Here we present a meta-analysis of literature sources and
original data on basal metabolic rates (BMR) in birds and mammals based on 1817
measurements (817 data points from mammals and 1000 from birds). We used these
data to estimate scaling coefficients and intercept for all endothermic animals
pooled and for birds and mammals separately to study the difference between
these classes, and also for the main groups of lower taxonomic status. We
considered BMR variation and the duration of activity in three mammalian
subclasses: monotremes, marsupials and eutherians, and in three groups of
birds: palaeognaths, neognaths non-passerine and passerines, depending on the
evolutionary age of these groups. In all groups BMR varies with body size with
the different scaling coefficient which is statistically indistinguishable from
¾ in mammals and 2/3 un birds, but with significantly different intercepts. The
data for all the considered groups were combined and the common exponent for
six allometric equations between the BMR and body weight was established b =
0.7248. Reduced to the common slope, the relative metabolic rate forms the
following series: Neognathae - Passeriformes - 1.00, Neognathae -
Non-Passeriformes - 0.75, Palaeognathae - 0.53, Eutheria - 0.57, Marsupialia -
0.44, and Monotremata - 0.26. The main finding is that the metabolic rate in
the six main groups of mammals and birds consistently increases as the
geological time of the group’s divergence approaches the present.
Activity duration varies between the main groups of
endotherms. Overall sleep duration is a parameter inversely related to the
overall activity. A high level of activity is related to high BMR. Placentals
and paleognaths have similar BMR, i.e. terrestrial lifestyle without flight is based
on nearly equal BMR.
We calculated sleep duration in the main groups of
endotherms on the basis of literature data. BMR in a taxon correlates with its
evolutionary age: the later a clade diverged, the higher is its metabolic rate
and the longer is activity period. BMR of eutherians and flightless paleognaths
is similar, and these groups evolved at practically the same time. We suggest
that each group formed its taxon-specific BMR depending on the ability to
maintain thermal homeostasis under the environmental conditions that prevailed
during its emergence. Monotremes were the first to branch off from the basal
mammals and have the lowest BMR among the endotherms we consider. This level is
minimally sufficient for maintaining homeostatic body temperature under a very
limited range of environmental conditions. The next level is typical of
marsupials and allows them to maintain thermal homeostasis under a broader
range of conditions and have a more protracted period of activity. Finally, the
metabolic rate typical of placentals and paleognaths formed in the
mid-Cretaceous and allowed these groups to occupy a broader range of
terrestrial niches. Immediately when the development of blood circulation and
respiratory systems made it possible to reach the BMR that allowed maintaining
a body temperature of 37 oC, the explosive radiation of mammals and birds
started. In the mid and late Cretaceous birds and mammals started to occupy the
leading positions in the ecosystems. And at last, some 50 mya passerines that have
the highest BMR (nearly 50% higher than eutherians and paleognath birds)
adapted to the forest habitats and gained body temperature of ca. 40oC, which
is at the upper physiological limit. The duration of activity and body
temperature increased in parallel to the BMR. Ecological expansion of birds and
mammals resulted in their worldwide geographic distribution. A relatively
similar energetic analogue of passerines in mammals are humans, who have BMR
ca. 35% higher than predicted by the scaling equation for mammals, have amazing
reproductive potential, large brains and longevity much exceeding the live
duration which is typical for their body-mass specific mean. Sleep duration in
humans is 30% shorter than the mammals-specific mean, and it is one of the evolutionary
youngest species. At the moment the global population of humans exceeds 8
billion people.
Author(s) Details
Valery M. Gavrilov
Department of Vertebrate Zoology and Zvenigorod Biological
Station, M.V. Lomonosov Moscow State University, Moscow 119991, Russia.
Please see the book here:- https://doi.org/10.9734/bpi/ibs/v4/3716G
