For example, Hone and Naish (2013) claim that “multifunctionality for many such [bizarre] structures is probable, given extant analogues”; however, “invoking species recognition as the primary selective mechanism driving the evolution of such structures is problematic given the lack of evidence for this in extant species”; that “fossil animals must have been subject to the same selection pressures as extant ones” (how can one know “selection pressures” in extinct animals?); and that “non-avialan dinosaurs were likely similar to extant animals and probably used multiple signals as identifiers” (our emphases).

These arguments violate the concept of uniformitarianism, the philosophical foundation of science. True uniformitarianism holds only that the laws of nature are immanent, and by no means can one assume that the processes and patterns that governed extinct creatures and their environments are restricted by what is available to observe in the present day. So it is incorrect to claim that because we have not yet observed (or looked for, or intensively studied) a process in today's world, the process could not have existed in the past or was not important.

Moreover, arguments cannot be justified solely on the basis of analogies, because they do not provide evidence for anything, and are merely rhetorical devices. To substantiate analogies, specific structures between analogized groups must be identical in order to postulate similar functions. For example, dinosaur cranial ornamentations are vastly different than they are in dimorphic birds and mammals. On the other hand, Darwin's (1859) classic analogy between artificial and natural selection works because he was able to show that both processes depend upon natural, inherited variations that succeed differentially under given circumstances; in fact, his comparison was ultimately based on the homology of genetics.

For example, Hone and Naish (2013) state that “there has yet to be any documented case in any extant species where a crest or similar structure functions primarily in species recognition” (and therefore this was impossible in extinct taxa); “[t]hat no extant species, including the thousands of extant dinosaurs [i.e., birds], has yet been demonstrated to use exaggerated morphological structures for the purposes of ‘species recognition’ argues against the idea that we should assume such a role among Mesozoic taxa”; “[g]iven that extant taxa do not appear to be using these [bizarre] structures for species recognition, a plausible mechanism is required to explain their origin, retention and propagation, and to our knowledge none has been proposed”; and that “[t]here is currently no evidence that in extant taxa, exaggerated structures have evolved primarily through species recognition” (again, our emphases).

In trying to minimize the possible importance of species recognition, Hone and Naish misrepresent the work of Harrison and Poe (2012), who they say “specifically tested the species recognition hypothesis with respect to the presence of exaggerated structures and found it wanting.” In fact, Harrison and Poe tested several hypotheses that could explain sizes and patterns in female dewlap structures in one genus of lizard (Anolis) and merely could find no evidence that some of them, including species recognition, were in play in this particular case. This is not an invalidation of the general concept. On the other hand, Vanhooydonck et al. (2009) found that species recognition was one of the best hypotheses that explains dewlap patterns in both males and females of A. sagrei. Harrison and Poe concluded that much probably depends on the size of the habitat and the presence of other species, but the situation is complex and requires more study.

Beyond this, however, species recognition is understood to be important in the differentiation of lineages, both by itself and in conjunction with other processes. For example, in their influential review, Panhuis et al. (2001, p. 369) stated: “Species recognition is important when interpopulation or heterospecific mating is costly, producing relatively inviable or infertile offspring. Traits facilitating species recognition might then diverge under natural selection when populations or closely related species have overlapping geographical ranges.” And, in the same passage, “Several studies have shown (e.g. butterflies; fish and frogs) that male display traits can be both sexually selected and used in species recognition.”

Ritchie (2007), in another influential review in which he is fairly skeptical of the ability of sexual selection alone to cause speciation, notes that “[s]exual selection probably contributes most effectively alongside ecological selection or selection for species recognition than as a solitary process”, a view echoed by Panhuis et al. (2001). Ritchie (2007, p. 83) further emphasizes that “[t]he issue of defining species recognition and sexual selection as different processes is critical when discussing speciation by reinforcement (selection against deleterious hybridization) or the evolution of behaviors to avoid signal confusion or mating competition from heterospecifics. In these cases, behavior is evolving to be species-specific; therefore, species recognition is clearly a valid description of the function of the behavior.”

Even earlier, Ryan and Rand (1993) argued that species recognition and sexual selection may act in concert in the evolution of animal communication. Their discussion is highly nuanced and relevant here, because they recognize the importance of a phylogenetic legacy in the selection of certain behaviors involved in both mating and species recognition, a theme that we stressed (Padian and Horner, 2011a) in criticizing pan-adaptational hypotheses for “bizarre” structures in dinosaurs. For example, on occasions where female frogs preferred males of other species, Ryan and Rand note that it was because those males added a “chuck” to the ends of their calls that were not voiced by the males of the females’ species, but was a feature of the common ancestor of both species.

All of these cited authors point out, somewhat critically, that species recognition and sexual selection have often been treated either as antagonistic forces (a view they rightly reject) or as sites on a continuum (equally critically viewed) (Panhuis et al., 2001, Ritchie, 2007 and Ryan and Rand, 1993). Much of this depends on the definitions of these terms (Ryan and Rand, 1993), which have been many (Mendelson and Shaw, 2012). Nevertheless, all these authors recognize the importance of the process of species recognition, especially in tandem with other processes. In Table 1, modified from Padian and Horner (2013) to include the concept of social selection, we argue that terms such as these are better understood not as part of a continuum but as a hierarchy (see below). We recognize, however, as Darwin (1871) did, that structures and behaviors can be used at more than one hierarchical level.

It is generally admitted that species recognition has not been adequately studied in living animals (Hone and Naish, 2013, Mendelson and Shaw, 2012, Padian and Horner, 2011a and Paterson, 1985), and that in studies where attempts have been made to assess it, there has been such a strong mate attraction signal entailed (or interpreted) with these behaviors that it has been difficult to tease apart (Panhuis et al., 2001, Ritchie, 2007 and Ryan and Rand, 1993).

Main et al. (2005; see also Padian and Horner, 2011a) showed that hypotheses of mechanical function (adaptation) and various kinds of display frequently fail when trying to explain “bizarre structures” in extinct animals because either they are (a) untestable or (b) could not have worked in other members of that clade with similar structures (e.g., thermoregulation in all stegosaurs, jousting in all ceratopsids, sound amplification in all lambeosaurs). They proposed instead that these structures may have differentiated in closely related lineages and later served to reinforce species recognition among conspecifics (Fig. 1). Support for this came from two lines of evidence. First, members of a clade successively closer to the species with the adaptation in question (such as Stegosaurus) did not show progressive improvement in the proposed adaptation (Galton and Upchurch, 2004). Second, the expression of many “bizarre” features, such as the various accessory spikes and ornaments on the skulls of ceratopsids, show no obvious trends in morphology when the species are mapped phylogenetically. Main et al. (2005) hypothesized that these variations were favored and maintained in lineages as a way to reinforce specific distinction and to allow conspecific recognition.

A critical consideration, however, is the scale at which these evolutionary processes work. Main et al. (2005) and Padian and Horner, 2011a and Padian and Horner, 2011b emphasized that structures selected to enhance species recognition occur in lineages of closely related species. Hone and Naish (2013), in criticizing the potential importance of species recognition, erred at the level of phylogenetic scale in stating: “it is not at all clear that distribution of ornamentation (elaborate head, neck and tail feathering, wattles) is in any way ‘directional’ or phylogenetically ‘logical’. Rather, ornamentation could be considered ‘relatively random’, albeit with members of specific lineages representing variations on a theme.” Hone and Naish (2013, fig. 3) picked for their example a scattered sample of genera from the Phasianidae, which comprise some 150 living species representing lineages that have been diverging continually (Zhao et al., 2012) for at least 30 million years (Mayr et al., 2006). This is an inappropriate comparison to Mesozoic non-avian dinosaurs–first, because peafowl, pheasants, quail, grouse, turkeys, and other members of this group are a much larger and more long-lived clade than the closely related dinosaurs are, so species recognition is inappropriate to invoke at this taxonomic level; and second, these bird species are known to be highly sexually dimorphic, with the females and juveniles closely resembling those of related species. To draw a reasonable comparison to non-avian dinosaurs, it would be appropriate to use a group with distinct ornaments, but low or no sexual dimorphism, whose members have diversified only in the past few million years. That is what centrosaurine and chasmosaurine ceratopsians, pachycephalosaurs, and lambeosaurs did in the last few million years of the Late Cretaceous, independently and repeatedly (Horner et al., 1992).

This debate shows no sign of abating, regardless of historical precedent (Padian and Horner, 2011a, Padian and Horner, 2011b and Padian and Horner, 2013). It would seem necessary to establish two concepts of sexual selection: “Darwinian sexual selection” for what Darwin actually said when he established the concept, and “Neo-Darwinian sexual selection” to denote the alterations that distorted Darwin's definition several decades ago and have confused many studies since then, including those by Hone and Naish (2013), Knell and Sampson (2011), Knell et al. (2012), and Hone et al. (2012).

During the 1970s, Darwin's concept of sexual selection was radically altered by biologists who (we believe inadvertently) changed it from an explanation of why bizarre structures occur in one sex but not another (and that would be difficult to attribute to natural selection) to a mathematical description of reproductive success (see Padian and Horner, 2011b for a brief historical review). This revisionism is epitomized in Ryan and Rand's (1993, among many other authors) definition of sexual selection as “variance in reproductive success that derives from variation in the ability to acquire mates”. But this merely reflects the confusion that is rife in today's literature on sexual selection (Carranza, 2009 and Clutton-Brock, 2007). There is no way out of this if biologists insist on describing every factor that increases mating success as “sexual selection.” More importantly, this disregard for history results in relegating to a minor role the main phenomenon for which the concept was invented.

Darwin, 1859 and Darwin, 1871 established the concept of sexual selection not to describe differential reproductive success but to explain why bizarre structures were selected for in one sex, even though they provided no advantage to the survival of that individual. Such individuals would presumably have better access to mates (a testable hypothesis, not a dogma), and would leave not necessarily more offspring than others, but offspring that bore the same features that repelled rivals and attracted mates. Darwin knew exactly what he was doing. This is a complex concept that cannot be reduced to a mere advantage in fertility or production of offspring. And, despite the impression of many present-day workers, sexual selection cannot be “a kind of” natural selection (Padian and Horner, 2011b and Padian and Horner, 2013).

However, confusion is persistent. Knell et al. (2012), for example, claimed that “in neither The Origin of Species nor The Descent of Man can we find an unambiguous definition of sexual selection requiring sexual dimorphism.” Here is Darwin introducing the concept in the first part of the first edition of The Origin (1859: 89–90):

“Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection; that is, individual males have had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms; and have transmitted these advantages to their male offspring.” [emphasis added]

This could scarcely be clearer, and Darwin used more than 500 pages in The Descent of Man, and Selection in Relation to Sex (1871) to document what he was talking about. Large claws in beetles, antlers in cervids, horns in bovids, songs and feather colors in birds, dewlaps in lizards, and myriad other features are present in (usually) males but not females, and they are used to repel rivals or attract mates (Darwin, 1871), to the end of achieving greater access to mates. They may also be used in other contexts. Pursuant to the passage quoted above, Darwin (with his typical wisdom) noted that it is often difficult to tell whether a particular structure was shaped in the context of sexual selection or natural selection or both, and he realized that sometimes the females have the unusual structures; but he did not feel that his concept would fail based on these completely reasonable variations. What is critical is how the feature or behavior is used to gain access to mates.

(Dimorphism is not simply difference; size difference is not dimorphism in Darwin's sense because it does not describe a structure, function, or behavior that one sex has and the other does not. It is possible for larger males [e.g., in crocodiles] to use their size difference in repelling rivals and attracting mates, but if they also use it for greater success in trapping prey, then more information is needed to determine why it evolved or how it is maintained).

The situation is considerably worse for extinct animals. Knell et al. (2012) maintained that “Many fossil animals bear traits such as crests or horns that probably functioned as sexually selected signals or weapons. Interpretations of these structures as functioning in mate choice or intrasexual contests are often controversial, with interpretations based on biomechanics or physiology being favoured by many. Although testing hypotheses based on sexual selection can be difficult, especially given that there is no single, reliable means of recognising sexual selection, we argue that it is not impossible; indeed, there are now several cases where sexual selection is strongly supported.” (emphasis ours). This passage shows the confusion that ensues when Darwin's definition of sexual selection is ignored: sexual selection means whatever one wants it to mean, and notions of what is “probable” or “supported” have no objective testability.

We have previously (Main et al., 2005, Padian and Horner, 2011a, Padian and Horner, 2011b and Padian and Horner, 2013) shown that although Darwinian sexual selection is well established in many clades of living animals, its application to extinct dinosaurs and their relatives is unsubstantiated. We base this on the absence of any evidence that these taxa showed Darwin's requirement of sexual dimorphism. Most recent authors who have discussed sexual selection in extinct animals (Hone and Naish 2013; Knell et al., 2012) accept the Neo-Darwinian view that sexual dimorphism is not required for sexual selection; however, Darwin invented the concept to explain unusual dimorphic structures, not to provide yet another measure of numbers of offspring left in the next generation.

Various reasons are offered for why it is permissible to change the definition of a concept. Knell et al. (2012) argue that we have learned a lot since Darwin's time, and therefore we are entitled to alter his definitions. Darwin, after all, did not know everything. Of course, no one does, but it is astonishing how much Darwin perceived beyond others in his day and how well so many of his concepts remain valid, including his original formulation of sexual selection. Those who think it is fair game to alter definitions based on recent reformulations of concepts fall into Humpty Dumpty's trap (Carroll, 1871): “When I use a word, ‘Humpty Dumpty said in rather a scornful tone,’ it means just what I choose it to mean — neither more nor less.” This quotation is sometimes mistakenly ascribed to Carroll's Red Queen; but of course, the Red Queen learned herself that in evolutionary biology, her original formulation by Van Valen (1973) as a statement of how all organisms compete for diffuse forms of energy could be transformed into an emblem of sexual competition (Bell, 1982) and even dichotomized into the relative influences of biotic and abiotic factors on evolution (Barnosky, 2001). There is nothing wrong with this research, but it distorts and distracts from the original formulation of the concept, which was of at least equal value.

Hone and Naish (see also Knell et al., 2012) point to concepts such as sperm competition and “mutual sexual selection” to show how the understanding of sexual selection has “improved” since Darwin's time, and therefore that we should feel no obligation to honor his definition. Sperm competition is uncontroversial as an example of sexual selection, because the males have structures that the females lack, and they use them to repel rivals (in this case, to remove sperm previously deposited by other males and therefore to deny them access to the female's reproductive system), which is just what Darwin was talking about. Finding new examples does not invalidate a definition.

As for “mutual sexual selection,” Darwin never used the term (contra Jones and Hunter, 1993, and others). Darwin knew that there were some species that had exaggerated structures in both sexes (cassowaries, for example). He excluded them from his concept of sexual selection. This is because he needed to explain bizarre structures that were in one sex and not the other. This is why “mutual sexual selection” is an oxymoron. Sexual selection requires critical differences between males and females, and “mutual sexual selection” entails the two sexes selecting the same feature in the other sex, which is distinctly different from Darwin's definition, where the critical feature differs between the sexes. The concept of “mutual sexual selection” is best restated as “mutual mate choice.” (Mate choice does not equal sexual selection: Table 1). Moreover, the structures used in mutual mate choice could be the same as those that facilitate species recognition. There is no evidence that these structures were specifically selected as aids to repel rivals (which both males and females would have to use) or attract mates. In most cases it is simply assumed that this is what they are used for. Jones and Hunter (1993), in their classic study, showed experimentally that both sexes of the crested auklet prefer larger crests to artificially shortened ones (i.e., mutual mate choice), although they did not (and perhaps could not) test whether these preferences actually shaped the evolution of the crest.

Studies of extinct tetrapods (Hone and Naish, 2013, Hone et al., 2012, Knell et al., 2012 and Tomkins et al., 2010) that tout sexual selection have no experimental evidence and have not tested alternative hypotheses. These authors use “mutual sexual selection” as a default hypothesis based on poorly constrained analogy to isolated living examples in which “mutual sexual selection” (i.e., mutual mate choice) has not even been adequately established. Could these dinosaurs have used their bizarre structures in mutual mate choice? We have no objection to the hypothesis, but it cannot be directly tested any more than species recognition can, and neither one can be regarded as a “default hypothesis.” Indirect diagnostic tests of “mutual sexual selection” are still wanting.

If one does not accept that dimorphism is required for sexual selection (regardless of Darwin's exhaustive documentation), it becomes difficult to separate Darwin's original concept of sexual selection from other related ideas with which it is frequently conflated (Table 1). Reducing the concept to “success in reproduction related to success in obtaining mates” guts Darwin's entire theory of how bizarre, non-adaptive (and even maladaptive) structures evolve in one sex and not the other, a concept he needed to stave off objections that not all structures were shaped by natural selection (Padian and Horner, 2011a and Padian and Horner, 2011b). To follow the “Neo-Darwinian” course leaves no term for the very important class of observations that Darwin (1871) documented so copiously.

Main et al. (2005) and Padian and Horner (2011a) proposed, first, that under species recognition, sister species diverging in sympatry (or close allopatry) would be expected to show little in the way of directional trends, compared to traits under natural selection or sexual selection. This mechanism was discussed above. Hone and Naish (2013), following some traditional authors, claimed that the bizarre features of non-avialan dinosaurs showed trends in evolution that “could well be interpreted as representing” functional improvement or sexual selection, but they provided no evidence for this claim.

The second proposed test was that, under selection for species recognition, apparently “random” character divergences would be expected to occur in closely related sympatric lineages. Under selection for species recognition there is no obligation but to be different in some respect from closely related populations that are sympatric or parapatric; some lineages may even do this without trying to differentiate themselves from neighboring populations (e.g., Triceratops, stegosaurs, and many populations of living birds; see below).

Hone and Naish (2013) bring up a case of stegosaurs with poorly resolved taxonomy and phylogenetic relationships and claim that this is evidence against species recognition; we suspect that if the taxonomists have not yet figured out what names to use, the material is inadequate or their approaches to it are. They also claim that “[a]n additional argument against the use of exaggerated structures in species recognition is that some structures differ little between sympatric species.” Of course; cryptic sympatric or parapatric species are found all the time, and see Hutchinson (1959) for a classic example of Italian water beetles. But this is beside the point. It is also difficult to understand the evidence (if any) for their claim that “[i]f the primary selective process driving the presence of such structures was species recognition, we would predict that species would differ with respect to the form of a single structure–additional or elaborate structures would be redundant and pose additional costs.” We address “costs” below.

Turning to the empirical evidence (Horner et al., 1992, Scannella and Horner, 2010 and Weishampel, 2007), it is easily seen that, at any given time in the Late Cretaceous of the North American Western Interior, several closely related taxa of chasmosaurine and centrosaurine ceratopsians were sympatric or parapatric. They differ mainly in the minor “bells and whistles” features of their frills and horns, just as we would predict.

Among crested hadrosaurs (lambeosaurines), Corythosaurus and Lambeosaurus show a similar differentiation (Fig. 2, from Evans, 2010), and these taxa may have “morphed” stratigraphically into other distinct forms, just as we have seen with Triceratops (Scannella and Horner, 2010; see also Horner et al., 1992). Again, no sexual dimorphism is found. However, in sharp contrast to Triceratops (Goodwin et al., 2006), in most lambeosaurines the specifically distinct crest features do not begin to develop until the animals reach at least half of their linear growth (by which we presume that they are sexually mature), and they have not yet differentiated into species-specific forms (Evans, 2010). The crests do not become close to fully elaborated until the animals reach 80–90% of their full size, well past the initiation of sexual maturity (Fig. 2). And again, no sexual dimorphism is found. If these features were under “mutual sexual selection” they would have developed much earlier. We hypothesize that these animals were gregarious and that the elaboration of the crests reflects social status as much as species recognition.

Horner et al. (1992) described an exceptional constellation of Cretaceous dinosaurs (ceratopsians, lambeosaurs, pachycephalosaurs, and tyrannosaurs) from the Western Interior of the US that showed apparently anagenetic transformations in cranial ornamentations. Fig. 3 shows the stratigraphic positions of these successive forms, which again do not show sexual dimorphism but do include transitional individuals. The figures treat the specimens as if they were distinct species (i.e., in a cladistic analysis, which is not a phylogeny and cannot show possible linear transitions), but Horner et al. explained in their text that the transitions were real: “Anagenesis, rarely documented in terrestrial vertebrates and almost unknown in Mesozoic vertebrates, seems to be clearly evident in these dinosaurs.” It is clear that these transitions could occur quickly and that some forms had sympatric relatives. Given the lack of sexual dimorphism it is difficult to see how sexual selection could be involved, but the closely tracked changes in morphology speak directly to species recognition and coherence.

Scannella and Horner (2010), following the lead of earlier authors, were able to reduce some 18 named taxa of Triceratops and Torosaurus to ontogenetic stages of a single lineage, represented by two species that succeed each other in time, and with no sexual dimorphism. Using new discoveries of dozens of specimens closely tied to stratigraphic position, they found that Triceratops prorsus is succeeded by T. horridus in the Hell Creek Formation (Maastrichtian, US Western Interior), and that Torosaurus is an adult form of Triceratops (Goodwin et al., 2006 and Horner and Goodwin, 2006). Hone and Naish (2013) argue that other authors (Farke, 2011 and Longrich and Field, 2012) dispute this interpretation, but Scannella and Horner (2010) showed that the putative autapomorphies of “Nedoceratops” that Farke (2011) provided (to argue that the specimen represented a different species) were also found in other specimens of Triceratops, and also that the skull in question had severe pathologies. Longrich and Field (2012) used only 36 specimens with no stratigraphic or geographic control, and evaluated the relative ontogenetic age of their specimens by eye, rather than histologically (in contrast to Horner et al., 2011). It will be interesting to see if other authors can falsify the OMH, or support a different hypothesis, after they perform a full analysis of all the data, including a much larger sample size with precise stratigraphic and geographic control. Such a study was performed by Horner et al. (2011), reporting the results of over a decade of fieldwork in the Hell Creek formation that obtained over 240 articulated or associated dinosaur specimens of seven taxa, all recorded in precise geographic and stratigraphic context. We note that even in the limited sample (36 individuals) studied by Longrich and Field (2012), their fig. 5 shows that five of the six specimens attributed to Torosaurus fall into their “adult” category, and the sixth is a very large “sub-adult” (their characterizations), which actually supports the OMH.

Hone and Naish (2013) claim that the unusual non-linear allometry of many features of these ceratopsian skulls, which were interpreted in the context of “status recognition” of different ontogenetic stages (Horner and Goodwin, 2006 and Padian and Horner, 2011a), do not support species recognition, but they are conflating two kinds of differences. Some characteristics are diagnostic of individual species, but other characteristics diagnose ontogenetic stages. Non-avialan dinosaurs achieved sexual maturity before completing skeletal growth–in fact, by about two-thirds of their adult linear size and half of their adult mass (as recognized by the formation of an External Fundamental System: see Horner and Padian, 2004, Lee and Werning, 2008 and Schweitzer et al., 2005). Although Hone and Naish (2013) claimed that “[c]orrectly identifying a conspecific of the correct status (social or reproductive) gets harder, not easier, when several transforming morphs are present,” it really does become easier, because there are more intermediate stages in an increased sample, and the ontogenetic stage of individuals can be assessed independently by histology. The growth patterns suggest that some features could advertise sexual maturity and some could advertise social status, and that they are not mutually exclusive.

In Table 1 (modified from Padian and Horner, 2013 to include social selection), we describe how intraspecific behaviors related to mating and other functions are hierarchical. It seems self-evident that it is not possible to choose a mate among possible mates until and unless conspecifics have been identified. This means that species recognition is prefatory to other intraspecific activities, although species recognition per se has not been studied in most animals in which mating behavior has been. Some biologists maintain that this process is not important, and should only be considered when an individual makes “mistakes” in mating with a non-conspecific, creating a “cost.” In graphic terms, a dog can attempt in vain to mate with another male dog, a fire hydrant, or his owner's leg; but a day, two days, or several hours later he may encounter a receptive female and achieve procreation. Is there really a “cost” of this profligacy? Can it be calculated, or is it just hypothetical modeling based on mathematical assumptions? Can the innate ability to recognize conspecifics be unimportant? In the end, animals congregate with others of their species for reasons other than reproduction, and that presumes that they know who is in their species.

The general issue of “cost” of advertising features for species and (or) mate recognition seems to be an argument rooted more in theoretical speculation than in practice. This is not to say that organisms suffer no “costs” of metabolic and sexual activity, but such alleged “costs” cannot be assumed to be debilitating, especially when lineages have been successful for long periods. Whether neoceratopsians, for example, evolved their horns, frills, and other adornments in the context of mate recognition or mate choice or of species recognition (or some combination thereof), they devoted metabolic effort to developing and reworking these structures ontogenetically. But obviously, as for many other lineages, it worked fine for them over millions of years.

“Costs” of reproduction sometimes seem outlandish, but they are apparently not excessive to the animals. Take the innumerable species of marine crustaceans that broadcast their eggs and sperm into the water column. What is the “cost” of the waste of hundreds of thousands of gametes merely to replace a breeding pair? These clades have survived and prospered for hundreds of millions of years. An Irish Elk male typically grew and shed 20 kg of antler bone each year. Did the lineage become extinct from metabolic exhaustion? “Cost-benefit” arguments at these levels are spurious.

Among animals that actively seek and choose mates, Table 1 suggests that one normally differentiates which members of the opposite sex are appropriate mates, then chooses among them based on various characteristics. One may simply take the first possible mate to hand, in which case mate choice is not so important. The same is true with mate competition: it may be important or not in a given situation.

Sexual selection, as Darwin invented it, is only a small part of this whole complex of behaviors. There must first be mate recognition, mate choice, and mate competition. Here, the competition is of a very particular kind. One sex must possess a feature that the other lacks (or does not use), and it is used to repel rivals or attract mates. Sexual selection therefore requires dimorphism. But it is not universal in the struggle to acquire mates, and should not be synonymized with any general sort of competition for mates or of processes of mate choice.

One may, of course, maintain that Darwin, 1859 and Darwin, 1871 did not know what he was talking about, or was vague about his concepts, or that his examples were inconsistent (the animal kingdom is complex). But historians recognize that he devised the concept of sexual selection to explain the “bizarre structures” that (usually) males of some species had and that the females lacked, and that were used to attract mates and repel rivals. This explanation was critical to answering skeptics of his theory of natural selection, who could have rightfully claimed that structures such as the peacock's tail could not have evolved under natural selection to improve the adaptive fitness of its bearer. We think that not all intraspecific sexual interactions should be labeled “sexual selection.” We do not think it is acceptable to assert by analogy alone that superficially similar structures in living and extinct animals were shaped by sexual selection, without being obliged to test this hypothesis or others (Knell et al., 2012). Analogy is not a test of anything. It is merely a rhetorical device.

Species recognition is a viable hypothesis to explain many “bizarre structures” in non-avialan dinosaurs because the differences among closely related contemporaneous and sympatric or parapatric species are non-adaptive, non-directional in their trends, and non-dimorphic. No other hypothesis explains these features better, and all have substantial flaws (Padian and Horner, 2011a and Padian and Horner, 2011b). Corrupted definitions and inattention to the history of the field of evolutionary biology have brought confusion to what can be sorted out, if the profusion of conflicting descriptions of concepts is replaced by a sensible hierarchy (Table 1).