements and shock-absorption during the blow [68] (figure 3). We might also consider extreme selective conditions, which may prioritize selection on a function of interest over other functions. For example, variation in beak structure may be more closely tied to seed crushing in a bird species in a desert environment where resources are limited and selection for is strong, while selection on song quality and mate choice is relaxed (e.g. [93]).
Second, trade-offs between multiple functions performed by the same trait may be navigated differently across different species. Some species may generate extreme forces because their biology results in different form-function relationships between a trait and different fitness contexts. For example, woodpecker skulls can withstand very high decelerations (on the order of 1000 g) when hitting their head on trees [94,95]. The same beak used for acquiring energy is also used to advertise to mates because woodpeckers attract mates by banging their heads on things (drumming), not by singing, as we see in finches [96]. This alignment of the formfunction relationship across selective contexts suggests that perhaps the biological trait is more likely to be 'optimized' with respect to an immediate function of interest (withstanding force), relative to a species where this relationship varies across fitness contexts ( figure 7). Often, over evolutionary time, the emergence of specialization around a particular biological function may further shift the tradeoff landscape, reducing possible tradeoffs. For example, the evolution of organs specialized for crushing, such as a gizzard (figure 4), reduces interactions between potential functions of a trait. In other words, the emergence of a new trait (the gizzard), reduces the function of the beak towards crushing, altering the trade-off landscape.

Move beyond the direct analogy
Initial explorations in bio-inspired design often gravitate towards direct analogies between a design function and a biological function, such as a machine crushing and a beak crushing. However, we can expand the range of biological models we might consider by moving beyond a direct analogy (figure 8). We often discover relevant traits by considering the opposite function to the function initially considered [32]. For instance, 'withstanding force' may be relevant to improving jackhammer design as much as 'generating force' . The shape and materials in turtle shells or mussel shells may give ideas on how the structure of a hammer handle could absorb the shock of impact (figure 8 [97,98]).
To creatively expand your list of possible biological analogies, we can think about biological models where the evolved function of a trait is entirely unrelated to the design function. For example, tree roots are capable of moving and crushing rock and cement as they snake their way through the ground in search of nutrients or water [99,100]. While roots obtaining resources does contribute to the individual's fitness, the design function of 'crushing' did not evolve due to selection on crushing. To emphasize this point, we use a somewhat absurd example. Tree branches are capable of crushing entire cars when a tree comes crashing down in a windstorm (figure 8). In the first example, crushing arose as a byproduct of selection on root 'foraging' [101], whereas in the second exampling, crushing arise as a byproduct of a very large organism that is susceptible to falling in windstorms [102].
In the tree crushing examples (figure 8), the designer might still be inspired by the biological trait, even if the evolved function is not aligned with the design function. For example, perhaps the architecture of tree roots provides an idea for generating force. Here, a designer is building on a biological trait in a novel design context [103]. For example, NASA has recently engineered sound absorbing devices based on clusters of reed stalks, which happen to be highly effective sound absorbers [104]. However, these wetland plants did not evolve to absorb sound; instead, the acoustic properties of reeds are a byproduct of selection on reed structure and emerges in a group of reeds. In finches, beaks contribute to fitness by generating forces when cracking seeds to acquire energy and nutrients. However, they contribute to fitness in other functional contexts where force generation is less important (preening, singing). In this example, there are tradeoffs for the trait characteristics such as beak depth between the function of interest in an engineering context (force) and other aspects of trait function from an evolutionary perspective (preening, singing). In other words, evolution is not necessarily optimizing 'generating force': the improvement of one function performed by this trait will likely come at the expense of the deterioration of another function performed by the same trait. All finch images are by Lizzie Harper. Illustration by Lizzie Harper www. lizzieharper.co.uk ©2022. (B) Functional alignment: In contrast, for woodpeckers, beak traits related to force generation are related to fitness in similar ways in both a foraging context and a mate selection context, as they drum to advertise for mates, not sing. Thus, we might say the functions are 'aligned.' Additionally, woodpeckers often rely on non-beak traits for parasite removal, such as anting behavior, resulting in no link between the trait of interest and performance in a disease avoidance context. In this case, evolutionary selection for better food acquisition and mate attraction are aligned around beak and skull characteristics related to generating force. Top and bottom woodpecker images are by Lizzie Harper. Illustration by Lizzie Harper www. lizzieharper.co.uk ©2021. Middle is CC-4.0 by the von Wright brothers (Svenska Faglar). Reproduced from rawpixel.com. Image stated to be in the public domain. CC0 1.0.
Identifying and clarifying examples of such coopted and emergent function is important in bioinspired design in part because it allows the designer to move into new creative space. Determining the evolutionary origins of a trait, relative to the design applications, helps to refine how a bio-designer will search biological space for inspiration. For example, to explore other examples of organisms incidentally crushing things in their environment (like the tree crushing a car), one might first explore the evolution of large size in animals [105] or wind resistance in trees [106,107]. Such a search might lead to studies of how elephants withstand forces while running [108] or how whales experience forces while jumping out of the water [109], which could generate insights related to withstanding force. In these examples, the biological model did not evolve to crush, but considering crushing as an emergent property of that trait can link it to the focal problem. Bio-inspired design can benefit from clarifying the functional alignment between design and evolution [110].
Finally, we note that in many cases, the evolutionary context for a biological trait may be uncertain. The immediate function of a trait may be clearbutterfly wing scales reflect light in a particular waybut the evolutionary function of the trait is unclear (e.g., does it function in mate choice or predator avoidance?). We may be able to surmise function based on other examples, or we may need to study the model further. And in other cases, knowing the full evolutionary story of a trait may not always be necessary for the utility of the trait in bio-inspired design. For instance, the ultimate function of shark denticles that inspired 'sharklet' is unclear, but the product is still useful [111]. Regardless, a more thorough understanding of the related biological traits opens more creative doors for bio-inspired design and reduces some of the limitations of copying biological traits that may be 'imperfect' from an engineering perspective.

Conclusions and next steps
In this review, we have developed a companion guide to the idea of 'function' for bio-designers to consider alongside the steps of a biomimetic process ( figure 1 [58]). In the first steps of the bio-inspired design process, we translate our challenge of interest to design functions. These functions are used as a bridge between application and biology. We then offer key steps to generate a greater range of biological models for a given function: (1) explore analogous functions in the biological world, (2) explore across fitness contexts, (3) explore across extremes of a function, (4) consider tradeoff structure, and (5) explore indirect analogies. Considering the immediate versus the ultimate function of a biological trait can give clues to tradeoffs across different functions. To avoid the limitations of copying biological traits that may not be optimized for a particular function of interest, explore a range of biological models, as they each come with different tradeoff structures.
The topics reviewed here allow us to navigate the ideas of function as a bridge between biology and engineering in the biomimetic process. While this paper gives an overview of these steps, we have also built a set of activities that can be used in the classroom or in a design exploration (see appendix). The process reviewed here is the first step in building a diverse set of biological ideas for inspiration in the design process. The next step, as we detail in the next paper in this series, involves expanding this list even further, by building a toolset for navigating the vast space of biological diversity. While this paper has focused on the top-down, or challenge-to-biology approach in biomimetics, it is important to note that the concept of function also works to move from biology to challenges, or the bottom-up approach in biomimetics. In considering the immediate and ultimate functions of biological traits and adaptations, we can brainstorm which human applications may benefit from further studying that organism or trait.

Data availability statement
No new data were created or analysed in this study.

Acknowledgments
This work was supported by a grant from the John Templeton Foundation on Function as a Bridge between Biology and Design, within the broader "Science of Purpose" program (Award 10996). We are grateful to students in ESR's course in bioinspired design (GCC3015/5015) and animal behavior (EEB3412W) for input and comments over the years on the concepts included in this manuscript. We are grateful to comments and critiques provided by the Snell-Rood lab, and members of the broader Templeton project, including Mary Guzowski, William Weber, Jessica Rossi-Mastracci, Amanda Hund, Mike Travisano, Ruth Shaw, Alan Love, Mark Borrello, and Gillian Roehrig.
Certain images in this publication have been obtained by the author(s) from the Wikipedia/Wikimedia website, where they were made available under a Creative Commons licence or stated to be in the public domain. Please see individual figure captions in this publication for details. To the extent that the law allows, IOP Publishing disclaim any liability that any person may suffer as a result of accessing, using or forwarding the image(s). Any reuse rights should be checked and permission should be sought if necessary from Wikipedia/Wikimedia and/or the copyright owner (as appropriate) before using or forwarding the image(s).
Author contributions E S R led conceptualization and funding acquisition. Content was developed by both authors, with writing led by E S R. D S led content critiques and revisions, with both authors editing the manuscript.

Key terms
Biological trait: A feature or subunit of an organism, such as a leg, liver, or behavioral response.
Evolution: A change in gene frequencies in a population over time or space.
Natural selection: Variation across individuals in survival and reproduction underlain by differences in traits (aka "phenotypes"). Can lead to evolution by natural selection when variation in fitness is tied to underlying genetic variation.
Fitness: The genetic contribution of an individual to future generations, generally a function of reproduction (of self or relatives) and survival (of self, offspring and relatives).
Immediate function: What is the trait doing right now for an individual? For instance, the immediate function of 'hunger' is to drive an individual to eat.
Ultimate function: The evolutionary or longerterm function of a biological trait-how does a trait contribute to fitness, and how does this explain how it came into existence? For instance, the ultimate function of 'hunger' is to obtain nutrients, which contribute to fitness; thus, genes tied to this physiological drive increased in frequency in populations over time.
Emergence of function: When a function of interest in a human application emerges from a system of evolving parts, but without selection on that particular function. For instance, ecosystems may store carbon but are not necessarily selected to do so.

Appendix: Companion activities to 'Function as a bridge in Bio-inspired Design'
In this activity, students will go through the initial steps of a bio-inspired design process, using a topdown or challenge-to-biology approach. We will draw on a more extensive exploration of the biology to help expand the idea space in creative ways. The current activity is written for a non-majors undergraduate course lab period, but can be modified depending on time constraints and participant background.

Problem analysis 1. Mind map a big problem.
Choose an overarching problem-depending on the class, this could be 'climate change,' 'pandemics,' or 'building envelopes' . Explode this problem into components through a mind map-with the problem at the center of a page, start drawing out as many pieces of this problem as one can think of, linking related pieces with lines.

Choose and refine a sub-problem.
Highlight common themes in sub-problems (either within or between mind-maps). Choose one of these subproblems to explore in more detail. Find a relevant Wikipedia page or article to learn a little more about the sub-problem to refine it further. For example, you might go from 'climate change' to 'green energy' to 'solar panels' to 'solar cells that work well at high latitudes' .

Generate a list of design functions.
After learning more about a sub-problem, list as many related 'functions' that you might be trying to build into a new product or application. Think about what you want this product to do-what verbs come to mind? For the solar cell example, this may be things like 'capture or harvest light,' especially in low light conditions. Consult a thesaurus to help expand your list of design functions.
Biological analogies: explore immediate and evolutionary function 4. Start a list of biological analogies. For your focal problem, begin a list of biological analogies-what biological traits come to mind that are performing an analogous function for an organism? List what initially pops into your head, and then use various resources to expand this list, such as the database 'Asknature,' field guides to different taxonomic groups, a walk in the woods (or natural history museum), or talking to someone with expertise in different organisms. We will continue to add to this list of 'biology analogies' .

Choose one trait and map to fitness.
From your initial list, choose one analogy that plays out at the level of an individual organism, not a system or group of organisms. For instance, in the solar cell example, you might focus on the pigments of a butterfly wing, but avoid system-level analogies like how a forest reflects light. How does this trait contribute to the survival and reproduction (fitness) of this organism-consider the function that originally led you to this organism, but push yourself to think of how this same trait would apply to other fitness contexts (see table 1). It may help to do a little research on the biology or natural history of the species using field guides, Wikipedia, or other resources.

Explore function across fitness contexts.
Take a look at your initial list of biological analogies. Can you assign each one to the different ways in which a trait may contribute to fitness (table 1). Is there a category that is missing from your list? If so, continue to explore; for instance, perhaps most of the ideas that initially come to mind have to do with energy or nutrition-can you find examples having to do with defense? In looking at your list, are there traits that show up where the evolutionary function is unclear?
Biological analogies: explore function extremes 7. Find the record holders. Start to push your list of biological analogies into the extremes. Can you find the organisms that stand out with respect to your function of interest. This may require some literature searching (see step #12) beyond a Google search, which often first hits on the charismatic record holders that get attention in the media.

Map the alignment between form and function for a few of these systems.
Choose a handful of organisms from your growing list and try to sketch out the relationship between form and function-for instance, as beak depth goes up, 'function' of the beak in terms of bite force goes up, but performance of the beak in terms of singing rate goes down. In many cases, you will likely be limited by research on these formfunction relationships and you may have to hypothesize a relationship based on knowledge of related species, physical interactions, etc. Does the likelihood of tradeoffs apply differently across your species?
Move beyond the direct analogy 9. Explore the opposite function. Return to your list of design functions (#2). Expand this list by considering the opposite of these action verbs. Does this add to your list of biology analogies?

Explore when there is no biological function.
In your list of biology analogies, which stand out as having no analogous evolutionary function-for instance, the traits 'do' something that may be of a design interest, but it is unclear whether they evolved to do this. (In some cases, it may be unclear!) Can you expand this list of biological models where a function of interest is a byproduct of selection on something else?

Move beyond basic databases-using literature searches.
Many bio-inspired design databases are currently the tip of the iceberg of biological diversity. You can expand your list of biological analogies further with searches of the biology literature. The database Wed-of-Science is particularly useful as you can search using Boolean operators, and also select databases that go far back in the literature. Google scholar can be complementary as it searches full text, but you have less control over your search. An example search for papers related to bird beaks that produce force might be: (beak or beak) and (function * or performanc * or 'form-and-function') and (bite * or force * )

Move beyond literature searches-ask a biologist.
While there is a lot we can do with the help of the internet, sometimes it is even more helpful to ask an expert. How do you find a biologist to talk with? One of the easiest ways is through Web of Sciencefor a given search, look at the 'authors' side tab; you may want to restrict your search to the most recent ten years to get people actively working in an area. You can further narrow the list to researchers in a given location. Send them an email (or two)-biologists often love to talk about their organism!