Ataru and Spike: "H marks the spot?"

In some strange twist of fate, Ataru has gone treasure hunting with Spike. However, while following the map to the X, their final destination has left them in a rather puzzling situation...

Since this is Spike's first appearance, here's some info. Spike is Ataru's older brother, and he lives with their Mom and Dad out in the faraway savanna. Spike has had a history of picking on his little brother for his shortfallings, and sometimes tortures him by exercising his greatest fears. Why is Spike so mean towards his little brother? Is he the reason why Ataru's so timid?