Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum
This article discussed the manner in which climate affects
body size as related to equines and how this information could be useful in
revealing how mammals will react in the future as warming occurs once again. The
authors found results intimating that changes in temperature were the most
likely major factor to changes in mammalian body size. They identified other
factors, such as precipitation and elevated atmospheric partial pressure of CO2
that may contribute to changes in body size but determined that temperature was
the main component driving body size changes.
The authors studied ᵹ 18 O in Coryphodon tooth enamel as a proxy for
change in MAT, ᵹ 18 O mean differences between mammals that are
sensitive to aridity and mammals that are insensitive to aridity, and paleosol
major oxides. The last two were used as proxies for changes in MAP, and the
results of these two proxies showed that the decrease in Sifrihippus body size was not caused by increased aridity during
the PETM. Body size in Sifrihippus was
negatively correlated to ᵹ 18 O values in Coryphodon tooth enamel, suggesting that warming temperatures had a
strong effect on decreasing Sifrihippus size.
This adheres to Bergmann’s rule which states that body size decreases as
climate warms. Their study discovered that approximately 38% of the genera that
were found prior to the PETM and persisted into the PETM displayed a decrease
in body size across this time period. They also determined that approximately
40% of the post-PETM genera displayed increased body size including seven
genera that originated during the PETM. All of this information supports the
authors’ conclusion that temperature seems to be a determining factor in
changing the body size of mammals.
Questions
1. Quite
a few mammals don’t adhere to Bergmann’s rule, what factors do you think
contribute to this?
2. Since
the relationship between body size and increased temperature weakens around 11
degrees Celsius, what are some factors that could explain why Sifrihippus still displays a decreased
body size despite higher temperatures? Why do you think the correlational value
reverses after 11 degrees Celsius?
They propose that body size could have decreased during the PETM because of elevated partial pressure of CO2. How would this affect C3 and C4 plant distribution? Could there have been a shift in the Sifrhippus's diet and this could have caused them to become smaller?
ReplyDeleteThe 11 degrees Celsius implies a threshold to the Bergmann rule.The correlation value reverses after 11 degrees Celsius could be due to other factors such as food availability, habitat, competition,metabolism, anthropogenic etc. It may mostly like be a combination of the factors listed or more.
ReplyDeleteThe mammals that don't adhere to Bergmann's rule are probably better adapted to get rid of heat in the presence of a warmer environment so the rule wouldn't apply variable to the surface volume ratio. Also the species that don't follow the rule could have a different eating pattern than ancestral organisms due to the change in vegetation making more food easier to consume. I think that the increase in biomass of plant leaves but decrease in nitrogen levels causing harder digestion for mammals to be more difficult would be a good explanation for the change in size; meaning if animals adapted to digest and use the nutrients easier then they could grow bigger in size.
ReplyDeleteAs indicated by some of the other comments, some mammals may not adhere to Bergmann's rule given an increased ability to dissipate heat via methods other than a reduction in size. Relatedly, given that ~40% of the genera that originated during the PETM displayed an increase in body size (contrary to Bergmann's rule), it is possible that these organisms were able to adapt to the environmental conditions in such a way that allowed for an increase in body size. Further, these organisms may have also found themselves occupying ecological niches that may have been more robust and thus less susceptible to the effects of the PETM. Finally, metabolic rates can vary with temperature (to a certain degree). Given this, perhaps some organisms in the PETM evolved to exploit the higher temperatures with respect to their metabolism. Thus, potentially improving the efficiency of their metabolism, allowing for an increase in body size while other organisms become smaller.
ReplyDeleteThis was yet another article that shows us the importance of using stable isotope analysis to determine something about the past, in this case mammal size and how this could be affected by climate. Additionally, this is another example of how stable isotope analysis helps construct models for changes in the past and how these models can be useful in predicting changes that we could see in the future.
ReplyDeleteThe paper mentioned that PETM warming models are similar in magnitude to models predicted within the next century but that the warming that took place during the PETM occurred much slower than at present time. Given that climate warming is taking place at a much faster place currently, what might be some other ways that mammals could respond in terms of physiological or ecological change as a response to rapid warming?
I would have to agree with the previous comments. It's clear that the mammals who do not follow the Bergmann's rule would have a better method in giving off heat. Therefore having more efficient metabolic rates and even smaller body sizes to compensate for the increase in temperature. I liked Derek's example in that we have larger plant biomass which would allow the nitrogen to be more spread out which would make it a little harder for mammals to digest the plant material. But being able to have a smaller body size would allow for more efficient digestion and make it that much easier to absorb the proper nutrients from the plant material.
ReplyDeleteEven though they were experiencing decreased body size due to temperature it could also be due in part to the surrounding habitat and the diet they consumed,but possibly also in part to predation big body size means you move slower so maybe a way they got around extreme predation was through a decrease in body size.
ReplyDeletetemperture may have been the ultimate source of changes in body size but no rule can be established to say that warming results in decreased body size. the controling factors of body size must be controlled by ecological conditions such as food availability, species interactions, etc. this paper found a strong correlation but did not determine a causation. i propose the development of a model which estimates evolutionary reactions under seperate ecological conditions that may be used to determine the effect on body size at different tempertures.
ReplyDeleteBergmann's rule is one of the factors that dictate body size on an animal, but it is by no means the only variable that factors into this. re: CHD, I think that changes in diet could potentially explain why these ancient horses didn't adhere to Bergmann's Rule across the PETM. It is also possible that habitat availability could have factored into this as well. If the landscape became increasingly more densely forested, than having a larger body size may have been a hindrance to the horses.
ReplyDeleteMammals that do not follow this rule would actually be better off by having a more efficient metabolism and smaller bodies when the temperature increases.
ReplyDeleteThe mammals that do not adhere to the rules have a more efficient way of releasing body heat in order to deal with the increasing temperatures. As said previously this could be to a more efficient metabolism. But I also ageee with Jared just because we found a correlation does not mean that temperature is the cause of smaller body size alone, there are always other factors such as food sources.
ReplyDeleteChanges in body size are determined by multiple factors, in this case, how certain climatic conditions. Those that do not adhere to Bergmann's rule are influenced by other factors such as diet. These other factors are probably why early horses still decreased in body size, even though temperatures were high. I like Jared's proposal to determine the effect of different ecological conditions.
ReplyDeleteInteresting how the authors tested aridity as a possible contribution to body size during the PETM using the two proxies listed (delta18OE and paleosol oxides). Although we haven't talked much (if at all) about aridity in our EoP class, I'm interested to know if there are any cases where aridity does play a role in body size. Is there any reason aridity would affect body size? Are there other proxies to test this?
ReplyDeleteI think animals that do not follow this rule would have better or simply different adaptations to deal with climate change. Not all animals have a strong correlation between climate and body size that can be caused by a number of different adaptations.
ReplyDeleteThe mammals that don't adhere to Bergmann's rule could have other factors influencing such as their internal temperatures which can be different depending on the mammal in order to maintain homeostasis. Another factor may be what the diet is of the mammal and the areas. Not all mammals would adhere to the rule because they aren't all the same.
ReplyDeleteI thought it was interesting we keep coming back to size and temperate and how that affects the size of organisms. I wonder in this study how much is due to other variables such as habitat and availability of resources. Bergmann's rule was an interesting concept that obviously some animals have to be better at dissipating heat rather than others. I'd like to see more data that deals with the variables organisms face in the wild.
ReplyDeletethe first thing this paper made me think of was felisia's paper on the wood rats and the correlation between size and temperature,I thought this paper was pretty well worded and in particular the figures helped me understand what was being discussed a little better. I agree with Krystals hypothesis that diet could ultimately influence their internal temperatures and ability to maintain homeostasis, interesting idea.
ReplyDeleteFactors that could contribute to non-adherence to Bergmann's rule could be adaptations for heat retention/dissipation, resource availability and risk of predation. Perhaps the odd patterns observed around 11 degrees C could be due to succession of less nutritious forage.
ReplyDeleteDont adhere to the rule since there are other mechanisms that they were selected for to survive; more efficient metabolisms, heat dissipation, homeostasis, fighting off predation or being the predator again. They selected for bigger size in higher temps due to higher resource availability
ReplyDelete