Editing Tyrannosaurus (section)

===Life history===
[[File:Baby T-rex 0496.JPG|thumb|Illustration of a juvenile ''Tyrannosaurus rex'']]
The identification of several specimens as juvenile ''T. rex'' has allowed scientists to document [[ontogeny|ontogenetic]] changes in the species, estimate the lifespan, and determine how quickly the animals would have grown. The smallest known individual ([[Los Angeles County Museum of Natural History|LACM]] 28471, the "Jordan theropod") is estimated to have weighed only {{cvt|30|kg}}, while the largest adults, such as [[Field Museum of Natural History|FMNH]] PR2081 (Sue) most likely weighed about {{cvt|5650|kg}}. [[Histology|Histologic]] analysis of ''T. rex'' bones showed LACM 28471 had aged only 2 years when it died, while Sue was 28 years old, an age which may have been close to the maximum for the species.<ref name="ericksonetal2004" />

[[File:Tyrannosauridae growth rates.svg|thumb|left|A graph showing the hypothesized growth curve, body mass versus age (drawn in black, with other tyrannosaurids for comparison). Based on Erickson and colleagues 2004]]
Histology has also allowed the age of other specimens to be determined. Growth curves can be developed when the ages of different specimens are plotted on a graph along with their mass. A ''T. rex'' growth curve is S-shaped, with juveniles remaining under {{cvt|1800|kg}} until approximately 14 years of age, when body size began to increase dramatically. During this rapid growth phase, a young ''T. rex'' would gain an average of {{cvt|600|kg}} a year for the next four years. At 18 years of age, the curve plateaus again, indicating that growth slowed dramatically. For example, only {{cvt|600|kg}} separated the 28-year-old Sue from a 22-year-old Canadian specimen ([[Royal Tyrrell Museum of Palaeontology|RTMP]] 81.12.1).<ref name="ericksonetal2004" /> A 2004 histological study performed by different workers corroborates these results, finding that rapid growth began to slow at around 16 years of age.<ref name="hornerpadian2004">{{Cite journal |last1=Horner |first1=J. R. |last2=Padian |first2=K. |date=2004 |title=Age and growth dynamics of Tyrannosaurus rex |journal=Proceedings: Biological Sciences |volume=271 |issue=1551 |pages=1875–80 |doi=10.1098/rspb.2004.2829 |pmc=1691809 |pmid=15347508}}</ref>

A study by Hutchinson and colleagues in 2011 corroborated the previous estimation methods in general, but their estimation of peak growth rates is significantly higher; it found that the "maximum growth rates for T. rex during the exponential stage are 1790 kg/year".<ref name="Hutchinsonet.al.2011" /> Although these results were much higher than previous estimations, the authors noted that these results significantly lowered the great difference between its actual growth rate and the one which would be expected of an animal of its size.<ref name="Hutchinsonet.al.2011" /> The sudden change in growth rate at the end of the growth spurt may indicate physical maturity, a hypothesis which is supported by the discovery of medullary tissue in the [[femur]] of a 16 to 20-year-old ''T. rex'' from Montana ([[Museum of the Rockies|MOR]] 1125, also known as [[B-rex]]). Medullary tissue is found only in female birds during ovulation, indicating that B-rex was of reproductive age.<ref name="schweitzeretal2005">{{Cite journal |last1=Schweitzer |first1=M. H. |last2=Wittmeyer |first2=J. L. |last3=Horner |first3=J. R. |s2cid=30264554 |date=2005 |title=Gender-specific reproductive tissue in ratites and ''Tyrannosaurus rex'' |journal=Science |volume=308 |issue=5727 |pages=1456–60 |bibcode=2005Sci...308.1456S |doi=10.1126/science.1112158 |pmid=15933198 |url=http://doc.rero.ch/record/14767/files/PAL_E1895.pdf |archive-date=March 12, 2023 |access-date=November 23, 2022 |archive-url=https://web.archive.org/web/20230312052627/https://doc.rero.ch/record/14767/files/PAL_E1895.pdf |url-status=live }}</ref> Further study indicates an age of 18 for this specimen.<ref name="LW08">{{Cite journal |last1=Lee |first1=A. H. |last2=Werning |first2=S. |year=2008 |title=Sexual maturity in growing dinosaurs does not fit reptilian growth models |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=2 |pages=582–587 |bibcode=2008PNAS..105..582L |doi=10.1073/pnas.0708903105 |pmc=2206579 |pmid=18195356|doi-access=free }}</ref> In 2016, it was finally confirmed by Mary Higby Schweitzer and Lindsay Zanno and colleagues that the soft tissue within the femur of MOR 1125 was medullary tissue. This also confirmed the identity of the specimen as a female. The discovery of medullary bone tissue within ''Tyrannosaurus'' may prove valuable in determining the sex of other dinosaur species in future examinations, as the chemical makeup of medullary tissue is unmistakable.<ref>{{Cite journal |last1=Schweitzer |first1=M. H. |last2=Zheng |first2=W. |last3=Zanno |first3=L. |last4=Werning |first4=S. |last5=Sugiyama |first5=T. |date=2016 |title=Chemistry supports the identification of gender-specific reproductive tissue in Tyrannosaurus rex |journal=Scientific Reports |volume=6 |issue=23099 |page=23099 |doi=10.1038/srep23099 |pmc=4791554 |pmid=26975806|bibcode=2016NatSR...623099S }}</ref> Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes.<ref name="ericksonetal2006">{{Cite journal |last1=Erickson |first1=G. M. |last2=Currie |first2=P. J. |last3=Inouye |first3=B. D. |last4=Winn |first4=A. A. |s2cid=34191607 |date=2006 |title=Tyrannosaur life tables: an example of nonavian dinosaur population biology |journal=Science |volume=313 |issue=5784 |pages=213–7 |bibcode=2006Sci...313..213E |doi=10.1126/science.1125721 |pmid=16840697}}</ref>

[[File:Ontogram of Tyrannosaurus rex.png|thumb|Diagram showing growth stages]]
An additional study published in 2020 by Woodward and colleagues, for the journal ''Science Advances'' indicates that during their growth from juvenile to adult, ''Tyrannosaurus'' was capable of slowing down its growth to counter environmental factors such as lack of food. The study, focusing on two juvenile specimens between 13 and 15 years old housed at the Burpee Museum in Illinois, indicates that the rate of maturation for ''Tyrannosaurus'' was dependent on resource abundance. This study also indicates that in such changing environments, ''Tyrannosaurus'' was particularly well-suited to an environment that shifted yearly in regards to resource abundance, hinting that other midsize predators might have had difficulty surviving in such harsh conditions and explaining the niche partitioning between juvenile and adult tyrannosaurs. The study further indicates that ''Tyrannosaurus'' and the dubious genus ''Nanotyrannus'' are synonymous, due to analysis of the growth rings in the bones of the two specimens studied.<ref>{{cite journal|last1=Woodward |first1=Holly N |last2=Tremaine |first2=Katie |last3=Williams |first3=Scott A |last4=Zanno |first4=Lindsay E |last5=Horner |first5=John R |last6=Myhrvold |first6=Nathan |date=January 1, 2020 |title=Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy "Nanotyrannus" and supports ontogenetic niche partitioning in juvenile Tyrannosaurus |volume=6 |issue=1 |pages=eaax6250 |journal=Science Advances |pmid=31911944 |doi=10.1126/sciadv.aax6250 |pmc=6938697 |bibcode=2020SciA....6.6250W }}</ref><ref>{{cite web|url=https://www.nationalgeographic.com/science/2020/01/nanotyrannus-fossils-really-are-teenage-t-rex-dinosaur-growth-rates/ |archive-url=https://web.archive.org/web/20200101225622/https://www.nationalgeographic.com/science/2020/01/nanotyrannus-fossils-really-are-teenage-t-rex-dinosaur-growth-rates/ |url-status=dead |archive-date=January 1, 2020 |last=Greshko |first=Michael |date=January 1, 2020 |title=These sleek predatory dinosaurs really are teenage T. rex |work=[[National Geographic]] |access-date=January 2, 2020}}</ref>

Over half of the known ''T. rex'' specimens appear to have died within six years of reaching sexual maturity, a pattern which is also seen in other tyrannosaurs and in some large, long-lived birds and mammals today. These species are characterized by high infant mortality rates, followed by relatively low mortality among juveniles. Mortality increases again following sexual maturity, partly due to the stresses of reproduction. One study suggests that the rarity of juvenile ''T. rex'' fossils is due in part to low juvenile mortality rates; the animals were not dying in large numbers at these ages, and thus were not often fossilized. This rarity may also be due to the incompleteness of the [[fossil record]] or to the bias of fossil collectors towards larger, more spectacular specimens.<ref name="ericksonetal2006" /> In a 2013 lecture, Thomas Holtz Jr. suggested that dinosaurs "lived fast and died young" because they reproduced quickly whereas mammals have long lifespans because they take longer to reproduce.<ref name="HoltzLecture2013">{{Cite AV media |url=https://www.youtube.com/watch?v=sqkqkxYGNZc  |archive-url=https://ghostarchive.org/varchive/youtube/20211211/sqkqkxYGNZc| archive-date=December 11, 2021 |url-status=live|title=The Life and Times of Tyrannosaurus rex, with Dr. Thomas Holtz |date=March 19, 2013 |last=Holtz |first=T. R. Jr. |publisher=[[Burke Museum of Natural History and Culture]] |location=Seattle, WA |access-date=October 12, 2013 |orig-date=Lecture held March 8, 2013 |medium=Lecture}}{{cbignore}}</ref> Gregory S. Paul also writes that ''Tyrannosaurus'' reproduced quickly and died young but attributes their short lifespans to the dangerous lives they lived.<ref name="Paul2008Highrisklife">{{Cite book |title=''Tyrannosaurus'', The Tyrant King |last=Paul |first=G. S. |publisher=Indiana University Press |year=2008 |isbn=978-0-253-35087-9 |editor-last=Larson |editor-first=P. L. |pages=307–345 |chapter=Chapter 18: The Extreme Life Style and Habits of the Gigantic Tyrannosaurid Superpredators of the Cretaceous North America and Asia |access-date=September 14, 2013 |editor-last2=Carpenter |editor-first2=K. |chapter-url=https://books.google.com/books?id=5WH9RnfKco4C&pg=PA307}}</ref>