Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 05/08/2013 13:34:49
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Really enjoy your programme.
Why don't we see insects as large as whales or cows ? Why is the ratio of largest to smallest so much bigger for mammals (blue whale vs kitti's hog nosed bat) than it is for insects (goliath beetle vs fairy fly) ?
Thanks,
Jim Schaefer
Ann Arbor, Michigan, USA
Asked by Jim Schaefer
Visit the webpage for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/naked-scientists/show/20130801-1/)
[chapter podcast=1000442 track=13.08.01/Naked_Scientists_Show_13.08.01_1001159.mp3](https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.thenakedscientists.com%2FHTML%2Ftypo3conf%2Fext%2Fnaksci_podcast%2Fgnome-settings-sound.gif&hash=f2b0d108dc173aeaa367f8db2e2171bd) ...or Listen to the Answer[/chapter] or [download as MP3] (http://nakeddiscovery.com/downloads/split_individual/13.08.01/Naked_Scientists_Show_13.08.01_1001159.mp3)
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jim schaefer asked the Naked Scientists:
Really enjoy your programme (http://www.thenakedscientists.com/HTML/podcasts/).
Why don't we see insects as large as whales or cows ? Why is the ratio of largest to smallest so much bigger for mammals (blue whale vs kitti's hog nosed bat) than it is for insects (goliath beetle vs fairy fly) ?
Thanks,
Jim Schaefer
Ann Arbor, Michigan, USA
What do you think?
The structures that allow insects to walk and fly only allow them to grow to a certain size. It has to do with ratios. Think of the average bicept (a muscle used by the arm to lift). Th lifting capability of such a muscle is a function of the cross section of the muscle, i.e. how thick it is. If the strength of the muscle increases then it requires an increase in the cross sectional area to increase by a fctor of two. However if the shape of the muscle remains the same and the cross section increases by a factor of two then the volume increases by a factor of eight. Eventually the insect is unable to support his weigth using the same structural material and then that has to change to make thing larger. A similar argument applies to flight. An animal can only be so large and still be able to fly for similar reasons.
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I believe it also has to do with breathing. Insects do not have lungs, and there is a limit to the volume of body that insect respiration can oxygenate.
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This doesn't explain the cow or whale-sized insects, but it's interesting nonetheless: http://www.pnas.org/content/109/27/10927
Basically it's what Bill said--insects don't have an advanced respiratory and circulatory system like us, so they rely on absorbing oxygen fairly passively from the air. That means they're limited in size by the concentration of oxygen. Moreover, a large bird has far more efficient lungs so they can generally hunt much more actively than a similarly sized insect. Once birds evolved, insects stopped increasing in size, even though oxygen levels increased, indicating small size was an evolutionary advantage (they could move quickly and reproduce in large numbers).
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I believe it also has to do with breathing. Insects do not have lungs, and there is a limit to the volume of body that insect respiration can oxygenate.
A common breathing organ in insects is the spiracle, and so, many insects grow "longer" in order to be bigger — centipedes, millipedes, caterpillars, beetles, flies, etc. Maybe that's one reason why segmentation is so common among insects.
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There was a time when Earth had big insects.
Meganeura (http://en.wikipedia.org/wiki/Meganeura)
Don't know how correct it is but oxygen content must have played a role.
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(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.dmancini.com%2Fpictures%2Fother%2Farthropleura.JPG&hash=359275b5dbf730e0ec6e9f249dba25e3)
http://en.wikipedia.org/wiki/Arthropleura
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Perhaps the rigid exoskeleton restricts growth of insects - and shedding the exoskeleton in order to grow further represents a phase where it is susceptible to attack? Perhaps an internal skeleton allows steady growth without these major pauses?
Another factor is the strength of skeleton required to support a creature's weight. For Elephants, the leg bones take a large fraction of the diameter of the leg; presumably the ratio would be even more extreme for Apatosaurus.
However, seawater provides support, so the whales and plesiosaurs grew to quite large sizes.
One could speculate that insects could grow larger in seawater - if the sea and lakes weren't already populated by fishes with an internal skeleton, and with flow-through respiration using gills.
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Also, they have too little brain compared to animals that can grow larger.
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Also, they have too little brain compared to animals that can grow larger.
Interesting. Meaning that animals need to control their bodies so they don't become blundering, self-injuring and unable to defend themselves (and extinct) ... and that insects have limitation(s) in their nervous systems that evolution cannot overcome? Hmmm. The brain burns a lot of energy, so maybe the respiration, oxygen requirements,brain size, and body size are all related?
On a different line of thought, the mollusca phylum has diverse members, characterized by a shell, external for many species (clams etc), but internalized for many others (octopi, squids, etc). Maybe it's my perception, but the external-shelled animals seem mostly smaller and less intelligent, and the internalized-shell animals seem able to grow larger and more intelligent. Does being more vulnerable (lack of shell, maybe large size, etc) force an animal to compensate — if not by sheer number of offspring, then by intelligence (or both) — in order to survive? This might be the start of a new thread.
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That picture was scary RD :) He wouldn't just be in my boot, I suspect he rather would carry it on his, eh, feet?
Brains are highly convoluted are they not? In the way they are topologically I mean. Somewhat similar to how a lung can be stretched, folded out. A nervous system though is more like a analogue counterpart to some electrical description, able to produce logical reactions to the environment, as when one burn yourself.
Could one imagine a highly complicated array of a nervous-system to act similar to a brain, and if not, what will be missing? Isn't there animals that seem to have 'brains' in several locations too? Think I saw something about it, somewhere.
Where does a nervous system become a brain, the question might be :)
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Sure, a distributed nervous system. I'm thinking that, spread out as they are, they lack sufficient connectivity and hierarchy ... and maybe including consciousness.
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Sure, a distributed nervous system. I'm thinking that, spread out as they are, they lack sufficient connectivity and hierarchy ... and maybe including consciousness.
Octopuses have quite distributed nervous systems, but also show high intelligence and, potentially, consciousness (it's hard to know). Not sure if they've done the mirror test, but they can learn complex novel tasks by observation...
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jim schaefer asked the Naked Scientists:
Really enjoy your programme (http://www.thenakedscientists.com/HTML/podcasts/).
Why don't we see insects as large as whales or cows ? Why is the ratio of largest to smallest so much bigger for mammals (blue whale vs kitti's hog nosed bat) than it is for insects (goliath beetle vs fairy fly) ?
Thanks,
Jim Schaefer
Ann Arbor, Michigan, USA
What do you think?
Because if they were that big we'd call them whales or cows or dragons.
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That question causes me to ask: Why are there on insect sized whales?
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Sure, a distributed nervous system. I'm thinking that, spread out as they are, they lack sufficient connectivity and hierarchy ... and maybe including consciousness.
I wonder about insects and consciousness. The other day I was eating lunch at a picnic table and there was a large dead bug on it. Another insect went up to it, circled around it, paused, and then (I am not making this up) shoved it off the table. Then I watched the bug crawl down the picnic table to where the dead insect was, and haul him away.) How is that not a conscious thought process? And even if you believe it's "instinct" or "programmed" behavior, what test or experimental method would you use to tell the difference between instinctive, automatic behavior and conscious problem solving?
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I wonder about insects and consciousness. The other day I was eating lunch at a picnic table and there was a large dead bug on it. Another insect went up to it, circled around it, paused, and then (I am not making this up) shoved it off the table. Then I watched the bug crawl down the picnic table to where the dead insect was, and haul him away.) How is that not a conscious thought process? And even if you believe it's "instinct" or "programmed" behavior, what test or experimental method would you use to tell the difference between instinctive, automatic behavior and conscious problem solving?
First you've got to decide precisely what you mean by consciousness in this context. People can mean anything from simple responsiveness to environmental stimuli, through complex adaptive behaviour, to introspective self-awareness. Most people working on concepts of consciousness accept that there is a continuum both of degrees and kinds of consciousness; it isn't a simple thing, you can be conscious to varying extents, and in some ways but not in others.
Personally, I'd be inclined to judge this example in terms of adaptability and creativity in problem solving. I'd do a number of different trials with varying parameters to try to establish how flexible and adaptable this particular behaviour was.
A widely quoted (and debated) example of this kind of thing involves a particular species of sphex wasp that is said to show such rigid action patterns that it can be tricked into repeating them ad-infinitum (see Sand Wasps & Fixed Action Patterns (http://www.cals.ncsu.edu/course/ent425/tutorial/Behavior/sphex.html)). But this isn't the whole story - other sphex species don't get locked into this repetitive loop, some only briefly, and there seems to be some variation between individuals of a species too; does this mean they're 'more conscious' than those that do? And, of course, fixed, repetitive behaviour is not necessarily diagnostic of lack of consciousness - ask OCD (obsessive-compulsive disorder) sufferers.
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There was a time when Earth had big insects.
Meganeura (http://en.wikipedia.org/wiki/Meganeura)
Don't know how correct it is but oxygen content must have played a role.
I'm pretty sure the issue here is the effect of heat on diffusion. Insects are cold blooded and don't self-regulate temperature (at least not much). Insects that live in hotter regions can grow larger because their bodies reach higher temperatures and that lets oxygen and other nutrients move through the body more quickly. Similarly insects living in hotter times could also get larger.
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First you've got to decide precisely what you mean by consciousness in this context. People can mean anything from simple responsiveness to environmental stimuli, through complex adaptive behaviour, to introspective self-awareness. Most people working on concepts of consciousness accept that there is a continuum both of degrees and kinds of consciousness; it isn't a simple thing, you can be conscious to varying extents, and in some ways but not in others.
Personally, I'd be inclined to judge this example in terms of adaptability and creativity in problem solving. I'd do a number of different trials with varying parameters to try to establish how flexible and adaptable this particular behaviour was.
A widely quoted (and debated) example of this kind of thing involves a particular species of sphex wasp that is said to show such rigid action patterns that it can be tricked into repeating them ad-infinitum (see Sand Wasps & Fixed Action Patterns (http://www.cals.ncsu.edu/course/ent425/tutorial/Behavior/sphex.html)). But this isn't the whole story - other sphex species don't get locked into this repetitive loop, some only briefly, and there seems to be some variation between individuals of a species too; does this mean they're 'more conscious' than those that do? And, of course, fixed, repetitive behaviour is not necessarily diagnostic of lack of consciousness - ask OCD (obsessive-compulsive disorder) sufferers.
I've also heard of experiments that demonstrate instinctive, automatic behavior, where the animal seems caught in a loop, like a computer program. But when animals respond to novel situations in a sophisticated way, it becomes trickier to prove it is not instinct. In the past, scientists were accused of anthropomorphizing other species that seemed to be problem solving or displaying emotions. But it seems more unscientific to me to suggest that human behaviors suddenly popped into existence with us and have no roots in other species. It is fascinating, however, when animals or insects demonstrate similar behaviors with very tiny or different brains.
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In the past, scientists were accused of anthropomorphizing other species that seemed to be problem solving or displaying emotions.
Yes, but I think it's a general problem for scientists. It is very easy to jump to conclusions about what appears to be obvious and discover you're mistaken after wasting a lot of time and effort based on the mistaken assumption. Anthropomorhising is something we seem to do instinctively, even attributing agency and intent to inanimate objects and natural events. So we must by hyper-vigilant in this respect, and the default approach is not to accept the idea until it has been defined, tested, and verified. Another problem is the difficulty of obtaining a consistent consensus definition for consciousness - how can you test for something you don't have a solid definition for?
In this particular field, there was also the legacy of the quasi-religious idea that humans were quite different from, and superior to, animals - particularly in having consciousness and other advanced faculties (which was associated with the idea that animals were here for our benefit, and could therefore be exploited without qualms). It took a while to get past that worldview.
But it seems more unscientific to me to suggest that human behaviors suddenly popped into existence with us and have no roots in other species. It is fascinating, however, when animals or insects demonstrate similar behaviors with very tiny or different brains.
Yes; you only have to look at other primates and great apes to see we're on a related branch of the tree of life. The more we study other animals, not only mammals, the more we are surprised by their mental faculties and wonder how we could have ignored and missed them for so long. Even octopuses, invertebrates with a differently organised nervous system, show surprising intelligence (e.g. learning complex tasks simply by watching them done) and a degree of self-awareness.
Of course, something the size of most insects doesn't have much room for a highly sophisticated brain with complex adaptive behaviours, so their behavioural adaptability is limited, but can still surprise. I suppose that's why so many insects adopted the hive organism approach, where complex adaptive behaviours can be generated from the interactions of many relatively simple individuals.
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That question causes me to ask: Why are there on insect sized whales?
As with many scientific questions, asking the inverse question is often illuminating because it takes us back into the realm of the known.
There are bats and shrews that weigh less than 2 grams, and plenty of insects that weigh more, but there's no suitable ecological niche for a tiny aquatic mammal. The surface/volume ratio of small mammals is a limiting factor: tiny mammals (and hummingbirds) spend nearly all their time eating in order to keep warm enough to stay alive in air: they simply cannot eat enough to stay alive in sea water.
I suspect the opposite may be important in insect physiology: above a certain size, the heat generated by muscle movement might cook the digestive enzymes. Or it may be that with no effective heat sink (due to large size and exoskeleton) the muscles themselves may not work.
Mobility and escape from predators might be important. Yacht racing is ridiculously expensive because the speed of a boat depends on the square root of its length but the cost depends on the cube of its length, so you have to spend a hell of a lot of money to go a tiny bit faster. I suspect the same is true of terrestrial insects - the bigger you are, the thicker your exoskeleton needs to be (the thickness of a girder or roof beam increases with the square of its unsupported length) so armour weight increases disproportionately to the muscle mass available to move it.
But of course there is no size limit to an aquatic invertebrate. It just happens that the ecological niches for very large ones are occupied by creatures with more than six legs.