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How does the population size affect an individual’s physiology/ behavior?

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Shubhankar Kulkarni
Shubhankar Kulkarni Sep 01, 2020
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Have you performed/ come across any experiment that reports that population size affects some aspects of the individual’s physiology or behaviour? In what way does it affect? If the experiment is performed on animals, how do the results translate to human physiology and behavior?
Creative contributions

“Universe 25” by John B Calhoun

Apoorva Kulkarni Sep 01, 2020
A square closed universe was created with 16 compartments or cells. The universe had nesting areas and migration across the compartments was possible. Four pairs of 48-day-old Balb C mice were introduced to the universe. The mice had ad libitum access to food and water. The first litters were born after 104 days (the mice took time to adjust to each other since they were caged individually before starting the experiment). After this, the population doubling time was about 55 days. After the numbers reached 620 (after five doublings), the population growth rate decreased abruptly to a doubling time of approximately 145 days. The author observed that the mice now born into the universe reached sexual maturity early and reproduced early. Also, the births tended to be concentrated in some areas of the universe (although all areas were equally equipped with food, water, and nests). Social hierarchy was observed with a dominant male in the universe. Eventually, there were 470 mice in the universe that had experienced good maternal care and early socialization. Physical space in the universe was now occupied by 14 social groups. The population grew exponentially. Eventually, an unusually large number of young became adults and they contested for dominance. Males who failed withdrew physically and psychologically and became inactive. They disconnected interaction with their associates. Withdrawn males sometimes attacked each other, and the victim did not flee. Gradually, territorial defense and the area defended declined. Thus, nursing females were more exposed to invasion. So they became aggressive too. The young were often attacked and forced to leave the home area before normal weaning. Maternal behavior became disrupted. Conception declined and resorption of fetuses and fetal mortality increased. The societal organization collapsed. Deaths started exceeding births. After almost four years, there were only 122 mice (22 males, 100 females) left. Essentially, all young were prematurely rejected by their mothers. In a parallel, 2-cell universe, the same scenario was observed. Lesser females had viable embryos (detected upon autopsy), they died early, and fewer became pregnant. Male counterparts to these non-reproducing females were called the 'beautiful ones' by the author. These males never engaged in sexual encounters or fighting. Soon, the reproductive capacity of the universe terminated. The author mentions that these results might replicate when the usual causes of mortality reduce in a mammalian social-group-forming species. As crowding ensues, the young compete for the social roles with the older members. For a mouse, the most complex behaviors involve courtship, maternal care, territorial defense, and hierarchical social organization. The failure of healthy social interactions leads to the emergence of autistic-like creatures, incapable of these complex behaviors. The species then, eventually, die out. For an animal so complex as man, a comparable sequence of events might also lead to extinction. If there is competition for role fulfillment, only violence and disruption of social organization would be observed. Individuals born under these circumstances will lose the most-complex human behaviors. Reference: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1644264/pdf/procrsmed00338-0007.pdf
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The r and k reproductive strategies

Apoorva Kulkarni Sep 09, 2020
Organisms living in stable environments tend to produce few but expensive offspring. On the other hand, organisms living in unstable environments tend to produce many but cheap offspring. By expensive, I mean the cost involved in making the offspring and that involved in nurturing (parental investment) it. As an example, humans make expensive offspring who require care for years after birth. On the other hand, insects lay thousands of eggs that are cheap and mostly do not invest in parental care. The strategy involving producing expensive offspring is called the k-strategy and the other, r-strategy. Both strategies aim for the propagation of the species using the survival of the progeny. In stable environments, it is better to produce in a small number of offspring and invest more in their survival. In unstable environments, organisms aim at producing a large number of offspring, thereby increasing the chances that at least some of them survive. Refer to Figure 1.A.2 here - https://fdvbio.wordpress.com/2019/02/16/population-ecology/ In the population growth equation, “r” stands for growth rate and “k” stands for the carrying capacity of the environment. The terms "r-" and "k-strategies" describe the population growth strategies of the two kinds of organisms. In an unstable environment, population growth cannot reach the carrying capacity where density-dependent factors come into play. The population, thus, grows exponentially with the reproductive rate “r”. In a stable environment, the population of a k-strategist is near the carrying capacity “k”. Therefore, evolutionarily, the population density does affect the reproductive strategy (physiology) of the organism. Reference: http://www.bio.miami.edu/tom/courses/bil160/bil160goods/16_rKselection.html#:~:text=The%20two%20evolutionary%20%22strategies%22%20are,and%20live%20in%20stable%20environments.
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Role of population density in diseases?

Apoorva Kulkarni Sep 09, 2020
A case study of insulin resistance (type 2 diabetes). Even within a generation, the environmental factors or the population density might change. For the survival of the species, the organism, then, needs to switch between the “r-” and “k-” strategies. For an organism to have the plasticity to make that switch, it should have the ability to shift between two physiological states. Do organisms have such ability? It is known that insulin resistance is associated with decreased ovulation, decreased spermatogenesis, and subnormal sexual desire and function, all of which can lead to a reduced number of offspring. On the other hand, it is also known that insulin resistance reduces glucose uptake by the mothers, thereby providing more nutrition to the fetus. Evidence suggests that diabetic mothers have heavier babies. Also, contraceptive treatments are associated with insulin resistance. Taken together, if the number of offspring is reduced, the investment in the residual offspring should go up, and insulin resistance is the mechanism of doing so. Also, if insulin resistance sets in an organism, it should be accompanied by a reduced number of offspring. These findings suggest that population density can affect an organism’s reproductive capacity. Organisms do have the plasticity to switch between the two strategies. It is obvious that organisms do have limitations to the extent of this plasticity; the number of human offspring cannot be comparable to that of an insect. Nonetheless, humans can make that behavioral switch. This switch comes with consequences (insulin resistance). Type 2 diabetes, therefore, can be explained, although partly, by the changes in population density. Reference: Watve MG. Chapter 9, Doves, Diplomats and Diabetes: A Darwinian Reinterpretation of Type 2 Diabetes and Related Disorders. New York, Heidelberg, London: Springer, 2013. ISBN: 9781461444084.
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