Cyclical Metapopulation Mechanism Hypothesis: Generationally varying birth cohort specic hypothalamic hormone levels create physiological, cognitive, and behavioral dierences in cyclical animal populations, including human populations Text: Janne Miettinen | PDF-version | ResearchGate: project page Work-in-progress | First draft: Oct 2018 | Last update: Oct 2, 2021 Abstract Known animal population cycles are from approximately 4 years (small rodents) to 38 years (moose), but why the cycles exist remains unknown after a century of research. Even though the length of the cycle varies between species, the cycles of different species all share distinctly similar phase-dependent variations to average physiological and behavioral properties of individual animals, including size, age of reproductive maturity, and aggression. It is presented here that since hormone levels modulate the average physiological and behavioral properties in animals, the cyclical animal populations are a manifestation of a multiannual population-wide hormone cycle. This hypothesis details how the multiannual hormone cycle defines the average birth cohort specific levels of hypothalamic hormones of a cyclical population, thus creating generations with differing average hormone levels. The mechanism creating the generational hormone cycles is suggested to be a neural mechanism that accelerates evolution by varying hormone levels of succeeding generations, and that this occurs in synchrony between populations on a metapopulation level, as the cycles can be in synchrony over a 1000 mile range. The population cycles have a migration phase that creates new populations, and because the migration phase also increases gene flow between populations, the population cycles are viewed as a combination of recognized evolutionary mechanisms; the cycles are presented to be a holistic evolutionary metapopulation mechanism that accelerates evolution. Human populations are also presented to undergo a generational hormone cycle, and that the length of a human population cycle is approximately 80 years. Statistics from human populations are used to establish that the same generational hormone cycle exist in human populations as in other cyclical animal populations. A historical framework, the Strauss-Howe generational theory, details an 80 year long generational cycle repeating for centuries in the US population: four 20 year generations, with each generation having their own typical behavioral traits. This hypothesis presents evidence that the Strauss-Howe generational theory is in fact a description of a generational hormone cycle in the US population as well as in many other nations on the Northern hemisphere, where also the other cyclical animal populations exist. Table of Contents: 1 Hormones and cycles 1.1 Animal population cycles 1.2 Evolutionary benefits of the cyclical metapopulation mechanism 1.3 Cyclical human populations & review of contents 2 Generational history and hormone levels 2.1 Generational hormone theory 2.2 The Strauss-Howe generational theory 2.3 Historical oxytocin levels … 2.3.1 Oxytocin and parenting intensity … 2.3.2 Oxytocin and breastfeeding rates … 2.3.3 Oxytocin and maternal age … 2.3.4 Oxytocin and divorce rates … 2.3.5 Oxytocin and alcohol consumption … 2.3.6 A model of generational oxytocin levels 2.4 Historical dopamine and vasopressin levels … 2.4.1 Dopamine levels … 2.4.2 Group coherence and territoriality 2.5 Generational social hormone levels … 2.5.1 A model of generational social hormone levels … 2.5.2 Social hormones and political ideology 2.6 Other hypothalamic hormone levels … 2.6.1 Sex hormone levels … 2.6.2 Growth and thyroid hormone levels 3 Social hormones and group behavior 3.1 Social hormones and societal trends 3.2 Social hormones and societal group behavior 3.3 Neural in-group vs. out-group separation 3.4 Social hormones and populist nationalism 4 Group division and conflict 4.1 Group division 4.2 Societal paths of tightening group coherence 4.3 In-group empathy and scapegoating of the out-group 5 Initial conclusions 5.1 Possible societal trajectories 5.2 A review of societal actions 5.3 Unresolved questions References 1 Hormones and cycles 1.1 Animal population cycles Many animal species have population cycles that are very repetitive. For example, the length of population cycles are about 4 years for small mammals like lemmings and voles, 9 years for larch budmoth, 10 years for snowshoe hare and forest Lepidoptera, and 35 years for moose. (Myers, 2018​1)​ (Wang et al., 2009​2)​ (Krebs, 2010​3​)(Krebs et al., 2014​4​)(Hansson & Henttonen, 1985​5)​ There are over 1000 years of records of larch budmoth cycles that demonstrate the consistency of these cycles. (Esper et al., 2006​6​) Below is a statistic from the last century as an example of how highly regular the cycles can be in both length and amplitude. Values 0.001-1000 indicate larch budmoth population density. (S) Cyclical populations are very common in nature according to this study: “…we analyzed nearly 700 long (25+ years) time series of animal field populations, looking for large-scale patterns in cycles. Nearly 30% of the time series were cyclic.” … “The incidence of cycles varied among taxonomic classes, being most common in fish and mammal populations. Fully 70% of the fish and mammal species comprised at least one cyclic population…” (Kendall et al., 1998/2002​7)​ Predators, limited food supply, and diseases have been suggested to drive the animal cycles, but because the cycles do manifest even when these factors are excluded, it is probable that something else is creating and controlling the length of these cycles: “…lemmings on islands are known to be without predators and yet still undergo a 4 year population cycle.” (Ginzburg & Colyvan 8​, p. 79) “Numerous experiments have been done in attempts to delay the decline or stop the population cycles of lemmings and voles by feeding or excluding predators. These have had mixed results and Krebs concluded that predators can ‘modify’ population cycles, but that predator removal cannot stop cyclic dynamics. Similarly, food addition experiments can modify vole densities but not drive cycles.” … “Overall, experimentally stopping or starting population cycles has proven to be largely impossible.” (Myers, 2018​1)​ Increasing stress levels through increasing population density has also been used as a theory to explain the cycles, but this idea too has been disproven: “In 1967, Dennis Chitty proposed that larger and more aggressive voles would be selected for in increasing and high densities, and smaller voles with delayed reproductive maturity in low densities. The ‘Chitty Hypothesis’ predicted that variable selection would lead to a genetic shift over the 3 to 4 year cycle of voles. However, the genetic shifts predicted by this hypothesis have not been observed and the levels of heritability of traits required for the shift were unrealistically high.” (Myers, 2018​1)​ The end result is, that after a century of research into animal population cycles, not even one hypothesis exists that explains even one species’ cycles, while taking into account the findings made by Krebs and others about the environmental factors not starting or stopping the cycles, resulting in a situation where all explanations and theoretical models have been severely lacking in evidence and/or repeatability. (Andreassen et al., 2020​9​)(Oli, 2019​10​)(Myers, 2018​1)​ (Oli, 2003 11​) Since environmental factors cannot explain the cycles, what does explain the recurring appearance and characteristics of the cycles regardless of the species or the environment? It is presented here that the real explanation for the cycles lies within the endocrine system that controls animal hormone levels and therefore modulates both their behavior and biological traits throughout the cycle. Only a biological mechanism that controls the generational hormone levels can explain the populations cycles in a way that is not dependent on environmental factors like predators or food supply, while giving answers to all of the previously unexplained questions regarding the cycles. It is important to note that the term ‘ hormone levels‘ used throughout this text means that the hypothalamic neurons that secrete hormones are either small and inefficient or large and more efficient, like is presented in the lemming and vole studies below. In addition, the term ‘hormone’ will be used throughout the text, even though some of the same molecules also work as neurotransmitters in the brain, where the levels of these neurotransmitters modulate behavior. (S) Two Russian neurobiological studies have focused on the generational changes to the endocrine system in voles and lemmings during their four year population cycles, and these studies state that there are large variances between generational hormone levels, including dopamine, oxytocin, vasopressin, and other hormones secreted by the hypothalamus. (Arshavskaya et al., 1989​12/​ PDF)(Vladimirova et al., 2006​13​) This is due to generational changes/differences in the parts of the hypothalamus that produce and secrete hormones. These two studies will be used as a template for the suggested human generational hormone cycle. Other studies regarding cyclical populations have largely made similar findings using external measurements such as larger testicles during the increase phase and higher ACTH/cortisol levels during the decline phase. (Sheriff et al., 2011​14​) Below is a key excerpt from the mentioned lemming study as examples displaying