Evolution of Homeothermy in Mammals
All mammals and birds are endothermic, by generating and retaining heat internally they can achieve core body temperatures higher than the ambient temperature. This contrasts with other vertebrate classes where at most only a few species have this ability. All mammals are at least sometimes homeothermic, maintaining body temperature within a narrow range of a few degrees. Some mammals can become heterothermic, letting their body temperature decrease towards the ambient temperature by a regulated process, called (daily) torpor if it lasts less than 24 hours, and hibernation for longer periods. This site contains two "explorations," with each examining homeothermy in mammals from a novel perspective.
This site contains numerous hyperlinks between related material on different pages. Italics will be use when a more extensive discussion of the relationship is useful. Supplementary Material pages contain background material for the explorations or contain collected data for eventual use.
The Goldilocks Hypothesis
This hypothesis states that all mammals must be homeothermic during part of embryonic development . This requirement may have played a roll in the origin of the evolutionary constraint which results in almost all mammalian species having exactly 7 cervical vertebrae.
Circadian Clocks and Sloppy Models
Torpor in mammals utilizes the circadian clock mechanism to resume normal daily behavior after the return to the normal temperature. But how does this clock mechanism itself not be altered by the changing temperatures. Before looking in detail at mammalian circadian systems, it may be useful to examine the circadian mechanism in a blue-green algae, a prokaryote, with some intere sting properties.
This system which, among other things, regulates daily photosynthesis, continues 24 hour long oscillations even in the dark, and over a range of temperatures, i.e., it is temperature-compensated. A subset of this system also shows temperature-compensated behavior. It is, to my knowledge, the only temperature-compensated system yet to be studied a powerful technique used to study system organization, sloppy modeling.
Modeling suggests that the parameters that describe the actions of the molecular components and their interactions necessary to accomplish a particular cellular function have a property referred to as sloppiness. Some parameters can only vary slightly without affecting function, but most of the other parameters can vary more widely. This property presumably is favored during evolution because it makes the systems more robust by making many mutations less deleterious. Given this property, the ability to be homeothermic may have allowed more complex interactions between systems to evolve, such as the ability to be selectively heterothermic.
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The Seven Cervical Vertebrae Constraint
Pathologies Associated with Cervical Ribs
Pathologies Associated with Heterothermy
The Fossil Record
Development- Gastrula through Pharyngula stages
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