Species and courtship behaviourAQA A-Level Biology: Revision notes

Two organisms belong to the same species if they are able to produce fertile offspring. Fertile means the offspring can themselves reproduce.

If two organisms can mate but their offspring are sterile, they are different species. For example, a horse and a donkey can produce a mule, but a mule is sterile, so horses and donkeys are separate species.

Offspring produced by organisms of different species are called hybrids. The ability to mate or produce offspring is not enough: the offspring must be fertile.

Courtship behaviour is a set of actions carried out by one or both partners before mating. It is a necessary precursor to successful mating, because mating usually does not happen without it. Courtship can include calls, songs, displays, dances, flashing lights, chemical signals and the giving of food or other gifts.

Courtship:

• allows partners to recognise a mate of the right species
• shows that the partner is sexually mature and ready to mate
• stimulates the partner and synchronises mating and the release of gametes, so fertilisation is more likely

Courtship behaviour is often species-specific: each species has its own pattern of calls, flashes or movements. A potential mate responds only to the courtship of its own species.

This species recognition has an advantage: it ensures mating occurs only between members of the same species, so the offspring are fertile. Gametes, energy and time are not wasted on matings that would give no fertile offspring, which increases reproductive success.

Closely related species that live in the same place often have different courtship signals, such as different songs or flash patterns, which stops them mating with each other.

In exam questions, you may be given data on how often animals respond to the courtship signals of their own species and of another species. Quote the data and compare, for example $85 \div 6 = 14.2$ times as likely. Then link the result to species recognition and fertile offspring.

To decide whether two groups are the same species, apply the definition: can they produce fertile offspring? Courtship differences support but do not define the separation. If hybrids are healthy but sterile, the groups are different species.