Final answer:
A frog with vocal sacs is generally a male, as they use these sacs for mating calls to attract females and engage in external fertilization.
Step-by-step explanation:
A frog with vocal sacs is typically a male. The presence of vocal sacs is one of the primary sexual dimorphisms in frogs, which are used to amplify their mating calls. These calls are essential during the mating season to attract females and establish territory among other males. It is during this time that they also physically demonstrate their fertility by grasping the female from behind in an embrace called amplexus during external fertilization, where the male releases sperm over the female's eggs as they are laid.
In summary, vocal sacs are a characteristic feature of male frogs used for their croaking mating calls that attract females to suitable breeding sites and deter rivals. This behavior, alongside the physical act of amplexus, highlights how frogs reproduce through external fertilization.