I first became interested in homeothermy by a rather indirect path. I was teaching a course to medical residents that had a section on carcinogenesis. While researching new developments, I read about a strange anatomical anomaly observed for some childhood cancers: the incidence of small ribs on one or both sides of the 7th cervical vertebra is several times that observed in adults, which is around 1 percent. This anomaly is also observed in about 1/4 of autopsies of miscarriages and neo-natal deaths.
This anomaly has been studied in considerable detail by Galis and her colleagues and others, and the mortality associated with it is likely linked to an evolutionary constraint in mammals. An evolutionary constraint is said to be present when some characteristic is constant among a related group of organisms when it seems evolution would have produced more variety. Of the around 5500 species of mammals all but a handful have 7 cervical vertebrae. This contrasts with the variability observed among birds where the number ranges from ? to as many as 25 in swans, Mammals also show considerable variation in the number of vertebrae in the other regions of the spinal column, thoracic, lumbar, sacral, and tail.
Is there some characteristic that is common to most mammals, but not to birds, that might be linked to this evolutionary constraint? It would have to be something related to events happening at the same time that anterior/posterior patterning determining the boundary between the cervical and thoracic spinal column during embryonic development. This boundary is determined by the most anterior expression of the Hox6 genes, which are first expressed during the gastrula stage of development. In all mammals this stage occurs within the mother's body, so if the mother is homeothermic, so would be the embryo. In birds, eggs are laid well before this stage, and a parent sits on them to raise the embryo temperature above ambient temperature. Even when the parent is homeothermic, we would expect bird embryos to be more heterothermic than mammalian embryos.
I have called the idea that for proper development most mammals must maintain homeothermy for some period during embryonic development the Goldilocks Hypothesis. This would likely begin sometime during gastrulation and extend through the formation of organs whose precursors arise near the cervical/thoracic boundary. This period also likely overlaps the phylotypic stage of development, when there are extensive interactions between different regions of the embryos and extensive cell migrations. If the Goldilocks Hypothesis is true, we would not expect torpor to occur during the critical period, at least not without severe effects on the offspring. With this in mind, I carried out a survey of field and labratory reports of torpor in mammals during gestation, with the goal of determining at what stage of embryonic development the torpor occurred.
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