Evolutionary drivers of reproductive output variation among amniotes, and the origins of large offspring in birds

Journal article

Birds and mammals mostly produce clutches with small numbers of large individual offspring compared to non-bird reptiles, including dinosaurs on the bird-stem lineage. Existing hypotheses for this variation propose links between relative offspring size and large brains or high metabolic rates, but remain incompletely tested. We characterize the allometries of reproductive output traits (clutch mass, clutch size and individual offspring mass) and evaluate their correlates using phylogenetic regressions across 2857 living and extinct amniote species. Across amniotes, species with larger relative brain sizes have larger individual offspring, fewer offspring or both. This signifies a higher maternal investment per offspring in large-brained species, exemplified by birds and mammals. In dinosaurs, large egg sizes evolved shortly before the origin of the bird crown group, coinciding with an evolutionary increase in relative brain mass and suggesting that evolutionary increases in egg/neonate size evolved due to a requirement for greater investment per offspring with increasing brain size. These egg size increases may explain changes to dinosaur nest structure and pelvic anatomy along the bird-stem lineage. Our results provide insights into the evolutionary pressures shaping reproductive strategies and brain size across amniotes, underscoring the significance of these traits in the broader context of amniote diversification.

Authors: Lechki, SC; Benson, RBJ

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: The Royal Society
• Journal: Royal Society Open Science
• Volume: 13
• Issue: 6
• Article number: 251708
• Publication date: 2026-06-24
• Acceptance date: 2026-04-01
• DOI: 10.1098/rsos.251708
• EISSN: 2054-5703
• ISSN: 2054-5703
• Language: English
• Keywords: brain size evolution; energetic investment; dinosaur–bird transition; scaling relationships; dinosaur eggs; reproductive output; parental care