The energy costs of pregnancy have been called underestimated by an order of magnitude
Australian researchers have carried out a series of calculations on 81 animal species and concluded that the energy expenditure of a mother when carrying offspring is much higher than predicted by existing theories, which means that the total energy required for reproduction greatly exceeds available estimates. This is especially true for viviparous mammals and humans in particular. The paper is reported in the journal Science.
Total energy expenditure for reproduction includes direct energy, which goes directly to the growth of the new organism, and indirect energy, which is necessary for the mother's body to carry it to term. The former are well studied and described, while there is no clear data on the latter, but there are several theories that suggest they are small, ranging from 0 to 25 per cent of the total energy needed directly for reproduction from conception to birth. That said, knowing the energetic cost of pregnancy is critical to understanding evolutionary processes, particularly selection on metabolic rate and body size.
To obtain more objective data on this issue, Samuel Ginther and colleagues at Monash University have developed a quantitative modelling scheme using classical methods for estimating energy expenditure based on biochemical (reaction energy) and physical (heat production) principles. They calculated direct energy expenditure as the product of the mass of offspring from one reproductive act and the average specific energy of its tissues. And indirect ones - as the product of the increase in the level of maternal metabolism during pregnancy and its duration divided by two (based on the assumption that the level of metabolism with the course of pregnancy in general increases linearly from zero after conception to a maximum before delivery). In these calculations, they substituted available data for 81 animal species (Metazoa) ranging from planktonic primary-cavity rotifer worms (Rotifera) to humans (Homo). Due to lack of available data, birds were not included in the analyses.
Absolute values of reproductive energy expenditure among the studied species differed by 11 orders of magnitude, from 0.000003 kilojoules in the rotifer Brachionus plicatilis to 470037 kilojoules in the white-tailed deer (Odocoileus virginianus). Indirect energy expenditure was found to exceed direct energy expenditure in most (48 out of 81) species, and could be as high as 98 per cent of the total energy expended on reproduction. In ectotherms (cold-blooded animals) their share was lower than in mammals. In live-bearing ectotherms, indirect energy expenditure was slightly higher than direct energy expenditure (average ratio of 1.21 to 1, or 55 per cent), while in egg-laying and egg-laying animals it was lower (average ratio of 0.44 to 1, or 31 per cent). In mammals, the ratio averaged 8.62 to 1, i.e. almost 90 per cent of indirect energy expenditure (excluding lactation). In humans, this ratio was one of the highest (96 per cent), while the lowest ratio among the mammals studied - the little brown nightjar (Myotis lucifugus) - was still not lower than 75 per cent.
Indirect energy expenditure in each group of animals was directly proportional to the energy composition (specific energy of tissues) of the offspring, but the energetic dynamics of reproduction differed greatly among them. Cumulative masses of offspring from one reproductive cycle were similar in ectotherms and mammals, but their specific energy and direct energy inputs were lower in the latter. In indirect energy expenditure the difference was much more pronounced: although on average mammals took less time to bear offspring, their metabolic rate during pregnancy increased six times more than that of ectotherms.
Thus, the share of indirect energy inputs for reproduction may be an order of magnitude higher than is commonly believed. This means that animal ontogeny must have evolved, and continues to evolve, to anticipate and provide for the high energy costs of reproduction. It could also have affected the intensity of offspring care in mammals (high energetic investment in nurturing offspring increases interest in their survival) and other evolutionary aspects, the authors of the paper conclude.
Earlier, an international group of researchers analysed the intensity of metabolism in more than six thousand people and found that energy expenditure varies greatly depending on age, and human life can be divided into four metabolic periods, which are very different from household ideas.