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The nocturnal bottleneck and the evolution of activity patterns in mammals [2013]

In 1942, Walls described the concept of a ‘nocturnal bottleneck’ in placental mammals, where these species could survive only by avoiding daytime activity during times in which dinosaurs were the dominant taxon. Walls based this concept of a longer episode of nocturnality in early eutherian mammals by comparing the visual systems of reptiles, birds and all three extant taxa of the mammalian lineage, namely the monotremes, marsupials (now included in the metatherians) and placentals (included in the eutherians). This review describes the status of what has become known as the nocturnal bottleneck hypothesis, giving an overview of the chronobiological patterns of activity. We review the ecological plausibility that the activity patterns of (early) eutherian mammals were restricted to the night, based on arguments relating to endothermia, energy balance, foraging and predation, taking into account recent palaeontological information. We also assess genes, relating to light detection (visual and non-visual systems) and the photolyase DNA protection system that were lost in the eutherian mammalian lineage. Our conclusion presently is that arguments in favour of the nocturnal bottleneck hypothesis in eutherians prevail.

Evidence has accumulated that the visual and non-visual systems of photoreception in eutherian mammals in the Mesozoic era showed changes characteristic for a nocturnal lifestyle. The specific, and permanent loss of UV protection (e.g. by photolyases) is an additional argument for an initial restricted exposure to solar radiation in eutherian animals. Endothermy, although not limited to Mesozoic mammals, allowed an active use of the night, in combination with a small body size. Uncertainties in the radiation of eutherian species within or after the Mesozoic era are manifold and questions remain with regard to later developments of regaining diurnal function in behaviour and colour vision. The palaeontological search will continue and reveal new insights, as was the case in the past decade. Further comparative analysis, implicating other tetrapod taxa such as reptiles and birds, and above all marsupial mammals, will be rewarding to further explore the impact of what we see as the very probable occurrence of a Mesozoic nocturnal bottleneck of eutherian mammals.

 

The evolution of mechanisms involved in vertebrate endothermy (2020)

Endothermy, i.e. the endogenous production of metabolic heat, has evolved multiple times among vertebrates, and several strategies of heat production have been studied extensively by physiologists over the course of the twentieth century. The independent acquisition of endothermy by mammals and birds has been the subject of many hypotheses regarding their origin and associated evolutionary constraints. Many groups of vertebrates, however, are thought to possess other mechanisms of heat production, and alternative ways to regulate thermogenesis that are not always considered in the palaeontological literature. Here, we perform a review of the mechanisms involved in heat production, with a focus on cellular and molecular mechanisms, in a phylogenetic context encompassing the entire vertebrate diversity. We show that endothermy in mammals and birds is not as well defined as commonly assumed by evolutionary biologists and consists of a vast array of physiological strategies, many of which are currently unknown. We also describe strategies found in other vertebrates, which may not always be considered endothermy, but nonetheless correspond to a process of active thermogenesis. We conclude that endothermy is a highly plastic character in vertebrates and provides a guideline on terminology and occurrences of the different types of heat production in vertebrate evolution.

Endothermy, i.e. the ability for an organism to generate body heat through a specifically dedicated metabolic activity, is considered to have appeared at least twice in the evolution of vertebrates, in the mammalian and avian lineages

vertebrate endothermy consists of multiple strategies that appeared independently through time

Indeed, multiples ‘endothermies’ have evolved separately in the vertebrate lineage, each of them with its own adaptations to vastly different lifestyles and different pathways to regulate heat production.

we consider that inferences of endothermy in a specific fossil lineage should always include a mention of a specific inferred mechanism, rather than simply defining several taxa as endotherms or ectotherms based on similarity in BMR values, which is imprecise and might be meaningless in a phylogenetic context. Discussions of hypotheses regarding such strategies in such a context have allowed many recent physiological and palaeontological studies to unveil a new, clearer picture of the evolution of vertebrate endothermy, of which this review is only a short summary. Future methodological and conceptual developments will likely lead to important changes in the understanding of such processes in the next few years

 

“has evolved multiple times among vertebrates”?   Wow!

“simply defining several taxa as endotherms or ectotherms based on similarity in BMR values, […] is imprecise and might be meaningless in a phylogenetic context.”

 

Vertebrate palaeophysiology (2020)