h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

My son recently asked me why we have few large land animals these days, compared to the height of the dinosaurs and then the huge mammals such as Indricotherium.

I assume it must be to do with different quality food sources

Can anyone help?

Lost Rhino

Evolution.

Smaller animals need to eat less to survive, therefore have a greater chance of surviving ice ages.

Classic Darwinism.

B

there's also the oxygen issue - oxygen levels have fallen since those massive animals roamed the earth.

I can understand the greater chance to survive the ice age argument, but did the recent ice ages affect the tropics that much, I would assume the mechanism that drove animals to giant size would still be applicable to the animals living in largely unaffected areas.

The Oxygen argument - I didn't think that the oxygen levels had changed that much certainly since the last large mammals

Is the emergence of mankind not an important reason certainly the Native Americans are meant to have wiped out a large number of megafauna. However I disagree with the heat agrument as I understood that generally if there are 2 species who only (or predomenantly) differ only in size then the large one is generally in the colder climate.

Large mammals need less food than smaller ones relative to their body size and larger body masses are more efficient in maintaining body temp. Also the megafauna of northern areas had clear adaptations to colder climate and there extinction coincides with the warming after the end the last glaciation period and the spread of peoples into there environs.
Scarcity of food may have been a factor but this would be more do with climate change and sudden changes in its distribution and not because large animals need to eat excessively.

Dinosaurs lived in a much hotter period of geological history which suggest plants would have grown faster and they would need to eat less plant mass for fuel and dinosaurs could have drawn heat from the environs as modern reptile do.

Humans certainly have had some impact. Elephant birds in Madagascar and the giant moa in New Zealand were both hunted to extinction by people in the last millenium.

yes, definitely you humans never did anything good for the large sized animals .. like me, or cousin elephant ..

but we still like you .. most often