Behavioral Isolation | Definition & Example

The different mating rituals of animal species create extremely powerful reproductive barriers, termed sexual or Behavioral isolation, that isolates apparently similar species in the majority of the groups of the animal kingdom. In dioecious species, males and females have to search for a partner, be in proximity to each other, carry out the complex mating rituals and finally copulate or release their gametes into the environment in order to breed.

Mating dances, the songs of males to attract females or the mutual grooming of pairs, are all examples of typical courtship behavior that allows both recognition and reproductive isolation. This is because each of the stages of courtship depends on the behavior of the partner. The male will only move onto the second stage of the exhibition if the female shows certain responses in her behavior.

Behavioral Isolation Definition

He will only pass onto the third stage when she displays a second key behavior. The behaviors of both interlink, are synchronized in time and lead finally to copulation or the liberation of gametes into the environment. No animal that is not physiologically suitable for fertilization can complete this demanding chain of behavior. In fact, the smallest difference in the courting patterns of two species is enough to prevent mating (for example, a specific song pattern acts as an isolation mechanism in distinct species of grasshopper of the genus Chorthippus). Even where there are minimal morphological differences between species, differences in behavior can be enough to prevent mating. For example, Drosophila melanogaster and D. simulans which are considered twin species due to their morphological similarity, do not mate even if they are kept together in a laboratory. Drosophila ananassae and D. pallidosaare twin species from Melanesia. In the wild, they rarely produce hybrids, although in the laboratory it is possible to produce fertile offspring. Studies of their sexual behavior show that the males’ court the females of both species but the females show a marked preference for mating with males of their own species. A different regulatory region has been found on Chromosome II of both species that affects the selection behavior of the females.

Pheromones play an important role in the sexual isolation of insect species. These compounds serve to identify individuals of the same species and of the same or different sex. Evaporated molecules of volatile pheromones can serve as a wide-reaching chemical signal. In other cases, pheromones may be detected only at a short distance or by contact.

What Is Behavioral Isolation

In species of the melanogaster group of Drosophila, the pheromones of the females are mixtures of different compounds, there is a clear dimorphism in the type and/or quantity of compounds present for each sex. In addition, there are differences in the quantity and quality of constituent compounds between related species, it is assumed that the pheromones serve to distinguish between individuals of each species. An example of the role of pheromones in sexual isolation is found in ‘corn borers’ in the genus Ostrinia. There are two twin species in Europe that occasionally cross. The females of both species produce pheromones that contain a volatile compound which has two isomers, E and Z; 99% of the compound produced by the females of one species is in the E isomer form, while the females of the other produce 99% isomer Z. The production of the compound is controlled by just one locus and the interspecific hybrid produces an equal mix of the two isomers. The males, for their part, almost exclusively detect the isomer emitted by the females of their species, such that the hybridization although possible is scarce. The perception of the males is controlled by one gene, distinct from the one for the production of isomers, the heterozygous males show a moderate response to the odor of either type. In this case, just 2 ‘loci’ produce the effect of ethological isolation between species that are genetically very similar.

Sexual isolation between two species can be asymmetrical. This can happen when the mating that produces descendants only allows one of the two species to function as the female progenitor and the other as the male, while the reciprocal cross does not occur. For instance, half of the wolves tested in the Great Lakes area of America show mitochondrial DNA sequences of coyotes, while mitochondrial DNA from wolves is never found in coyote populations. This probably reflects an asymmetry in inter-species mating due to the difference in the size of the two species as male wolves take advantage of their greater size in order to mate with female coyotes, while female wolves and male coyotes do not mate.

Behavioral Isolation Example

Each group has its characteristic courtship ritual. This elaborate, complex mating ritual isolates them from closely related species and helps them find the correct mating partner. The blue-footed booby (Sula nebouxii), for instance, shares its habitat with several other species of the genus Sula, but despite all their similarities, it never mates with them. In this case, behavioral isolation is facilitated by the elaborate courtship ritual that is unique to this species. The female blue-footed booby selects a mating partner only after watching the entire courtship ritual.

How Does Behavioral Isolation Lead to Speciation?

Suppose we have one population of very similar organisms that can reproduce with one another. All of these organisms would be considered one species. However, if some of the females only mate with males in the population who can make a specific sound this is called behavioral isolation.

Behavioral isolation can lead to speciation over time. As more time goes by, the subgroups continue to become different until they are no longer able to interbreed. They become two different species.

Once speciation occurs, the isolating behaviors ensure mating only happens between members of the same species by making it easy to recognize potential mates. This also prevents the waste of energy and time on looking for and mating with individuals that won’t produce fertile or viable offspring.

Why is behavioral isolation beneficial?

Limiting the number of potential mates and narrowing the gene pool is not always beneficial to an organism. However, while behavioral isolation can have a negative impact on an individual, it will benefit the entire species as a whole. Environmental pressures that an entire population faces lead to small changes that can cause behavioral isolation and then (possibly) speciation.

Birds are known to have so of the weirdest mating rituals. Watch the video below to see just one example!