Which of the following is a pre-reproductive isolating mechanism?

Pre-reproductive isolating mechanisms are strategies that prevent the mating or fertilization between different species, thereby contributing to speciation. Behavioral differences serve as a significant type of pre-reproductive barrier because they affect how individuals of different species interact during courtship and mating.

When two species exhibit distinct mating behaviors, such as different courtship rituals, vocalizations, or timing of reproduction, these differences can hinder individuals from recognizing each other as potential mates. This means that even if they inhabit the same area, these behavioral cues are crucial in ensuring that mating does not occur between species, thus maintaining species boundaries.

In contrast, gamete mortality, zygote mortality, and hybrid sterility happen after fertilization has occurred. Gamete mortality refers to the failure of gametes from different species to survive long enough to fertilize or create a zygote. Zygote mortality occurs when the fertilized egg does not develop properly into a viable organism. Hybrid sterility refers to the inability of hybrid offspring to reproduce, which is also a post-reproductive barrier. Therefore, these options do not prevent mating from occurring initially.