Average Animal Size vs Time

Discussion in 'Biology & Genetics' started by EndLightEnd, Nov 8, 2008.

  1. EndLightEnd This too shall pass. Registered Senior Member

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    Does anyone have a plot showing the correlation of the average size of all animals on the Earth from lets say around the Paleozoic Era to present as a function of time? I looked and looked but could not find. Thanks in advance.

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  3. Ophiolite Valued Senior Member

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    No.
    So we are clear, would you care to define an animal? Do you include forams, for example?
     
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  5. ElectricFetus Sanity going, going, gone Valued Senior Member

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    Their are "jellyfish" 10 meters wide for all we know those things have been around for 400+ million years!
     
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  7. Roman Banned Banned

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    Hox homologs.
     
  8. nietzschefan Thread Killer Valued Senior Member

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    Good luck with that.
     
  9. DeepThought Banned Banned

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    I imagine it would look like a bell curve.
     
  10. ElectricFetus Sanity going, going, gone Valued Senior Member

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    I wonder if the size of terrestrial animals could be correlated with oxygen levels?
     
  11. synthesizer-patel Sweep the leg Johnny! Valued Senior Member

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    possibly - marine inverts in high latitudes tend to be much larger - which is partly due to a higher partial pressure of dissolved o2 in cold water
     
  12. Ophiolite Valued Senior Member

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    Why on Earth (pun intended) should it look like a bell curve? What mechanism are you proposing that would produce a progressive increase in average size over time, followed by a progressive decrease over time? You must have some mechanism in mind to make such a bizarre proposal.
     
  13. ElectricFetus Sanity going, going, gone Valued Senior Member

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    I said terrestrial animals, but that is a good point for marine animals.
     
  14. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Well, there is some "law" (more like a loose rule of thumb) that says animals tend to increase in size, right? For example, the largest dinosaurs were around right before the last major extinction.

    In other words, evolution tends to work small->large as opposed to large->small.

    But again, I'm no expert.
     
  15. synthesizer-patel Sweep the leg Johnny! Valued Senior Member

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    Not heard that one - got any more info? - size in vertebrates is often closely coupled to their proximity in the food web to primary productivity (i.e photosynthesis) - so for example certain whales / whale sharks / manta rays etc etc get so big because they feed on zooplankton -which in turn feed upon phytoplankton - so we have a very short food chain from phototsyntesis (i.e turning inorganic stuff into organic stuff in the first place) to vertebrate - the fewer the trophic levels the less energy lost at each step - thus the more efficient it is and the bigger the animal can grow. Note that the biggest whales & fishes etc are all planktivorous
     
    Last edited: Nov 8, 2008
  16. DeepThought Banned Banned

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    Mechanism?

    All the evidence suggests that life evolved from single celled organisms, reached a peak size during the dinosaur era, and is now decreasing in size.
     
  17. synthesizer-patel Sweep the leg Johnny! Valued Senior Member

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    although the largest animal ever to have existed (as far as we know so far); the Blue Whale, is still around - and hasn't been around for all that long (nor is it likely to stick around much longer)

    There's a whole bunch of variables that are responsible for size - diet, trophic position, oxygen etc - these are likely to complicate simplistic correlations
     
  18. Ophiolite Valued Senior Member

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    The question relates to the average size of all animals. The majority of animals are small, indeed almost microscopic. The few large animals are insignificant in terms of total mass.
     
  19. synthesizer-patel Sweep the leg Johnny! Valued Senior Member

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    actually Ben, if I apply a little thought to that it kind of makes sense - if we accept that size is often closely coupled to a trophic level that is close to primary production - and in general it is - think of all the really big animals that ever lived: diplodocus etc - in general they are herbivorous or at least only one or two trophioc levels from PP.
    Then consider that all of the major extinction events (like the KT extinction) have a close coupling to catastrophic climate change events - which effect food webs from the bottom up - from primary production and up the food web from there, then we have at least a viable hypothesis for natural selection selecting against very large organisms - I could give you some paradigms in the dynamics of phytoplankton speices succession throughout the seasons, but that's probably only interesting to marine biology geeks like me

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    Last edited: Nov 8, 2008
  20. Anti-Flag Pun intended Registered Senior Member

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    Wouldn't the average size be pretty consistent across time? I'm thinking along the basis of the earth only being able to sustain a maximum number of creatures, and that there would be less creatures at the top of the food chain(presumably large creatures) and many more smaller ones. Given the number of insects on the planet I imagine the average overall size would actually be little more than a large beetle.
    Or are we discounting insects?
     
  21. iceaura Valued Senior Member

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    The dinosaur that used to be called "brontosaurus" was around for many tens of millions of years - large size is not selected against, in normal circumstances.

    But it takes time for large size to be selected for - scaling up involves a lot of changes. Compare a hyrax to an elephant, or a catalpa tree to a garden bean plant. Even generations times are longer - slower evolutionayr feedback. So after an extinction event, large size takes a while to happen.

    And larger animals are inherently more vulnerable - there will be fewer individuals of them, simply on energy budget and space considerations, so they can be wiped out more easily. They also need far more room within a particular habitat for breeding populations etc. On islands, animals are smaller than on continents. The largest elephants, tigers, bears, wolves, ruminants, etc, were all found on the largest continents in the widest expanses of a given ecozone (until the very recent major extinction event).

    The continents are smaller and more separated now than in dinosaur times. The oceans are large and contiguous. The ocean animals alive now are apparently the largest that have ever lived.
     
  22. GeoffP Caput gerat lupinum Valued Senior Member

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    No, it certainly would not.

    By average size of animals I think you're looking for a maximal size relationship - "are they getting bigger overall". From a biomass perspective, no. But maximal size will, with variation within taxons, probably look like a series of sigmoidals with wide asymptotic tails as each major extinction event knocks out the large, K-selected types leaving smaller ones able to persist. But the inertia of maximal body size is probably very low. Evolution drives average maximal size back to the mean before the event quite quickly.
     
  23. DeepThought Banned Banned

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    But during the dinosaur era even insects were huge compared to today.

    Is there any reason to believe that 'microscopic' has always been the same thing in terms of size? Primitive cells may have been larger compared to todays more efficient organisms.

    Doesn't a human increase in biomass as it grows, reaching a peak and then falling into decline with age?
     

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