Higher animals and plants most likely have heterogametes. Isogametes are gametes that are morphologically similar and of the same size, while heterogametes are gametes that are morphologically different and of different sizes. In higher animals and plants, the male gametes (sperm or pollen) are typically smaller and more motile than the female gametes (eggs or ovules). This difference in size and motility is due to the different roles that male and female gametes play in reproduction. Male gametes are responsible for delivering the genetic material to the female gametes, while female gametes are responsible for providing the nutrients and environment necessary for the development of the embryo.