[Terr_]

> arbitrary chemical environments

Temperature is another factor. IIRC amphibian embryos have to develop in a wide range of temperatures (an egg might be stuck to a leaf), so their cells have many more variants of proteins, where each variant is most-effective in a different temperature band.

In contrast, a mammal blastocyst or embryo already has the multicellular mother keeping temperature within a narrower band.

[tyre]

Another interesting example is sea turtles, whose eggs are in a relatively stable environment (sand), but its temperature changes year to year. Based on the temperature of the eggs, you see a different distribution of offspring sex.

[vanderZwan]

Yeah, ages ago I read in a book about evolution that mammalian genes are actually simplified (or optimized, if you will) compared to amphibians because we don't have to accomodate as wide of a temperature range due to being warm-blooded and giving live birth.

I also recall seeing in a documentary that the temperature of crocodile eggs will determine if it's a male or female. Wikipedia seems to back that up:

https://en.wikipedia.org/wiki/Temperature-dependent_sex_dete...