Essay: Constraints on maximum body size / consequences of a change in size of terrestrial mammals

The giant asteroid that hit the Earth 65 millions years ago might have marked the end of the dinosaur era but the evolution of mammals has exited long before then. This opened the door to the diversification of terrestrial animals. The evolution of these animals has changed so much since the Cretaceous/Paleocene era. The size is one of the biggest differences between the mammals then and the mammals now. For the first 140 million years, mammals were small and only occupied a small niche and there was only a narrow range of body sizes (Smith et al 2010). The body mass estimates for about 1,534 fossil mammal species and on average it shows that new species are 9.1% larger than the older species that are in the same genre. This change is partially explains how the new species are emerging and how they are making a mid size gap between the lower size limit and the upper size limit (Alroy 1998). The body size has numerous consequences for the structure and function. The size of an organism is vital in recognizing the importance of the question of how It manages to survive.
The ultimate questions are: what are the constraints on the maximum body size and what are the consequences of a change in size of these terrestrial mammals. There are many factors to consider when trying to find what the constraints are. For example, there are factors that the animal simply cannot live without but the mammal also cannot change anything about it. The ultimate constraint may as well be the design of the oxygen supply system or even the pumping capacity of the heart (Schmidt-Nielsen 1984). The ultimate goals of animals are to survive and reproduce. So the mammal has to be at the maximum size as possible for it to succeed. Body mass is directly correlated with metabolic and psychological variables and with some ecologically characteristics. Some of the ecological characteristics include history traits, diet, population density, and home range size.
The skeletal support also plays a major role in the limitation of the body size. Since terrestrial animals have to adapt to gravity their body is shaped according to this adaptation. The endoskeleton of a mammal is a rigid external covering for the body that provides protection and it also is a form of support since the muscles are attached to it. As the animal becomes larger, the endoskeleton also increases in the thickness, which may interfere with the body size. The size of the animal is then determined by the amount of the skeletal system it needs that way it does not lose support of the other tissues or the amount of muscle it requires (Damuth and MacFadden 1990).
According to Alroy (1998) if there are higher rates of extinction or lower rates of origination, this will suppress the diversity of the larger animals, which may be the reason why the body size has reached the limit of the specific size range. He also mentions that the reason as to why the size is beyond the limit, it could be because there are newly evolved species that have moved in the areas where recently extinct species have lived prior. This displacement could be pushing lineages away from the center of distribution, which could also be the reason for the upper limit of body size being reached (Alroy 1998). In the study by Smith et al (2010), they found a similarity in the rate, trajectory, and the limit of size across continents, orders, and all despite differences in climate.  It was also concluded in their analysis for the evolution of the giant mammals was to diverse to fill up the niches, other factors may have come into play. For example, the environmental temperature and/or land area may have constrained the maximum size achieved.
A study was conducted to develop a model for the effect that a species’ body size has on fitness. Brown and others defined fitness as a reproductive power and the rate of conversion of energy into offspring. The model predicts overall that in each taxon, the relationship between life, history, and ecological characteristic as longevity, home range size and population density. If fitness has an energetic definition then it also has the potential to unify areas of ecology and evolutionary biology (Siblya and Brown 2007).
By studying the limitations that these species have, it will then in turn increase the fitness of the species. Not only will it be beneficial to increase the fitness but by studying the consequences of the change in size, it will also estimate the metabolic rate of mammals. This is important to the ecological and evolutionary consequences. The effect of having the size limit surpassed of a mammal will increase the volume and mass of the species but the certain area where they are living will not increase (Feldhamer et al 2015). Now the question is how does the relationship between body mass and metabolism relate to the geographic dispersal of mammals. Feldhamer and others mentioned a certain rule called Bergmann’s rule where Carl Bergmann argued that on a big scale, the larger species live farther up north and the smaller species live in the south. This rule implies that the energetic advantage may be gained through the decrease surface-area-to-volume ratio. The study continued on to discuss how the amount of heat loss depends on the animal’s surface area and their surroundings. A larger animal has a larger total surface¬ area per unit volume but has less heat loss than that of a smaller animal (Schmidt-Nielsen 1984). The study of the terrestrial mammals and what their constraints on size will ultimately help in the conservation of animals. Terrestrial animals help maintain the ecosystem, without these animals everything else would not survive. They help control populations in certain species and plants as well. The human race cannot live on earth without mammals and without plants. If there is even a slight chance that the human race can help in preserving and making sure no more animals will go extinct, I believe we should all make an effort I preserving our Earth.