Pleistocene Mammals

[coquina]

Many mammals of the Pleistocene were gigantic compared with modern species - what factors existed in their environment that allowed them to attain such huge proportions that are not present today?

[Deathby]

We are a factor, but don't forget elephants and rhinos still exist. At least till we got guns

 

I think the major factor was climate. With a huge mass, there is a smaller surface area to lose heat from. And I believe the most recent Ice Ages have been in the Pleistocene. Australia had some really crazy megafauna until the aborigines came and wiped them out. There were wombats the size of a hippo

[Aardvark]

I think the major factor was climate.

 

It's a curious coincidence that the mega fauna die off always happened at the same time humans turned up on the scene.

 

I think its pretty obvious what happened, but for political reasons people are afraid to draw the obvious conclusions.

[blike]

She didn't ask why they died off, she asked what factors existed which allowed them to reach the size that they did. Let's stay on topic please.

[Aardvark]

She didn't ask why they died off, she asked what factors existed which allowed them to reach the size that they did. Let's stay on topic please.

 

The main factor was lack of humans.

[Sayonara]

Isn't that just the answer to the question "why did they not die off until time X?"

[Deathby]

The lack of humans (or any other animal capable of killing off large animals) is a major factor, but don't forget that elephants, giraffes, rhinos, hippos, storks (a bit dodgy- not many large birds left) still exist in Africa. And emus and cassowaries (around human height) exist too in Australia.

 

I reckon its the climate. A large size generally means a lower surface-area to volume ratio, which means that less heat can be lost/gained. Ironic eh?

Also I just thought of this: what about migrationary habits? Not many smaller mammals migrate. Only birds (I know Monarch Butterflies do but they can fly as well). Elephants "migrate" a fair bit, as do antelopes. They're fairly "large" as far as modern mammals go. And the biggest whales all migrate. I reckon that food would be scarce during the Pleistocene so animals would have to migrate, or at least wander. We know that mammoths and wooly rhinos migrated don't we?

[Ophiolite]

Gigantism seems to me to be the norm for a proportion of any fauna existing at any time. Amphibians and reptiles all had many large representatives. Size brings the advantage of temperature stability noted above and the 'can't mess with me because I am bigger than you' stance.

The only time we don't have large fauna is after an Extinction Level Event, when the big guys all get it. It then takes a few million years for the little chaps to grow up. We are in the midst of an ELE right now, courtesy of homo sapiens, and once again the big guys got it first. Consequently we view elephants and blue whales as strange exceptions. They are not, and their large size is simply a consequence of the advantages noted above.

[ecoli]

I think it probably has to do with selective mating. Animals choose the biggest strongest animals to propagate the species. The mammals prefer bigger mate; better to defend young, themselves. Also many mammals fight for mates. The bigger, stronger mammals will win the fight, and the mate.

Why they became extinct?

 

I don't think there is only one answer. Humans make sense, but we should also consider that maybe the animals got too big for their bodies. Being big means more food, more body mass, more blood flow. Their bodies simply couldn't support themselves There is not enough food to support the population - fierce competition for food and resources. The result? Dieoffs. The bigger animals die off the smaller ones come in, to take their place. Add that with humans; resoursful animals who can use tools and weapons, and view the bigger animals as an excellent source of food, and you might have an accurate picture of what happened.

[coquina]

How does this set with you?

 

When I took biology in the dark ages - I learned that only one organism can occupy the exact same niche.

A niche is ... it is the totality of all biological and environmental factors that affect a population. ...A niche can be defined as a hypervolume because it encompasses every thing you can think of that allows populations to live, grow and reproduce. This means for a life time....

 

Let's exclude human interferance for a moment...

 

Stephen Gould's "punctuated equilibrium" theory was that after major extinction events, many niches became vacated, and through chance mutation, organisms adapted that would fill them. However - since speciation is a slow process, maybe the animals that did fill a particular niche were able to grow larger because the population was lower.

 

We can see that the reverse is true. Take for example the pygmy mammoths, whose fossils have been found in the channel islands, of the coast of California. http://www.nps.gov/chis/pygmy.htm

 

Mammoths swam across the Santa Barbara channel, when sea level was at its lowest and when the islands were one larger land mass. As sea level rose, the geographical area decreased. As a result, small size was selected for.

 

I wonder if there is a "pygmy gene" present in many species that gets "turned on" by lack of food. Maybe there is a "giant gene" present that gets turned on by an abundance of it.

 

In other words - maybe there is more than sheer chance in adaptation towards an environment. From what I have read, when genomes are mapped, there seem to be large sections that appear to be unused. It seems a waste for an organism to manufacture all that, for no good reason. Maybe those sequences are "species catastrophic insurance policies" that get "turned on" by changes in the organisms niche.

[Sayonara]

There are some interesting situations under which multi-species niche occupation occurs. Iirc sympatry is one of them.

[Mokele]

I wonder if there is a "pygmy gene" present in many species that gets "turned on" by lack of food. Maybe there is a "giant gene" present that gets turned on by an abundance of it.

 

Maybe, but I'm much more inclined to chalk it up to simple variation and natural selection. Most animal populations display a pretty wide range of individual sizes, even when you take out environmental factors. Given that, if selection begins to favor one size extreme, you could expect the population to evolve in that direction at a fairly brisk pace, evolutionarily speaking.

 

Mokele

[AzurePhoenix]

okay, maybe this is far off, but i'm just thinkig out loud.....

 

early African humans faced a tough environment, so their numbers didn't get high enough to threaten the species that evolved alongside them. That's another factor. The african species evolved alongside them, so were better able to react to them in such a way as to avoid death.

when people left africa, they entered less harsh environennts, and flourished. Their numbers grew large enough to do severe damage, bringing harm to new species, unprepared to deal with these hairless little monsters. Also, the foreign disease thing would apply here.

 

..... go ahead, cut me down where I stand. Just be quick about it, I would rather not have to suffer.

[Blackfin]

I think the major factor that contributed to large species is simply the kind of biome they were living in. Grasslands predominated during the Pleistocence. Ever taken a look at the grasslands in Africa? The largest terrestial mammals live in them, and that's simply because a grassland supports a huge biomass of grazing species. Large animals live well in grasslands. And large predators evolved to fill the ecological niche left open by prey that grew too large to be caught by earlier predators. You couldn't have had a Cave lion without an aurochs and other similarly huge species. ~_^

[coquina]

I think the major factor that contributed to large species is simply the kind of biome they were living in. Grasslands predominated during the Pleistocence. Ever taken a look at the grasslands in Africa? The largest terrestial mammals live in them, and that's simply because a grassland supports a huge biomass of grazing species. Large animals live well in grasslands. And large predators evolved to fill the ecological niche left open by prey that grew too large to be caught by earlier predators. You couldn't have had a Cave lion without an aurochs and other similarly huge species. ~_^

 

Another interesting post - You mentioned the African grasslands. I had read about a giant kangaroo in Australia, but I since I didn't know that much about kangaroos - I thought I would test your theory. The following link goes through Australian fauna from the cretaceous to the present:

http://www.lostkingdoms.com/facts/index.cfm#pleistocene

Pleistocene Prey:

The Giant Short Faced Kangaroo:

http://www.lostkingdoms.com/facts/factsheet51.htm

"The Giant Short-faced Kangaroo had a flat face and forward-pointing eyes. On each foot it had a single large toe something like a horse's hoof. On these unusual feet it moved quickly through the open forests and plains, where it sought grass and leaves to eat. Its hands were equally strange: each hand had two extra-long fingers with large claws. Perhaps they were used to grab branches, bringing leaves within eating distance. "

 

Diprotodon:

http://www.lostkingdoms.com/facts/factsheet48.htm

"The hippopotomus-sized Diprotodon was the largest marsupial that ever lived. It browsed on tree leaves, shrubs and perhaps some grasses as it wandered through the open forest, woodlands and grasslands. It never strayed too far from water.

 

Diprotodon's closest living relatives are the wombats and Koala. "

Pleistocene Predator - Marsupial Lion:

http://www.lostkingdoms.com/facts/factsheet54.htm

"The Pleistocene Marsupial Lion is the largest meat-eating mammal to have lived in Australia, and one of the largest marsupial carnivores the world has ever seen. It would have hunted animals - including the giant Diprotodon - in the forests, woodlands, shrublands and river valleys, as well as around waterholes. "

 

So - I'd say your theory holds true for Australia as well as Africa.

[Blackfin]

That's cool, and I think you're right. I just didn't think of Australia immediately. Africa came more readily as an example.

 

God knows I watched enough episodes about wildebeest and buffalo on Wild Discovery when I was a kid. :3

[Hellbender]

.... Australia had some really crazy megafauna until the aborigines came and wiped them out. There were wombats the size of a hippo

 

along with carnivorous kangaroos, and truly gigantic lizards.

[Hellbender]

I think the major factor that contributed to large species is simply the kind of biome they were living in. Grasslands predominated during the Pleistocence. Ever taken a look at the grasslands in Africa? The largest terrestial mammals live in them, and that's simply because a grassland supports a huge biomass of grazing species. Large animals live well in grasslands. And large predators evolved to fill the ecological niche left open by prey that grew too large to be caught by earlier predators. You couldn't have had a Cave lion without an aurochs and other similarly huge species. ~_^

 

couldn't have said it better myself.