Armor protects against predators: threespine stickleback

"The dermal armor of G. [Gasterosteus] aculeatus serves to protect the fish in different ways, including: (1) resistance to penetrating attacks (e.g., bites from toothed predators), (2) increasing swallowing difficulty and damage to soft mouthparts of predators, and (3) increasing body size when spines are erect, which is an effective deterrent to gape-limited piscivores (Reimchen, 2000; Hoogland et al., 1956). Regarding (1), a penetrating attack generally results in a complex multiaxial stress field below the loading point, as well as potential displacement of the armor unit as a whole via the armor-to-armor unit interconnections (Bruet et al., 2008). Both the local structure of the individual armor units, as well as the microscopic geometry and articulating mechanisms contribute to the effectiveness of protection." (Song et al. 2010:329)

"Aside from evolutionary relevance, detailed studies of the structure and properties of biological armor hold broad applicability to the development of synthetic engineered, protective penetration-resistant materials (e.g., human body, vehicle, and building structure), protective coatings (e.g., exterior paint of automobiles, motorcycles, etc.), construction applications (e.g., pipelines that need resistance to rock penetration/abrasion), and sporting equipment (e.g., helmets, chest protection, etc.) (Arciszewski and Cornell, 2006; Bruet et al., 2008; Ortiz and Boyce, 2008; Yao et al., 2010). Particularly relevant are interlocking mechanisms, plate geometry, porosity, compositional gradients, surface topology, and their relation to penetration resistance and biomechanical mobility (mechanical mechanisms of movement in living organisms, e.g., joint degrees of freedom, ranges of motion, etc.)." (Song et al. 2010:320)