Expert: Walter Hintz - 9/28/2006

Question
I just read that animals with a backbone, mammals are generally regarded as the most dominant modern land animals, when compared to reptiles and amphibians. Then there  are "placental" mammals where their embryos get nourishment within the mother through the placental system of blood vessels. But by contrast, amphibian and reptilian embryos are nourished by substances contained in eggs the mother lays. What are any advantages mammals derive from the placental means of development? WHat are some of the disadvantages?

Answer
Hi Jamie
First of all "most dominant" is a relative term.  Mammals are not the most dominant land animals.  Insects are.  With regard to vertebrates  there are 4000 known mammals, 7870 reptiles and 8000 bird species.
When it comes to individual survival however placental mammals do have an edge.  When a mammal is born it is fairly well developed and then cared for by the mother. This is the big advantage. The disadvantage is that the young are completely dependent upon the mother for some time and if something happens to her they cannot survive, (Humans are an exception since we take of each other,or should)
Many other vertebrates also care for the young, like the crocodilians and the birds.  There is a relationship between the number of the young born and the odds of survival.