Sexual Selection & Reproduction

Baby tyrannosaurs

Fossils of hatchling tyrannosaurs were relatively small, and clutches were likely large compared to size-scaling trends of modern birds, although not to the degree of modern reptiles. Their bone tissue was remodeled and teeth worn, suggesting precocial offspring with fairly limited parental care. Their growth rate was likely above those of crocodilians, but lower than modern birds and mammals. Overall, their reproductive strategy was intermediate between reptiles and birds.

Study: Longrich, Makovicky, Tokaryk, Cooper, Saitta, Erickson, Szekely, & Snively 2026, Biology

Press Release: Oklahoma State University

Media: Nature Podcast, Discover, Gizmodo, IFLScience, Science Alert, Science & Vie (French), FOCUS Online (German), Geopop (Italian), elDiario.es (Spanish), Infobae (Spanish), Times of India, Phys.org, CNN Greece (Greek), FOX Weather, Naked Science (Russian), Olhar Digital (Portuguese), Descopera (Romanian)

Art: Raul Martin

Coauthor video summary
Coauthor interview with Dr. Brian Curtice of Fossil Crates

Intra- & inter-species variation in dinosaur growth

Models of dinosaur growth can overlook key variables. A growth-only bias may appear in the analysis and presentation of data itself. Recent work shows growth models can contradict evidence of inter-species variation. Sexual differences are also an underappreciated form of dinosaur variation.

Study: Saitta 2026, Cretaceous Research

Launch Interactive T. rex Growth Simulator V.2 (Unpublished; Added parameters; Improved method; Debugged)

Built with Claude AI

Lessons from the ‘replication crisis’ in science

Simulated data show that effect-size statistical approaches, rather than significance testing, are more reliable when searching for sexual dimorphism in the fossil record. Dinosaurs show many lines of evidence for sexual variation.

Study: Saitta et al. 2020, Biological Journal of the Linnean Society (Invited Review)

Press Release: Field Museum

Media: Discover Magazine, briefly discussed in Cosmos

My Summary: The Conversation

Presentation: Palaeontological Association, 2017

Simulated growth data of hypothetical populations of alligators (males in blue, females in red). As sexual dimorphism (black curves) increases in magnitude, our method (green curves) can estimate this variation without knowing individuals‘ sex. Although we overestimate dimorphism magnitude in populations with minimal sexual variation, we can still distinguish between populations with low vs. high sexual variation.
Presentation recorded by Palaeocast at the 61st Annual Meeting of the Palaeontological Association, Imperial College London, UK, in 2017.

Independent reproducibility of Saitta (2015):

Dimorphism can be established in at least three species of dinosaurs and a sauropterygian, namely Allosaurus fragilis, Hesperosaurus mjosi, Protoceratops andrewsi, and Keichousaurus hui” – Motani (2021)

Hypothesized sexual dimorphism in the plates of Hesperosaurus mjosi (Saitta 2015)

Stegosaur plate variation likely correlates with sex

Hesperosaurus mjosi plates show variation between wide and narrow varieties that cannot be solely explained by factors such as growth/development, ‘hidden’ species, or head-to-tail variation and is, therefore, consistent with sexual dimorphism.

Study: Saitta 2015, PLoS One

Press Release: University of Bristol

Media: BBC, The Guardian, Los Angeles Times, Popular Science, The Economist, Vice, NBC News, ABC News, IFLScience, International Business Times, Live Science, World Science Festival, Der Spiegel (German), The Naked Scientists, Princeton University, briefly discussed on I Know Dino (Episode 38 “Stegosaurus“ & Episode 50 “Hesperosaurus“) & PBS Eons, featured by Vladimír Socha, Martina Ležáková, & editors of ABC magazine (2022) Dinosauři Czech News Center (Czech), briefly discussed by Joël Ignasse (2025) Sur les traces du nouveau T. rex Eyrolles (French)

Presentation: Palaeontological Association, 2015

Presentation recorded by Palaeocast at the 59th Annual Meeting of the Palaeontological Association, Cardiff University and Amgueddfa Cymru – National Museum Wales, UK, in 2015.
Hypothetical sexual dimorphism in Centrosaurus (Art: James Ormiston)