WHY MAMMALS INCREASE THEIR BODY SIZE IN THE PROCESS OF EVOLUTION ?
SU JianPing , liu jike
2000, 20(1):
58-66.
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Many mammals have been proved by lots of observations to increase their body size in the process of evolution. This is a very interesting biological phenomenon , and on which theoretical explanations havebeen suggestedby paleontologists. Some of the explanations are based on the famous Bergman's law and accepted almost by all paleontologists who are interested in the phenomenon, although a lot of contrary evidence has been found. In this article, the authors presented many facts to show that the augmentation of body size is a process independent of cooling of earth during Quaternary refuting all explanations based on Bergman’s law. The most important fact in use was the late Pleistocene fossil mammalian faunas from Northeastern China Research group of advanced vertebrates of the Institute of Vertebrates Paleontology, 1959) showed in table 1. If the explanations based on Bergman’s law were correct, larger species would have higher survival rates after undergoing through the extreme cold of ice age in Quaternary than smaller ones. However, Table 1 showed that the survived species or subspecies after going through the ice age included all 8 species of Rodentia and Lagomorpha, with small body size 5 of 6 species of Carnivora, with moderate body size, and 10 of 22 species or subspecies of Proboscidea, Perrisodactyla , Artiodactyla , with large body size. These data indicated that augmented body size was not advantageous for mammals to live in extreme cold climate, and might increase their extinction.
The authors also used the evidence from the fossils of order, Proboscidea. The process of evolution in this order was about as follows. The most primitive family Moeritheriidae, occurred from the late Eocene to the early Oligocene with a body size of pig, the smallest size in this order; Deinotheriidae occurred during Miocene and Pliocene; Gomphotheriidae occurred from the early Oligocene to the early Pleistocene. Mammutidae occurred from the early Miocene to Pleistocene. And Elephantidae occurred in the late Miocene and was the only group still surviving today. The body size was gradually augmenting in this process, indicating that the augmentation of mammal’s body size could be a very long process which might have started at the beginnings of mammals and ended within the extreme cold ice age. So the problem should be discussed on a larger scale of time rather than only for Quaternary.
There are several possible mechanisms involved in the augmentation of mammal's body size. First, nature selection should favor larger species because they arc advantageous in competition with a smaller one utilizing the same resources. Second, species specialize on the resources relatively being propitious to large one to avoid competition with other species. Giraffe's evolution is this situation. Third, large body size makes herbivorous mammal's antipredator more successful, that is why the present largest mammals are herbivorous, such as elephants and rhinoceros. Fourth, larger body size means higher running speed and endurance, and thus higher survival rates of prey from predators more successful predation of predators. The predator -prey coevolution prompts both predator and prey augment their body size. In predator - prey interaction, prey have a higher stress than predator because prey will lost it’s life if it fails to escape from a predator, while predator only lost a meal if it fails to capture a prey. So a prey may increase it's body size more rapidly than it's predator.
In the last paragraph of this article, the authors discussed why augmented mammals had high extinction rates in Quaternary ice age. The following two factors together can account for this phenomenon. First, augmented mammals must exclusively occupy enlarged territory because a larger animal needs more resource food) than a smaller one. Thus population density, number of individual per unit area will decrease with augmentation of body size, and if it decreases to the minimum viable population it will be unavoidable to extinct. Second, the extreme cold climate in the ice age should result in a considerable decrease of plant productivity or a strong habitat succession , which must aggravate the rarefaction of population caused by augmentation of body size. The authors agree with the law of augmentation of size established by Charles Deperet.
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