Received 10 August 2015; accepted 19 April 2016; published 22 April 2016
Glucose in the blood is the principal energy source for brain functioning and but glucose can be used as the energy source for multiple other tissues. A relationship between metabolic rate and circulating concentration of glucose has been reported  with higher blood concentration of glucose with increasing metabolic rates in across vertebrates. Moreover, there is a negative relationship between blood concentrations of glucose and body weight (log) in mammals (blood:  ; serum:  ). In birds, an allometric relationship for circulating concentrations of glucose was reported in one study (blood:  ) but not in another (plasma/serum:  ). This leads us to question whether an allometric relation for circulating concentrations of glucose, in fact, exists in mammals. The presence of such a relationship would be consistent with the relationship between basal metabolic rate being proportional to either the body weight to the power two thirds (2/3) (reviewed:  ) or three quarters (3/4)   .
What is not clear is whether the supposed relationship between circulating concentrations of glucose and log body weight is real reflecting a true allometric relationship (and reduced needs for energy in larger animals) or represents an artifact of species included in previous analyses and the inclusion of domesticated animals in the analyses. Domesticated animals have been selected for growth and larger body size and consequently lower circulating concentrations of glucose  . There are also marked phylogenic differences in circulating concentrations of glucose within the Class Mammalia  . The present study re-examines the relationship between circulating concentrations of glucose and body weight in mammals with a large sample size (270) of wild species but with domesticated animals excluded from the analyses.
2. Materials and Methods
A database was assembled for serum/plasma concentrations of glucose in wild species of mammals using the published or calculated mean for the species based on rigorous and systematic series of searches of the literature  together with body weights principally from the Animal Diversity Web. This is presented in Table 1.
Table 1. Database of plasma/serum concentrations of glucose (from  ) and log body weight).
Allometric relationships (comparing both serum/plasma concentrations of glucose and log10 serum/plasma concentrations of glucose with log10 body weight) across the Class Mammalia were analyzed by linear regression (Microsoft Excel). The data were also analysed separately for major groups of mammals including Marsupial mammals, Eutherian mammals, for taxa within the Eutherian mammals, namely Glires, Euarchonta, Laurasiatheria, Afrotheria and Xenarthra (following the relationships advanced in  -  ) together with Orders and sub-orders within the Laurasiatheria and Euarchonta where there is sufficient data for analysis.
There was not a relationship between circulating concentrations of glucose and log10 body weight in wild mammals (circulating concentrations of glucose: adjusted R2 = −0.003; log10 circulating concentrations of glucose: adjusted R2 = −0.003) (Table 2 and Table 3) (Figure 1). Similarly, there was no allometric relationships when data for marsupial or eutherian mammals were analyzed separately (Table 2 and Table 3) or with major taxa within the Eutheria.
However, there was a strong relationship between circulating concentrations of glucose and log10 body weight in some taxa; specifically across species within the Order Primates (circulating concentrations of glucose: adjusted R2 = 0.480; p < 0.001; log10 circulating concentrations of glucose: adjusted R2 = 0.511 p < 0.001) (Table 2 and Table 3). Moreover there were moderate allometric relationships in the Orders Perissodactyla and Carnivora (Adjusted R2 = 0.085; p < 0.05) (Table 2 and Table 3). Within the Order Primates, there was an even stronger allometric relationship in the Sub-order Haplorhini (New World and Old World monkeys together with Apes) (circulating concentrations of glucose: adjusted R2 = 0.597; p < 0.001; log10 circulating concentrations of glucose: adjusted R2 = 0.657; p < 0.001) (Figure 2) but no allometric relationship in in the Sub-order Strepsirhini. Within the Order Carnivora, the moderate allometric relationship was no longer significant when the data were examined within Sub-orders (circulating concentrations of glucose: Sub-order Caniformia-adjusted R2 = 0.059, p = 0.064; Feliformia-adjusted R2 = 0.098, p = 0.128). Despite there being data on circulating concentrations of glucose in a large sample size (76 species) within the Order Cetartiodactyla, there was no allometric relationship observed. Moreover, if the data for Cetacean species was analyzed separately, again no allometric relationship was observed (circulating concentrations of glucose: Cetacea-adjusted R2 = −0.077, p = 0.948; Artiodactylia-adjusted R2 = 0.0007, p = 0.312).
There was no relationship between circulating concentrations of glucose and log body weight in wild species of
Table 2. Relationship between serum/plasma concentrations of glucose and log body weight in mammalian species.
Table 3. Relationship between log10 serum/plasma concentrations of glucose and log10 body weight across mammals and in mammalian taxa.
Figure 1. Relationship between plasma/serum concentration and body weight across mammalian species. Left: plasma/serum concentration (mM) and log10 body weight across mammalian species; Right: Log10 plasma/serum concentration and log10 body weight across mammalian species (mM).
Figure 2. Allometric relationship for plasma/serum concentrations of glucose in Primate species (left glucose, right log glucose concentration).
the Class Mammalia or Infra-classes Eutheria or (Placentalia) (Table 2 and Table 3; Figure 1). This is in contrast to previous studies in mammals   . The basis for the differences is not clear. The present study employed a much large number of species and deliberately omitted domesticated species. There were similarly no allometric relationships observed across species for many mammalian taxa including marsupials, eutherian mammals, Super-orders Afrotheria (e.g. aardvarks, elephants, sea cows), Glires (rodents, rabbits and hares), Laurasiatheria (e.g. carnivores, large herbivores and whales) and Xenoarthra (anteaters, armadillos, sloths) and in Laurasiatherian orders such as Chiroptera (bats) and Cetartiodactyla (even toed ungulates such as deer together with whales and dolphins).
There was a very strong allometric relationship between circulating concentrations of glucose and log body weight in species of the Class Primates (apes, monkeys and lemurs) (Table 2 and Table 3, Figure 2) and, particularly, in the Sub-order Haplorhini (New World and Old World monkeys together with the Apes). Body weight accounted for much of the variation in circulating concentrations of glucose in species in the Sub-order Haplorhini) (>60%) (Figure 2). Moreover, there was an allometric relationship with circulating concentrations in species in the Orders Perissodactyla (odd toed ungulates such as horses, rhinoceroses and tapirs) and Carnivora (Table 1). It is suggested that in these taxa that the relationship between circulating concentrations of glucose and body weight is consistent to the reductions in energy requirements per unit weight with increasing body weight. Despite the strong relationship between here was insufficient data on basal metabolic rate or brain weight in primates (or within the Haplorhini)  for an analysis with sufficient power for significance. Alternatively, circulating concentrations of glucose may play a critical role in determining optimal body weight.
The overall conclusions are that there is no allometric relationship between circulating concentrations of glucose (or log10 circulating concentrations of glucose) and log10 body weight across species of wild mammals. However, there was a strong allometric relationship in primates, particularly in the Haplorhini (monkeys and apes).
No relationship was observed between circulating concentrations of glucose and log10 body weight in a large sample size (270) of wild species but with domesticated animals excluded from the analyses. The absence of an allometric relationship for circulating concentrations of glucose was unexpected. A strong allometric relationship was seen in Primates.
The helpful discussions with colleagues are gratefully acknowledged.
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