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 the current gene pool of a species results from the adaptations of its ancestors that allowed it to survive successive changes in its environment.  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.  It is mainly a synthesis of two previously unconnected lines of thought.  One is the work of Galis and her colleagues (most recently Galis et al. (2022)

on the deleterious effects of slight alterations in the positioning of the cervical/thoracic boundary in the mammalian lineage.  They propose that these arise during a period of development when there is the greatest interaction between different parts of the embryo, the phylotypic stage, thus being prone to errors during organogenesis than earlier or later stages where development is more modular.  Lovegrove has also proposed that endothermy is this lineage arose through a series of steps either in response to changes in environmental temperatures or to take advantage of benefits of increased metabolic rate or faster speed.  This narrative proposes that many of these changes involved interactions between the neck and thoracic regions that were selected for despite being associated initially with deleterious effects.  It is also proposed that these deleterious effects were minimized by more homeothermic behavior in at least part of the phylotypic stage.

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       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 vertebrates, there is considerable variability in the number of vertebrae within each of the major groups (cervical, thoracic, etc,) with the exception of the mammals, where almost all have exactly 7 cervical vertebrae with some, but limited variability more posteriorly.  These constraints likely are related to the evolution of endothermy and later homeothermy in the lineage from which mammals arose.

       Prior to the advent of endothermy, embryonic development had to cope with variability of the environmental. temperature, which may have even increased with the advent of the amniote egg that allowed development to occur in a terrestrial rather than aqueous environment.  This situation may have begun to change in the therapsid lineage when larger size produced a stabler maternal temperature.  Possible evolutionary benefits could were then made possible by brooding behavior or progressively longer retention in the embryo, ultimately leading to full term gestation in eutherian mammals. Changes would have occurred first in the cervical region since patterning across from anterior to posterior,

       The constraint limiting to cervical vertebral number to seven among the ancestors of mammals appears to have taken place around 250 million years ago (mya) around the time of the Permian/Triassic boundary.  Around this time there was a very large rise in average global temperatures, followed by a slow decrease.  Lovegrove (2019) argued that this and subsequent increases in internal temperatures were selective for faster growth rates, mobility and increased parental care. Since tissues arising in the neck region are involved in head and forelimb movement and oxygen supply (heart and diaphragm), increasing restriction were placed on development process in this region. 

       The more stable temperatures of homeothermy allowed more refined and complex phenotypes.  However, precision may make development prone to production of non-viable progeny by a variety of causes, such as mutations (heritable or somatic), stochastic events, and excessive temperature fluctuations.  This may place further restrictions on or prompt evolutionary changes the organization of developmental systems, such as increased modularity of the neck, and preventing changes to reduce lethality.   The association of cervical ribs with developmental lethality in humans and some other mammals likely was the result.     

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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 stage.

     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.