Editing Protoceratops (section)

===Sexual dimorphism and display===
[[File:Protoceratops variation.png|thumb|Diagram featuring specimens of ''P. andrewsi'' and ''P. hellenikorhinus'', showcasing a wide range of variability]]
Brown and Schlaikjer in 1940 upon their large analysis of ''Protoceratops'' noted the potential presence of [[sexual dimorphism]] among specimens in ''P. andrewsi'', concluding that this condition could be entirely subjective or represent actual differences between sexes. Individuals with a high nasal horn, massive prefrontals, and frontoparietal depression were tentatively determined as males. Females were mostly characterized by the lack of well-developed nasal horns.<ref name=Brown1940/> In 1972 Kurzanov made comparisons between ''P. andrewsi'' skulls from Bayn Dzak and Tugriken Shireh, noting differences on the nasal horn within populations.<ref>{{cite journal|last1=Kurzanov|first1=S. M.|date=1972|title=Sexual dimorphism in protoceratopsians|journal=Paleontological Journal|issue=1|pages=91–97|language=ru}}</ref>

[[Peter Dodson]] in 1996 used anatomical characters of the skull in ''P. andrewsi'' to quantify areas subject to ontogenic changes and sexual dimorphism. In total, 40 skull characters were measured and compared, including regions like the frill and nasal horn. Dodson found most of these characters to be highly variable across specimens, especially the frill which he interpreted to have had a bigger role in [[Display (zoology)|displaying behavior]] than simply serving as a site of masticatory muscles. He considered unlikely such interpretation based on the relative fragility of some frill bones and the large individual variation, which may have affected the development of those muscles. The length of the frill was found by Dodson to have a rather irregular growth in specimens, as juvenile AMNH 6419 was observed with a frill length smaller than other juveniles. He agreed with Brown and Schlaikjer in that a high, well-developed nasal horn represents a male trait and the opposite indicates females. In addition, Dodson suggested that traits like the nasal horn and frill in male ''Protoceratops'' may have been important visual displays for attracting females and repelling other males, or even predators. Lastly, he noted that both males and females had not significant disparity in body size, and that [[sexual maturity]] in ''Protoceratops'' could be recognised at the moment when males can be distinguished from females.<ref>{{cite journal|last1=Dodson|first1=P.|date=1976|title=Quantitative Aspects of Relative Growth and Sexual Dimorphism in Protoceratops|journal=Journal of Paleontology|volume=50|issue=5|pages=929–940|jstor=1303590}}</ref>

In 2001, Lambert and team upon the description of ''P. hellenikorhinus'' also noted variation within individuals. For instance, some specimens (e.g., holotype IMM 95BM1/1) preserve high nasal bones with a pair of horns; relatively short antorbital length; and vertically oriented nostrils. Such traits were regarded as representing male ''P. hellenikorhinus''. The other group of skulls is characterized by low nasals that have undeveloped horns; a relatively longer antorbital length; and more oblique nostrils. These individuals were considered as females. The team however, was not able to produce deeper analysis regarding sexual dimorphism in ''P. hellenikorhinus'' due to the lack of complete specimens.<ref name=Helleniko2001/> Also in 2001, Tereschhenko analized several specimens of ''P. andrewsi'' to evaluate sexual dimorphism. He found 19 anatomical differences in the [[vertebral column]] and [[pelvic region]] of regarded male and female ''Protoceratops'' individuals, which he considered to represent actual sexual characters.<ref>{{cite journal|last1=Tereschhenko|first1=V. S.|date=2001|title=Sexual Dimorphism in the Postcranial Skeleton of Protoceratopsids (Neoceratopsia, Protoceratopsidae) from Mongolia|journal=Paleontological Journal|volume=35|issue=4|pages=415–425|hdl=123456789/25744|url=https://www.researchgate.net/publication/272152287}}</ref>

In 2012, Naoto Handa and colleagues described four specimens of ''P. andrewsi'' from the Udyn Sayr locality of the Djadokhta Formation. They indicated that sexual dimorphism in this population was marked by a prominent nasal horn in males—trait also noted by other authors—relative wider nostrils in females, and a wider neck frill in males. Despite maintaining the skull morphology of most ''Protoceratops'' specimens (such as premaxillary teeth), the neck frill in this population was straighter with a near triangular shape. Handa and team in addition found variation across this Udyn Sayr sample and classified them in three groups. First group includes individuals with a well-developed bony ridge on the lateral surface of the squamosal bone, and the posterior border of the squamosal is backwards oriented. Second group had a fairly rounded posterior border of the squamosal, and a long and well-developed bony ridge on the posterior border of the parietal bone. Lastly, the third group was characterized by a curved posterior border of the squamosal and a notorious rugose texture on the top surface of the parietal. Such skull traits were regarded as marked [[Genetic variability|intraspecific variation]] within ''Protoceratops'', and they differ from other populations across the Djadokhta Formation (like Tugriken Shireh), being unique to the Udyn Sayr region. These neck frill morphologies differ from those of ''Protoceratops'' from the Djadokhta Formation in the adjacent dinosaur locality Tugrikin Shire. The morphological differences among the Udyn Sayr specimens may indicate intraspecific variation of ''Protoceratops''.<ref name=Handa2012>{{cite journal|last1=Handa|first1=N.|last2=Watabe|first2=M.|last3=Tsogtbaatar|first3=K.|date=2012|title=New Specimens of Protoceratops (Dinosauria: Neoceratopsia) from the Upper Cretaceous in Udyn Sayr, Southern Gobi Area, Mongolia|journal=Paleontological Research|volume=16|issue=3|pages=179–198|doi=10.2517/1342-8144-16.3.179|bibcode=2012PalRe..16..179H |s2cid=130903035 }}</ref> A large and well-developed bony ridge on the parietal has been observed on another ''P. andrewsi'' specimen, MPC-D 100/551, also from Udyn Sayr.<ref name=Czepiński2020/>
[[File:Protoceratops andrewsi male & femake.png|thumb|left|Hypothetical male (left, AMNH 6438) and female (AMNH 6466) ''P. andrewsi'' compared]]
However, Leonardo Maiorino with team in 2015 performed a large [[Morphometrics#Landmark-based geometric morphometrics|geometric morphometric]] analysis using 29 skulls of ''P. andrewsi'' to evaluate actual sexual dimorphism. Obtained results indicated that other than the nasal horn—which remained as the only skull trait with potential sexual dimorphism—all previously suggested characters to differentiate hyphotetical males from females were more linked to ontogenic changes and intraspecific variation independent of sex, most notably the neck frill. The geometrics showed no consistent morphological differences between specimens that were regarded as males and females by previous authors, but also a slight support for differences in the rostrum across the sample. Maiorino and team nevertheless, cited that the typical regarded ''Protoceratops'' male, AMNH 6438, pretty much resembles the rostrum morphology of AMNH 6466, a typical regarded female. However, they suggested that authentic differences between sexes could be still present in the postcranial skeleton. Although previously suggested for ''P. hellenikorhinus'', the team argued that the sample used for this species was not sufficient, and given that sexual dimorphism was not recovered in ''P. andrewsi'', it is unlikely that it occurred in ''P. hellenikorhinus''.<ref>{{cite journal|last1=Maiorino|first1=L.|last2=Farke|first2=A. A.|last3=Kotsakis|first3=T.|last4=Piras|first4=P.|date=2015|title=Males Resemble Females: Re-Evaluating Sexual Dimorphism in Protoceratops andrewsi (Neoceratopsia, Protoceratopsidae)|journal=PLOS ONE|volume=10|issue=5|pages=e0126464|doi=10.1371/journal.pone.0126464|doi-access=free|pmc=4423778|pmid=25951329}}</ref>

In 2016, Hone and colleagues analyzed 37 skulls of ''P. andrewsi'', finding that the neck frill of ''Protoceratops'' (in both length and width) underwent positive allometry during ontongeny, that is, a faster growth/development of this region than the rest of the animal. The jugal bones also showed a trend towards an increase in relative size. These results suggest that they functioned as socio-sexual dominance signals, or, they were mostly used in display. The use of the frill as a displaying structure may be related to other anatomical features of ''Protoceratops'' such as the premaxillary teeth (at least for ''P. andrewsi'') which could have been used in display or [[intraspecific combat]], or the high neural spines of tail. On the other hand, Hone and team argued that if neck frills were instead used for [[Protection|protective]] purposes, a large frill may have acted as an [[aposematic]] (warning) signal to predators. However, such strategies are most effective when the taxon is rare in the overall environment, opposed to ''Protoceratops'' which appears to be an extremely [[Abundance (ecology)|abundant]] and medium-sized dinosaur.<ref name=Hone2016>{{cite journal|last1=Hone|first1=D. W. E.|last2=Wood|first2=D.|last3=Knell|first3=R. J.|date=2016|title=Positive allometry for exaggerated structures in the ceratopsian dinosaur Protoceratops andrewsi supports socio-sexual signaling|journal=Palaeontologia Electronica|number=19.1.5A|pages=1–13|doi=10.26879/591|doi-access=free|url=https://palaeo-electronica.org/content/pdfs/591.pdf}}</ref>

Tereschenko in 2018 examined the cervical vertebrae series of six ''P. andrewsi'' specimens. Most of them had differences in the same exact vertebra, such as the shape and proportions of the vertebral centra and orientation of neural arches. According these differences, four groups were identified, concluding that individual variation was extended to the vertebral column of ''Protoceratops''.<ref name=Tereschenko2018/>

In 2020 nevertheless, Andrew C. Knapp and team conducted morphometric analyses of a large sample of ''P. andrewsi'' specimens, primarily confluding that the neck frill of ''Protoceratops'' has no indicators or evidence for being sexually dimorphic. Obtained results showed instead that several regions of the skull of ''Protoceratops'' independently varied in their rate of growth, ontogenetic shape and morphology; a high growth of the frill during ontogeny in relation to other body regions; and a large variability of the neck frill independent of size. Knapp and team noted that results of the frill indicate that this structure had a major role in [[Animal communication|signaling]] within the species, consistent with [[Mate choice|selection of potential mates]] with quality [[Biological ornament|ornamentation]] and hence [[reproductive success]], or [[dominance signal]]ing. Such use of the frill may suggest that intraspecific [[social behavior]] was highly important for ''Protoceratops''. Results also support the general hypothesis that the neck frill of ceratopsians functioned as a socio-sexual signal structure.<ref>{{cite journal|last1=Knapp|first1=A. C.|last2=Knell|first2=R. J.|last3=Hone|first3=D. W. E.|date=2021|title=Three-dimensional geometric morphometric analysis of the skull of Protoceratops andrewsi supports a socio-sexual signalling role for the ceratopsian frill|journal=Proceedings of the Royal Society B: Biological Sciences|volume=288|number=1944|doi=10.1098/rspb.2020.2938|doi-access=free|pmid=33529562|pmc=7893235 }}</ref>