What high-pitched sound is often a signal for play or alertness in animals?

The high-pitched sound that typically indicates a signal for play or alertness in animals is barking. This vocalization is commonly associated with dogs, who use barking to communicate various emotions and intentions, including excitement, playfulness, and a call to attention. In a playful context, a high-pitched bark can serve as an invitation for interaction or playtime, showcasing the dog's eagerness and alertness to their surroundings.

Other sounds, such as squealing, yowling, and hissing, serve different communicative purposes. Squealing might indicate distress or excitement but is less common in the context of playful interactions. Yowling is primarily associated with cats and can signify discomfort or a desire to attract attention rather than playful behavior. Hissing is typically a defensive vocalization, indicating fear or aggression, rather than an invitation to engage in play. Therefore, barking stands out as the most relevant sound related to playful and alert behavior in animals.