Smith et al. (2010)
The Evolution of
Maximum Body Size of Terrestrial Mammals
By: Hailey M. Griego and Asia Alsgaard
Dr. Smith’s
paper focuses on the study that while diversification was the primary driver
for giant mammals, environmental temperatures and land area could have
constrained the maximum body size that was reached. According to the paper, for
the first 140 million years mammals have been fairly small. During the middle
Mesozoic, diverse feedings have evolved and larger mammals have begun to feed
on small dinosaurs but their body size ranged around three to five grams. This
small range constrained the ecological roles of these early mammals. Following
the K/Pg mass extinction was a “wholesale” of ecological communities that was
marked the onset of rapid morphological, ecological and phylogenic diversity in
mammals that led to an expansion in mass by four orders of magnitude. This is
where maximum size was reached. The pattern of body size was similar across
continents, lineages, and trophic groups. On each continent body mass increased
rapidly during the Cenozoic and increased to three orders larger by the late
Eocene. The limit of 17 tons was reached in the early Oligocene in Eurasia and
again in the Miocene in Eurasia and Africa. North America has never supported
this size of an animal. Contrary to popular belief, body mass was not larger
when animals were more diverse. There were two hypothesis tested in this paper.
The first one was a growth model where maximum body size evolves following a
geometric Brownian motion. The second was growth saturating over time that
reflected the limits of resources or physiological, algometric, biochemical, or
ecological constraints. The results suggested that the random walk was not an
appropriate model to use. Results showed that large mammals evolved to fill
ecological roles. Smith et al. (2010) compared the global mammal mass increase
to the following abiotic factors: global temperatures, atmospheric oxygen
levels, and terrestrial land areas. They found that all of the abiotic factors
were significantly relate to mammalian body mass over the Cenozoic. The impact
of temperature and terrestrial land mass is consistent with previously
suggested principles: (1) Bergmann’s rule that larger mass helps to conserve
heat while smaller masses help to dissipate heat and (2) the available land
restricts the upper mammalian mass limit by either limiting the amount of
habitat or of available resources. Thus, the authors argue that mass
limitations for mammals are driven by their warm-blooded nature.
However, the authors warn that a number of issues may be
present within the data interpretation: (1) lack of resolution in the
environmental and paleontological records, (2) collinearity between two or more
of the abiotic factors (listed or not) making it more difficult to determine
the driving factor, and (3) having a limited statistical power and testing
ability. Even so, the authors argue that this is a valid pattern and that more
research is necessary.
In sum, the mammalian radiation and body mass increase was
not random, but was driven by processes independent of geography or physiology
and not based on a common ancestor or increased species richness. Ultimately,
body size decreased only as mammals were increasingly limited by abiotic
factors and had exhausted evolutionary possibilities for body size.
Questions:
1) Are
there any other abiotic or biotic factors that could be affecting mammalian body
size over time?
2) What
are some of the problems with trying to determine the cause behind such a large
physiological change in body size (either as mentioned in the article or from
other factors)?
3) What
are some of the implications this article suggests for what we should expect
coming out of the modern extinctions?
4) Do
you think these two hypothesis were appropriate for this study? Are there any
other ones that might be more appropriate?
1: biotic interactions interaction, which would be untestable. while the available reasorces may limit growth what drives growth is likely biotic. predator avoidance, prey capture in dependent on size. become large or small to avoid being eaten or become bigger so you can go after bigger prey.
ReplyDelete2: you can't determine the actual pressures that drove growth when working with the fossil record. extensive understanding of the life history of these animals would be necessary to determine the causes of morphological changes.
3: loss of habitat will harm thee larger mammals more than smaller animals as they require more habitat.
4: if possible it would be good to try and determine the biotic factors that effect growth. the aboitic factors establish a limit but fail to determine the cause of growth.
1) Interactions between organisms could influence body size. This would be difficult to find since living strategies such as behavioral patterns aren't recorded in fossils. Evidence of this could possibly be inferred through rapid change in a family size over time.
ReplyDelete2) We haven't seen such an event occur, so there are too many possibilities to look at. This includes their behavioral patterns, predatory rate, and fecundity; all things that aren't recorded in the geological record.
3) It doesn't suggest anything for the modern extinction, what can be said about it can only be inferred. What is inferred is that larger organisms will be adversely affected and the smaller organisms will be relatively unharmed. This will allow for diversification of species to fill the newly opened niches, resulting in an event similar to what was covered in the paper.
P.S. Stop asking questions like this. These assignments are meant for discussion of ideas, provide a starting point not direct questions.
I found the article lacked a good amount of mathematical data representation. The graphs shown are constructed from calculated points and it is stated that they fall on a given equation; one that should be plotted alongside them, but isn't. Not doing this allows for the overlooking of possibly significant data. Take figure 2 as an example: why does the overall body mass drop from Creodonta to Carnivora, when herbivore mass seems to reach a maximum at the time? Looking at what was provided: it seems as though this is possibly an error arising from the linking of two or more significantly different species, or that there is missing data at that point. Providing the projected data points, and not connecting the measured points together would allow for a better representation and explanation for the mass increase.
1) Changes in the availability of food and other resources could be another biotic factor influencing body mass over time.
ReplyDelete2) Some problems that were mentioned in the article were that seasonality and precipitation were not incorporated.
3) I think that the article suggests that more niches will open up for other animals because of modern extinctions.
Especially since we are going through a mass extinction so many niches will possibly open up and there might be size changes in the animals that occupy these new niches.
4)Two hypothesis were appropriate for this study because we needed to know if the increases in size were random or if they followed some sort of pattern.
Seeing evidence for abiotic and biotic factors limiting the growth of mammals makes sense because it reinforces other ideas and hypothesis we have discussed and also fits the trend of other animals not just mammals. There is a limit from both the abiotic and biotic environment organisms endure that limit everything about their physiology and ecology. If the environment can't support mega fauna then it is likely that natural selection would never select for those traits simply because the fitness of these organisms wouldn't be enough to get their genes introduced into the population. I think understanding these thresholds are extremely important for not only today's biologists but biologists in the future. Knowing that North America once supported much larger mammals and a lot more of them can help direct conservation and reintroduction of species.
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ReplyDeleteMammalian body size has shown to be largest when the temperature was coldest as well as the most land area available to large mammals so a possibility could be water sources as a limitation once all the available niches were occupied causing a stop of increase of body size. It seems difficult to try and determine the cause behind the large change in body size due to the lack of data and knowledge, however it does seem very consistent with Bergmann’s rule where body size would increase in the conditions given. Based on this article along with the idea of Bergmann’s rule if the modern extinction continues I don’t believe there will be any open niches occupied since humans are destroying the habitats but I do believe that it is possible to see a decrease in mammal size due to a rise in overall atmospheric temperature as well as increase in Carbon levels.
ReplyDeleteFor Figure 2, I am curious to know why at 60 million years ago that herbivores and carnivores were about the same body mass size? Was it because the food sources were comparable or because the abiotic factors (global temperatures, atmospheric oxygen levels, and terrestrial land area) were at the sweet spot?
ReplyDeleteAlso, why do carnivores come into the geological picture at 60 mya? Is this another direct cause of the K/Pg mass extinction since the dominant predators before were dinosaurs? Are the mammal carnivores only filling in a niche 5 million years later?
This article goes into the details of mammalian body size and it's relationship to the environment. Abiotic factors (temperature, amount of land mass, atmospheric oxygen levels) were measured and compared to body mass, and it was found that there is a significant relationship.
ReplyDeleteI liked what Derek said about the continuation of the modern extinction and if we apply Bergmann's rule to present day, we would probably see a decrease in body size in mammals. However, since humans are such a big influencer to the environment, Bergmann's rule probably will not fit and a new rule would be needed (if possible).
In response to Derek's post, I would agree that humans are closing niches with the destruction of habitats and this could also be a factor with the decrease in mammalian size. This could be due to the fact that without the proper niches being filled and habitats being destroyed the larger mammals food sources would be destroyed as well.
ReplyDeleteI don't believe water sources could be a factor in mammalian body sizes just because if mammals had such a large area to occupy in colder temperatures then there shouldn't have been a lack of water sources. I think it mostly has to do with food sources and the ability to regenerate those sources after they have been eaten.
Obviously this is a study about terrestrial mammals, but I wonder if the conclusions are also applicable to marine mammals (especially cetaceans). Consider, whales are the largest animals on the planet. Not only do they live in the largest contiguous environment, the ocean, but their maximum body size has also increased through time as things have gotten colder. Both of those observations feel somewhat tacky without a great big study clearly showing the correlation between ocean area or temperature through time plotted against cetacean body size through time though. But it may be, who knows? Whales? Dolphins mayhaps?
ReplyDeleteI would think that physiological limits are the main constraints on maximum body size in mammals. This seems to be evidenced by the plateau of their maximum body size at about 35-40Ma. However, temperature seems so have somewhat of an effect as evidenced by the slight jumps in body size immediately following rapid cooling periods like the Eocene-Oligocene glaciation and the Mid-Miocene Climatic Transition.
ReplyDeleteTOLL brought up a good point that was on my mind when reading this paper. The authors said that their data supports the idea that mammals become larger when Earth cools and land area is greater. I don't know too much about marine mammals, but are they at all effected by sea level regression? Also, do the environmental parameters tested here only apply to mammals (Bergmann's Rule, for example)? Could we apply this kind of technique to the Mesozoic?
ReplyDelete1) Perhaps predator-prey interactions, group efforts for survival or by themselves, or to overcome barriers to maintain homeostasis.
ReplyDelete2) There's not much to look at since the studies are on fossils. We can't determine their interactions, what they ate, etc. Did they look at anything other than the geographic areas and temperature? What about the seasons?
3) The more area that is lessened for the habitat of the mammals, the more they may change. There is an increase in temperature currently that can affect the size and survival for mammals.
4) Yes I think that the hypotheses were appropriate considering that there are habitat loss and an increase in temperature currently. We can try to determine what causes mammals to go extinct or change.
Personally, I found that both of the hypotheses presented in this paper were appropriate given the nature of the study. With that said, I did find the second model to be more appropriate as compared to the first. This is given that it seems to include more relevant factors that would appear to have important implications for body size. In terms of the modern extinction event, I think that it would be appropriate for one to expect to see a decrease in body mass given the global trend toward to warmer temperatures. This is because an improved ability to tolerate higher temperatures correlates negatively with body mass. Additionally, we may see a trend toward smaller organisms as the amount of land suitable for larger organisms is being reduced, generally, due to human expansion. Finally, I would expect that extinction rates would be higher in larger organisms given the difficulties they face as temperature increases and as available land decreases.
ReplyDeleteI really, really thought that reading the overall global trajectory of maximum body mass with abiotic factors such as global temperature, atmospheric oxygen levels, and terrestrial land area was such a brilliant way to tie what we have been discussing in class to a real life applicable model.
ReplyDeleteAs mentioned in the prior Smith article, body size and external temperature are inversely related. I liked how Dr. Smith used real world examples in order to tie it back into what we are facing today, it made it far more interesting for me and better understand some of the applications of the information we are learning in this class. From this I gathered that we could expect a present day trend more towards smaller animals due to rising ocean levels resulting in less land area as well as rising temperatures which also correlates to smaller body size.
ReplyDelete1) The rise of modern grasses and the subsequent decrease in forests to increases in savannas could have played a big role.
ReplyDelete2)Some problems with assessing the cause could be low resolution of paleoclimate and the inability to accurately measure factors such as ecological interactions.
3) We should expect an overall decrease in the size of mammals due to the size selectivity of human caused extinctions and the decrease in habitat and resources caused by human activities.
4) I think Bergmann's rule and resource use hypotheses were good matches, but there could be a hypthesis that could include more detail concerning habitat changes.
1. Depends on what is selected for; forests vs wide open grass lands favors larger mammals. Being bigger than your predator can help or hinder depending on the niche. Resource availability and fight vs flight survival tactics.
ReplyDelete2. Lacking fossil record; too many missing links and not enough fossil evidence.
3. Decrease in size; mostly because humans are driving them out of habitats.
4. Yes, could be expanded to account for more biotic/abiotic variables.
Though both hypotheses are well thought out, the second seems to be more plausible. Taking into account the idea that everything has a cost and a benefit, there will always come a point when the cost outweighs the benefit which has a limiting effect on all animals. The second hypothesis is more inclusive in factors that are evaluated in determining size limitations. There are many things the fossil record can't tell us which leaves current evidence open to new interpretations as knowledge and methods expand in the future.
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