Havers‐Halberg Rhythms in Canis familiaris

Our objective in this study is to assess variation in Havers‐Halberg oscillation (HHO) periodicities among domestic dogs ( Canis familiaris ). The HHO is hypothesized to be a biological rhythm controlled by the hypothalamus, acting as a central coordinator of multiple life history variables including body size, metabolic rate, and reproductive output. The periodicity of this clock varies among and within species, but not within individuals. It has been studied in detail among primates, though very little among other mammals. In monkeys and apes, the periodicity is strongly correlated with body mass, but lemurs show a restricted range of variation in HHO rhythm despite having a comparable range of body sizes. This raises the question of how HHO rhythms and body size evolution can be decoupled. Domestic dogs represent a potential means to shed light on this area, due to their wide range of body sizes accompanied by low variation in life history variables such as lifespan. Based on this, we hypothesize that dogs will resemble lemurs in HHO variation. To test this, we sectioned teeth of 11 previously deceased Canis familiaris from a university collection, representing a variety of breeds. Teeth preserve a histological marker of the HHO rhythm in the form of growth increments known as striae of Retzius, and the HHO rhythm can be quantified by counting the number of 24‐hour growth lines between them. Teeth were cleaned, embedded in acrylic resin, thin sectioned, and mounted to microscope slides following standard dental histology protocols. They were then imaged using circularly polarized light microscopy so that their growth increments could be measured to assess HHO periodicity. Next, HHO periodicity was regressed against body mass estimate data acquired via estimation from mandibular length measurements. As predicted, domestic dogs have a low range of HHO periodicity variation for their body size range, falling between 4 and 6 days (with a modal value of 5). This suggests that dog body size variation is attributed to mechanisms beyond the HHO; conversely, the narrow range of variation in HHO can potentially explain the lack of variation in life history factors such as lifespan among dogs. Since body mass variation among domestic dogs is postulated to result from a mutation in IGF‐1, this provides a possible explanation for the high body mass / low HHO variation seen in lemurs as well. Support or Funding Information Project funded by the 2010 Max Planck Research Award, endowed by the German Federal Ministry of Education and Research to the Max Planck Society and the Alexander von Humboldt Foundation in respect of the Hard Tissue Research Program in Human Paleobiomics.