The question of dinosaur endothermy — warm-bloodedness — is a vexed one. One of the cases against is derived from the 'lines of arrested growth' evident in the creatures' bone histology, indicating seasonal slowdown or cessation of growth. Similar features are seen in the bones of reptiles and amphibians, leading to the suggestion that these lines indicate ectothermy. Endotherms, it was assumed, grow throughout the year. The problem was that the endotherms most commonly studied — birds and small mammals — tend to grow to full adult size within a season, leaving little scope for a seasonal hiatus in growth. Now, Meike Köhler et al. present a comprehensive study of wild ruminants — large endothermic mammals — living in a wide variety of climate regimes, from tropical to polar. In all of the animals studied, including deer, antelope and reindeer, there is evidence for arrested growth during the unfavourable season. This precludes the use of lines of arrested growth as an argument in support of ectothermy in dinosaurs.
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