The pitch (fundamental frequency) of a mammal’s vocal signals tend to be correlated with body size, larger mammals produce lower pitched calls relative to smaller mammals (think of the rumble of an elephant to the squeak of a mouse). Although within adults of the same species the signals pitch can be an unreliable body size cue and the formants of the signal become more important. 

 

As body size changes with maturity however there is typically an associated lowering in pitch of vocal signals (the puppy’s yappy bark becomes a deeper bark in the adult dog). This is associated with an increase in size of the vocal apparatus.

 

The opposite however, appears to be true for the aquatic-calls of the leopard seal, where the older, much larger seals produce vocal signals (of the same call type) at a much higher pitch than the younger, smaller sub-adult seals.

 

The older leopard seals sound even more like a 'cricket‘.