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.

     My interest in homeothermy resulted from learning two facts which likely seemed to be connected, but in which the connection remains obscure.  One is the association in humans of fetal and neo-natal deaths with a minor anatomical anomaly, small ribs on the 7th cervical vertebrae.  The other that nearly all mammalian species have exactly 7 cervical vertebrae, in contrast to birds where the number is highly variable.

      A small rib on the last cervical vertebrae would seem to be a minor, even trivial, shift in anterior/posterior patterning.   That this minor shift was even noticed may be because it occurs at the position of a dramatic change in the segmentation of the vertebrae, between the ribless neck and rib-bearing thoracic segments.

      Dawkins (1988) suggested that groups of animals that are highly segmented, the annelids, arthropods, and vertebrates, may be more "evolvable" than other groups.  In going from the anterior to posterior, each segment differs structurally and functionally from its neighbors, except at a few positions where there are large changes.  The Hox genes play an important role in anterior/posterior (A/P) patterning in diploblastic animals.  Two features of the Hox genes are especially notable in vertebrates.  Like in the case of arthropods, the position of the Hox genes in chromosomes exactly matches their position of expression along the A/P axis.  However, expression of each Hox gene is controlled by different genes located elsewhere in the genome.  In vertebrates, the timing of the expression of Hox genes occurs according to their position along the chromosome as they are progressively become available for transcription.  This expression begins during the gastrulation phase of embryonic development.

      The assembly of this site was principally inspired by two authors.  The late Barry Lovegrove in his book "Fires of Life: Endothermy in Birds and Mammals (2019)" thoroughly covered the evolution of endothermy in terms of thermal physiology, anatomy, and behaviors of organisms.  Richard Dawkins, most recently (2024), has emphasized that although it is the organism that survives and reproduces, it is the gene pool of the species that evolves.  In this site I also want to also explore how the organization of the interactions between the gene products results in homeothermic behavior.

       Universal homeothermy likely arose much earlier in the mammalian lineage than in the bird lineage.  The embryonic temperature in mammals is also likely to be closer to that of the mother than would be case of birds where the parent(s) incubate the egg.  The following narrative explores the hypothesis that these two observations may be important to the evolution of homeothermy.

        I call it a narrative because at the moment it is just a story, plausible to me, but that may just reflect my relative ignorance of the subjects involved.  The aim of the site is to see if further examination will confirm, reject, or require refinement of parts or all of the narrative.

 

       At the time of their divergence from the cephalochordates, the ancestors of vertebrates were thought to have to have diploid sets of 13 distinct Hox genes.  Prior to the first vertebrates, it is thought that the whole genome underwent two rounds of duplication.  Some of the redundant genes were lost, but in all vertebrates each of the original 13 is present in from 2 to 4 somewhat different sequences (each with two possibly different alleles) as they each evolutionarily diverged.  This increased number of genes gave the possibility of complex A/P patterning among vertebrates.

        Among each of the classes of vertebrstes, their is considerable variability in the number of vertebrae within each of the major groups (cervical, thoracic, etc,) with the the exception of the mammals, where almost all have exactly 7 cervical vertebrae.  This evolutionary constraint arose prior the the last common ancestor of mammals.

        The ancestors to mammals may first have development some endothermy prior to the Permian/Triasssic boundary that occurred around 250 mya ago due to their large size (see Background ?),   At about the same time there was a long period of high worldwide temperatures following by a period of gradually lowering temperatures.  Lovegrove (    ) argued that the higher temperatures (and the resultant faster chemical reaction rates) put a premium on fast growth.  The increased demand for oxygen led to more complicated organs that have their developmental origins in what will become the neck and thoracic regions. beginning in gastrulation and ending in a functional circulatory system.  This period also is the time of development when there are the greatest cellular migrations and differentiation controlled which must be controlled by signaling molecules diffusive through large distances of intercellular space.   It contains the Phylotypic Period, thought to be already highly constrained in vertebrates than earlier or later in development.

     These evolutionary changes were also accompanied by longer retention of the developing embryo in the mother's body.  This allowed the take advantage of the increasing ability of the mother to control temperature variations (increased homeothermy) to make the developmental processes more precise.  However, for all systems, not just biological, increased precision may come at a cost of failure due defective parts of operation in changing environment.  In biological systems, a variety of causes such as mutations (heritable or somatic), stochastic events, excessive temperature fluctuation can result in failure.  The association of cervical ribs with developmental lethality may have resulted from such failures.

       The increases in endothermic and homeothermic capacity during the worldwide cooling period occurred at the same time as a shrinkage of body size in the mammalian lineage, making maintaining of heat problematic.  This problem could be somewhat elevated by not being endothermic all the time.  Facultative heterothermy, as still seen by some mammals using torpor, may have evolved at this time.  However, such heterothermy may not have been compatible at some stages during the mammalian life cycle, particularly around the Phylotypic Period,​​​​​​​​​​​

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     This site is made up of background sections and "explorations," which examine homeothermy in mammals from a novel perspective.  For reasons discussed above, these explorations focus on the stages in embryonic development around the time of what has been called the phylotypic period.

     I started the site mainly as a tool to organize what I was learning and to test out new ideas, but I hope others may find it interesting and/or useful.  The explorations contain some "trails with dead ends" either because I found further information that seemed to contradict the idea in question or it didn't seem that any more progress was possible.  I have included them (with a forewarning) because more than once I learned further information that revived an idea.

     This site contains numerous hyperlinks between related material on different pages.  Some links lead a blank page or just enough information to remind me why I thought the topic deserved further examination.  Italics will be use when a more extensive discussion of the relationship is useful.   The web editor I am using does not allow for testing if hyperlinks are working, so some pages will be published in the middle of revisions or updates.