JavaScript中简单类型方法的委托

我目前正在深入研究JavaScript中非常好的对象委托原型。据我所知，简单类型(数字，字符串和布尔值)以及函数和数组从Object.prototype获得其基本方法。toString方法就是一个很好的例子。因为javascript对象的可变性，所以可以改变这些方法。也因为它的动态性质，这些变化将立即适用于所有连接到Object.prototype的原型。

这是一个动态原型链接应该如何工作的基本例子:

var myFirstObject = {
  myMethod: function () {
    console.log('this is the method from myFirstObject');
  }
},
mySecondObject = Object.create(myFirstObject);
mySecondObject.myMethod(); // "this is the method from myFirstObject"
myFirstObject.myMethod = function () {
  console.log('this is a dynamic change of myMethod from myFirstObject');
};
mySecondObject.myMethod(); // "this is a dynamic change of myMethod from myFirstObject"
mySecondObject.myMethod = function () {
  console.log('this is the method from mySecondObject')
};
mySecondObject.myMethod(); // "this is the method from mySecondObject"
delete mySecondObject.myMethod;
mySecondObject.myMethod(); // "this is a dynamic change of myMethod from myFirstObject"

然而，对于toString:

var myFunction = function () {};
Object.prototype.myCustomMethod = function () {
  console.log('This is a custom function on the Object prototype');
};
myFunction.myCustomMethod(); // "This is a custom function on the Object prototype"
Object.prototype.toString = function () {
  console.log('Don''t mess around with the Object prototype default methods');
}
myFunction.toString(); // "function () {}" <-- ???
Function.prototype.toString = function () {
  console.log('Don''t mess around with the Function prototype default methods');
};
myFunction.toString(); // "Don't mess around with the Function prototype default methods"
delete Function.prototype.toString;
myFunction.toString(); // "Don't mess around with the Object prototype default methods" <-- ???

所以我的问题是:像toString这样的默认方法发生了什么样的神奇是Object.prototype和Function.prototype之间有一个真正的委托还是只是一个简单的复制?