Calling A Function With Prototype Property - Alert Message Returns In One Instance: 'someAnimal Is [Mammal "Mr. Biggles"]'

In the below code, the alert message returns in one instance: 'someAnimal is [Mammal "Mr. Biggles"]' I don't understand how this is possible. We pass as an argument 'Mr. Biggles' to the Mammal constructor. Then assign it to the object name property. But then it ends there. There is a function called toString which will return the alert. But the toString function is not being called like this: alert('someAnimal is '+someAnimal.toString()); If it was like how it is above, it would make sense to me but now it makes no sense to me because I don't see where that toString() is being called - I only see that it is prototyped to the Mammal object.

Code:
function Mammal(name){
this.name=name;
this.offspring=[];
}
Mammal.prototype.haveABaby=function(){
var newBaby=new Mammal("Baby "+this.name);
this.offspring.push(newBaby);
[Code]....