When male horses (stallions) and female donkeys (jennets) mate, they produce a sterile hybrid called a hinny. Hinnies occur much less frequently than do mules, but are just as healthy and robust as mules. Logically, which of the following best accounts for the relative rarity of hinnies, and what kind of prezygotic isolating mechanism is at work here?
A) Most hinnies die during fetal development; reduced hybrid viability.
B) Most hinnies die soon after being born; hybrid breakdown.
C) Most hinnies are reproductively sterile; reduced hybrid fertility.
D) Stallions and jennets are choosier about their mating partners than are mares and jacks; behavioral isolation.
E) Stallions and jennets are choosier about their mating partners than are mares and jacks; gametic isolation.
Answer: D
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