PhD ceremony: mw. K.A. Schubert, 13.15 uur, Academiegebouw, Broerstraat 5, Groningen
Thesis: Breeding on a budget: fundamental links between energy metab
olism and mammalian life history trade-offs
Promotor(s): prof. S. Daan, prof. H.A.J. Meijer
Faculty: Mathematics and Natural Sciences
In poor environments, animals reduce their energy expenditure rather than working harder, concludes Kristin Schubert in her thesis. In a study published in the Journal of Experimental Biology, she showed that in poor environments (experimentally-simulated), female mice make a range of behavioral, physiological and morphological adjustments to cope; all of these led to energy savings. Thus, the mice were not willing to push themselves to higher performance levels – and a higher energy turnover – when they could instead save energy. Many other researchers have been interested understanding physiological limitations on peak energy expenditure (for instance in top athletes) and the consequences of high energy expenditure for the animal’s current and future health. Schuberts results imply that mice avoid such negative consequences.
A companion paper published in Biology Letters showed that one way that mice saved energy was through daily torpor (i.e., lowering their resting body temperature). This was an exciting result, because torpor in mice has never before been elicited in this way. Moreover, controlled hypothermia may have therapeutic applications. Schuberts study design can be used in the future to compare natural torpor in mice to the torpor induced by pharmacological manipulations.
Another finding of Schuberts was that reproductive effort (the energy allocated to offspring) declines in poor quality environments. In a paper in American Naturalist, she provided a quantitative link between environmental quality and energy allocation to offspring. Her data suggest that, in natural populations, an increase in foraging costs would result in a disproportionate decrease in reproductive output. The implication is that even if adult animals have enough food for themselves, they will not invest resources in their young. This could be one reason why populations of animals in poor quality habitats are not viable; individuals survive but do not breed, and the population fails to replace itself.
Schubert also shows that early-life food deprivation does not automatically lead to obesity and disease. In humans, perinatal nutritional deprivation is believed to be a risk factor for the development of the metabolic syndrome (obesity, hypertension and type II diabetes). Rather than being specialized in energy storage – by becoming fat, for instance – animals reared in poor environments actually had higher metabolic rates in adulthood. This has never before been reported in mammals. We also found that a diet high in fat helped male mice to catch up their body size after nutritional deprivation.
In another study she showed that the natural lifespan of female mice is not shortened by breeding. This result is intriguing, because a fundamental concept from “life history theory” is that there should be trade-offs between traits like lifespan and reproduction. Some researchers have even suggested that having more offspring should shorten life in humans. Actually, whether having children shortens life may depend on circumstances such as wealth and access to medical care. Schuberts result in mice supports this idea. Under in ad lib laboratory conditions, animals might have enough energy to fully recover from the stress of breeding, or they may be able to allocate energy away from functions that are essential for survival under less benign conditions.
And last but not least, this is the first study, to the knowledge of Schubert, to look at lifetime-level changes in metabolic rate in laboratory mice. Her analysis revealed that individual changes in metabolism were functionally different for early-to-mid and mid-to-late life. Furthermore, mice with higher resting metabolic rates were more likely to die at any given moment relative to their peers. This striking result is in broad agreement with recent findings in humans. Thus, Schuberts work forms an important link between human studies and animal studies.
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