C# why does ternary operator require explicit casting?

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C#

.NET

conversion

casting

Why do I need explicit cast in this case:

class Program
    {

        static void Main()
        {
            bool boolVal = true;
            Animal a = boolVal ? new Dog() : new Cat();
        }

    }

    class Animal { }

    class Dog : Animal { }

    class Cat : Animal { }

At this line: "Animal a = boolVal ? new Dog() : new Cat();" I will get a compiler error "Type of conditional expression cannot be determined because there is no explicit conversion between Dog and Cat".
But If I add a cast to animal before "new Dog()" the compiler doesn't complain. Don't I also need to cast the second expression ? What's this strange behavior? :headache:

Also the same is the case with value types:

static void Main()
        {
            bool randomBool = true;
            short someRandomVal = 5;

            short s = randomBool ? someRandomVal : 0; // Won't work
        }

However here I need to cast the second value not the first value because the first is already a short.

What I have tried:

Looking at the documentation for ternary operator however I didn't see anything related to this behavior there.