# Why were dinosaurs so big? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [September 6, 2013, 3:08pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178 "2013-09-06T15:08:20Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![GRobLewis](https://avatars.discourse-cdn.com/v4/letter/g/b5e925/32.png) [@GRobLewis](https://boards.straightdope.com/u/GRobLewis)\ **Post date:** [September 6, 2013, 3:08pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/1 "2013-09-06T15:08:20Z")
Correct me if I’m wrong, but I believe there’s a general principle in biology that, given the opportunity, organisms will tend to evolve to larger body sizes. A big benefit of being big is that it affords protection from predators. Of course, the predators are meanwhile evolving bigger, badder bodies too (_T. rex_, for instance), and that’s a perfect example of the well-known phenomenon of _evolutionary arms race_. There are obvious limits to how big any species can grow: the strength of bones, the availability of food and habitat, and so on. And of course there are advantages to being small in some cases (for example, the pygmy elephants that evolved on small islands in the Mediterranean, where food and space were limited and major predators probably absent). Speculation: wasn’t there more oxygen in the atmosphere during the dinosaur era? That, too, could have enabled them to grow larger and still keep all their tissues oxygenated. ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [September 6, 2013, 9:16pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/2 "2013-09-06T21:16:03Z")
Probably referring to [this](http://www.straightdope.com/columns/read/985/why-were-dinosaurs-so-big) article by The Master(arr). > [@](#): > > Cecil replies: > > We have two questions here, Andrew — the one you asked, and the one you would have asked if the drugs hadn’t kicked in first. The best that can be said for question #1 is that if you mention it to a stranger on the bus you’re guaranteed to get the seat to yourself. Question #2 is more interesting: why were dinosaurs so big? Some theories: > > Dinosaurs had to be big to reach the leaves on the tops of the trees. This is the answer favored by fourth graders. It probably was a factor in the size of some dinosaurs, notably brachiosaurus, a plant-eater described as basically a crane on legs. But dinosaurs were huge whether they ate plants or not. One estimate puts their median weight at two tons. > > Dinosaurs had to be big because they were (a) cold-blooded or (b) warm-blooded. Paleontologists have been debating whether dinosaurs were warm-blooded or cold-blooded for 25 years, and the great size of the animals has been enlisted as an argument on either side. Cold-blooded advocates say dinosaurs were big because their sheer thermal mass protected them against the sudden temperature swings to which small cold-blooded creatures were vulnerable. Objection: “bulk homeothermy,” as it’s called, only works in animals weighing more than a ton; some dinosaurs were smaller. > > Warm-blooded advocates say dinosaurs had to be big because their uninsulated skin made them easily chilled. Great size gave them a lower ratio of skin surface to volume and thus reduced heat loss. Objection: come on, they couldn’t wear sweaters? There have to be easier ways to stop heat loss than putting on twenty thousand pounds. Equally to the point, how did they survive long enough to get big? > > Dinosaurs had to be big because the early mammals had taken all the small slots, ecologically speaking. It’s thought mammals were more agile than the relatively stiff-limbed dinosaurs and thus better equipped to survive. Objection: OK, but why couldn’t mammals have become huge too and beaten out dinosaurs at both ends of the size continuum? > > The question may never be resolved because so much about dinosaurs is unknown and, given the limitations of the fossil record, may be … well, I don’t know that it’s completely unknowable. But Cecil can appreciate that it’s a lot easier holding forth on your latest grand theory in the faculty room than excavating in some former swamp with a spoon. > > — Cecil Adams ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [September 6, 2013, 9:35pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/3 "2013-09-06T21:35:17Z")
Hmmm. That column was from 1994; we’ve learned a lot more about dinosaurs since then, and p’raps a review would be in order. ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [September 6, 2013, 11:24pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/4 "2013-09-06T23:24:08Z")
Because that’s what lady dinosaurs liked in a mate. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 7, 2013, 1:06am UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/5 "2013-09-07T01:06:15Z")
> [@C\_K\_Dexter\_Haven](#): > > Hmmm. That column was from 1994; we’ve learned a lot more about dinosaurs since then, and p’raps a review would be in order. My pet theory is that bipedal posture allowed the largest predator dinosaurs to grow much larger than any four-footed predator while remaining comparatively speedy. So the herbivorous dinosaurs increased in size to match. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [September 9, 2013, 3:54pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/6 "2013-09-09T15:54:51Z")
I tend to favour the idea that they grew big simply because they could. In that era the climate was warmer and CO2 levels higher. This meant a superabundance of food, so much like Americans (and others) today, they just kept growing. ---
**Author:** ![Spoke](https://avatars.discourse-cdn.com/v4/letter/s/a88e57/32.png) [@Spoke](https://boards.straightdope.com/u/Spoke)\ **Post date:** [September 9, 2013, 4:00pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/7 "2013-09-09T16:00:33Z")
I think I agree with you, **bob++**. Before human hunters came along, mammals also seemed to be headed in the direction of evolved gigantism. ---
**Author:** ![tullsterx](https://avatars.discourse-cdn.com/v4/letter/t/e9bcb4/32.png) [@tullsterx](https://boards.straightdope.com/u/tullsterx)\ **Post date:** [September 9, 2013, 4:13pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/8 "2013-09-09T16:13:00Z")
I don’t know, there were many small dinosaurs and mammals that survived by being small and fast and stealthy. So, why wouldn’t evolving smaller also be a natural trend. So, I believe the general trend/options would have been 1) get bigger 2) get smaller 3) develop some other defense. And basically human dominance has eliminated option #1. ---
**Author:** ![drewder](https://avatars.discourse-cdn.com/v4/letter/d/57b2e6/32.png) [@drewder](https://boards.straightdope.com/u/drewder)\ **Post date:** [September 9, 2013, 4:39pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/9 "2013-09-09T16:39:11Z")
I’ve heard that their large size was only sustainable due to the higher average global temperature(in the range of 10-15 degrees) ---
**Author:** ![extanker59](https://avatars.discourse-cdn.com/v4/letter/e/2bfe46/32.png) [@extanker59](https://boards.straightdope.com/u/extanker59)\ **Post date:** [September 9, 2013, 4:44pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/10 "2013-09-09T16:44:46Z")
This isn’t commenting on the size question but on the earth spinning faster by a fraction of a second each year. His idea is wrong then. The earth would have been rotating slower in the past. As far as the rotational speed needed to change the gravity around the equator by any noticable amount, pshaw. But Cecil could have commented on that. ---
**Author:** ![Lightnin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lightnin/32/180_2.png) [@Lightnin](https://boards.straightdope.com/u/Lightnin)\ **Post date:** [September 9, 2013, 6:00pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/11 "2013-09-09T18:00:44Z")
My theory is that the ecological niches for smaller creatures tend to fill in faster. Over time, existing species will radiate into untapped niches, but the niches that can be filled by larger creatures will be filled slower. In addition, megafauna tend to be more vulnerable to ecological changes- like, you know, incoming comets and angry primates. The dinosaurs had a very, \*very \*long time to evolve to exploit those niches, and enjoyed a reasonably stable environment for quite a while, resulting in more megafauna than we see today. In other words, the longer an environment is unchanged, the more species will evolve to fill the “large animal” ecological niche. That’s my theory, anyway. ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [September 9, 2013, 7:03pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/12 "2013-09-09T19:03:20Z")
No, the earth rotated faster in the past, and has been slowed by tides. It’s still slowing, in fact. ---
**Author:** ![extanker59](https://avatars.discourse-cdn.com/v4/letter/e/2bfe46/32.png) [@extanker59](https://boards.straightdope.com/u/extanker59)\ **Post date:** [September 9, 2013, 7:31pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/13 "2013-09-09T19:31:45Z")
Yeah, I thought I had read that too but couldn’t be sure. I was only commenting on his statement: “I read somewhere that the Earth’s day is getting shorter by a fraction of a second every year.” So, if it was getting shorter as he thought, then it’s speeding up. Unless it’s a typo by Cecil? Nah. Couldn’t be. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 9, 2013, 7:58pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/14 "2013-09-09T19:58:48Z")
It’s worth noting that while there have been giant mammal herbivores- mammoths, Paraceratherium, etc.- the upper limit on (land) mammal carnivore size has remained much smaller. We have no record of any mammal carnivore that ever exceeded one metric tonne in weight, and the vast majority were half that or less. The saber-toothed cats apparently adapted to hunting elephant-sized prey not by growing huge, but by evolving specialized fangs capable of fatally wounding large animals. Apparently a bigger four-footed predator would simply be too slow and lumbering even to ambush prey. The chief way dinosaurs were different was that the predator species were bipedal. If that made them faster, then the herbivores would have had a strong selective pressure to grow larger. Crocodilians were a special case, being semi-aquatic ambush hunters. The test case would be non-dinosaur reptilian predators of the Permian era, but I don’t know much about them- anyone? ---
**Author:** ![Oldeb](https://avatars.discourse-cdn.com/v4/letter/o/ea666f/32.png) [@Oldeb](https://boards.straightdope.com/u/Oldeb)\ **Post date:** [September 9, 2013, 11:16pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/15 "2013-09-09T23:16:09Z")
> [@Lumpy](#): > > Crocodilians were a special case, being semi-aquatic ambush hunters. The test case would be non-dinosaur reptilian predators of the Permian era, but I don’t know much about them- anyone? Bear-sized, with the only exception I could find being a semi-aquatic ambush hunter similar to later crocodiles. Using the largest sizes: [ul] [li]Dimetrodon - Early Permian: 3.2 meters[/li][li]Titanopheneus - Middle Permian: 2.85 meters[/li][li]Titanosuchus - Late Permian: 2.5 meters[/li][li]Inostrancevia - Late Permian: 3.5 meters[/li][/ul] All are Synapsids from lines that either produced mammals or with no extant descendants and all comparable to both the black bear (2.4 meters) and the polar bear (average 2.4 - 3, record 3.39 meters) and not much more than a quarter the size of later Tyrannosaursids. The semi-aquatic ambush hunter was [Anteosaurus](http://en.wikipedia.org/wiki/Anteosaurus) of the Middle Permian coming in at 5 meters and believed to have hunted much like modern crocodiles. Not sure, but it may have been the largest non-marine Synapsid carnivore in history. And nitpick, but pregnant polar bears can frequently exceed the 1 metric ton weight without difficulty hunting and obviously you’ll want to exclude pinnipeds and cetaceans from that mammalian carnivore weight limit. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 10, 2013, 1:59am UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/16 "2013-09-10T01:59:36Z")
> [@Oldeb](#): > > And nitpick, but pregnant polar bears can frequently exceed the 1 metric ton weight without difficulty hunting and obviously you’ll want to exclude pinnipeds and cetaceans from that mammalian carnivore weight limit. Ok, I should have said “around” a metric tonne to not quibble a few kilos. But my point still stands: I’m unaware of any quadrupedal predators that significantly exceeded that weight, other than the semi-aquatic species mentioned. ---
**Author:** ![Coccinelle](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Coccinelle](https://boards.straightdope.com/u/Coccinelle)\ **Post date:** [September 12, 2013, 7:06am UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/17 "2013-09-12T07:06:08Z")
I’m happy to be able to contribute to this thread. I have heard, and would be glad to have this verified,that as a general principal that as the size of an organism increases it’s energy demands (or required calorie intake) does not increase proportionately. That is to say that if you double in size, your energy demands will be less than double. This would give a clear evolutionary advantage to getting big, should time and circumstances be in your favour. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 12, 2013, 12:25pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/18 "2013-09-12T12:25:47Z")
> [@Coccinelle](#): > > I’m happy to be able to contribute to this thread. I have heard, and would be glad to have this verified,that as a general principal that as the size of an organism increases it’s energy demands (or required calorie intake) does not increase proportionately. That is to say that if you double in size, your energy demands will be less than double. This would give a clear evolutionary advantage to getting big, should time and circumstances be in your favour. The question then would be why did this apply to dinosaurs more than any other group of animals? In particular, we can compare the dinosaurs to the reptiles that existed in the Permian period. The incredibly stocky [Pareiasaurs](http://en.wikipedia.org/wiki/Pareiasaur) look like they were maximizing their ratio of body mass to surface area but they were only about cow sized. ---
**Author:** ![Coccinelle](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Coccinelle](https://boards.straightdope.com/u/Coccinelle)\ **Post date:** [September 17, 2013, 8:04pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/19 "2013-09-17T20:04:05Z")
> [@Lumpy](#): > > The question then would be why did this apply to dinosaurs more than any other group of animals? In particular, we can compare the dinosaurs to the reptiles that existed in the Permian period. The incredibly stocky [Pareiasaurs](http://en.wikipedia.org/wiki/Pareiasaur) look like they were maximizing their ratio of body mass to surface area but they were only about cow sized. I wouldn’t suggest it applies to dinosaurs more than any other animal, except that perhaps surface area might play a part in facilitating the heating of cold blood. Most of the explanations in this thread make sense to me. Another way to put the question might be: Why weren’t dinosaurs bigger? Or any other animal for that matter. I knew a dino expert who told me that it was not conclusive that T-rex fossils have shown actual full-grown size. Apparently, when crocodiles get old their bones fuse (as they do in humans, and other animals), but as yet no T-rex skeleton shows these tell-tale signs of old age. Also, the life spans of crocodiles show continual growth right up to the point of death. ---
**Author:** ![jack](https://avatars.discourse-cdn.com/v4/letter/j/cc9497/32.png) [@jack](https://boards.straightdope.com/u/jack)\ **Post date:** [September 18, 2013, 8:18pm UTC](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178/20 "2013-09-18T20:18:52Z")
> [@John\_W.Kennedy](#): > > No, the earth rotated faster in the past, and has been slowed by tides. It’s still slowing, in fact. I knew my workday was getting longer [Next page](https://boards.straightdope.com/t/why-were-dinosaurs-so-big/668178.md?page=2)