Sex and behaviour

Sexual reproduction brings with it its own set of behavioural implications, as individuals in a species must interact for the purpose of reproduction.

Parental investment

Parental investment refers to the input required by parents to ensure the survival of their offspring. This can be diverged even before the offspring are brought to life, in terms of the preparations required, whether biological or environmental. For example, the production and incubation of eggs requires more investment of energy for fewer offspring, while the production of sperm cells requires less energy for more cells.

In organisms that are immobile i.e. sessile, such as barnacles, sex poses a distinct problem.

Barnacles are attached to their rock, and cannot approach one another for sexual reproduction. A solution for this is the very long barnacle penis that extends quite some distance from its body in the hope of reaching a receptive barnacle nearby. Another solution is the casting of sperm into the sea for other barnacles to passively pick up.

This highlights internal versus external fertilisation. Each has its own costs and benefits. External fertilisation such as that carried out by fish requires little energy in terms of finding a mate and protecting offspring, and can produce many offspring over a large area, but results in higher mortality of offspring and wasted gametes.

Internal fertilisation on the other hand has better offspring survival as the embryo is shielded away from predators and offspring taken care of by parents, at the expense of having fewer offspring and expending energy towards finding a mate and looking after offspring.

In r/K selection theory, the number of offspring a population can have in a habitat is used to contrast species that produce many offspring with those that produce few.

r refers to the rate of growth while K refers to the carrying capacity of the habitat. Species that invest less in their offspring but have many are on the r side of the equation, hence termed r-selected organisms.

Those that tend towards the K side have fewer offspring and invest more energy in each of them. These are K-selected organisms.

Several traits are associated with each reproductive strategy, such as organism size, the stability of the niche they occupy and the developmental speed of offspring. Large organisms have fewer offspring and take close care of them. The offspring reach maturity slower.

Smaller organisms have more offspring and don't look after them as much. These reach sexual maturity quicker.

This model can accommodate a spectrum, and some species can't be classified as r-selected or K-selected. For example, turtles are large and long-lived, but they have many offspring and release them without watching them closely.

Reproductive strategies range from polygamy to monogamy, where individuals may mate with multiple partners or just one, although the latter is rare, and more closely represented by monogamy with extra-pair copulations, common in bird species, where mating outside of an established monogamous relationship takes place.

Primates including humans form social groups based on a few distinguished models based on the size of the group and relationships between members: single female and her offspring, monogamous family group, polyandrous family group, one-male-several-female group, multimale-multifemale group and fission-fusion society.

The strongest group organisation in primates is the single female and her offspring which occurs commonly in orangutans. The monogamous family group is formed of both parents and their offspring and is represented by some New World monkeys, and is a prevalent group type in humans too, even in polygamous cultures.

Polyandrous family groups consist of multiple males mating and rearing offspring with a female, found in tamarins and some humans. One-male-several-female groups do what they say on the tin, and are exhibited by langurs, gorillas and some humans. In some species the females are the core part of the group and choose the male, while in others the male is dominant over the females.

Multimale-multifemale groups contain many individuals who mate freely within the group, but not outside the group. The group members do fit into dominance hierarchies, determined by the hierarchy of their mother before them. These groups are found in savanna baboons and macaques.

Fission-fusion societies involve the breaking away from the group and joining new groups by chimpanzees, mainly the females.


What is courtship behaviour? The acts it encompasses are as varied as life itself; a sound, a gesture, an action, etc. The overarching and general attribute all these behaviours have (which makes them courtship) is whether they appear to be connected to successful mating.

The first feature of courtship behaviour is that it enables organisms to identify members of their own species. The central part of the definition of a species is the members' ability to produce viable offspring. Hence, attempting to mate with members of a different species is not an advantageous behavioural trait in the context of reproductive success

Courtship behaviours also allow organisms to approach one another without aggression or invasion of their personal space.

Sometimes the outcome of courtship behaviour is the formation of a pair bond. This bond results in a better reproductive success, due to the increased survival probability of the offspring. In some species this is the case, while in others it isn't. This is tightly related to a specific organism's physiology. Fish are able to lay a huge number of eggs, while pigeons only lay one or two. Therefore, it is more likely that pigeons would from a pair bond, rather than fish.

Last, but by no means least, is the nature of courtship behaviour which makes it a tool for sexual selection. This is not a mere test of survival (natural selection takes care of that), but a test of relative superiority in a variety of attributes which vary between species, at different times, and even between individual organisms. These attributes can be anything, and in many cases they seem random or peculiar. In others, they seem very much expected.

Sexual selection leads to sexual dimorphism which means males and females have distinct features, especially notable in bird species. Males are colourful while females are bland, although there are species where this is reversed. In the former case, females assess signals from males that communicate their state honestly in terms of their genetic makeup, whether they have a low parasite burden and other fitness indicators.

Territoriality is another key facet of sexual selection and courtship behaviour. The location, resources, food and bells and whistles associated with the period leading up to mating is strongly dependent on territory. It is a base definer of the available resources to the individuals and the success of their subsequent offspring.

Only a minority of species are strictly territorial, as defined by not only pertaining to a defined territory, but actually actively defending it. Usually, animals only defend the smallest part of a territory needed for their survival, as defending any larger a territory would not make sense in their cost-benefit analysis.

Benefits associated with territorial behaviour include the resources found therein e.g. food, the suitability for the territory to serve as a nesting ground, as well as the presence of mates within the territory. In this sense, an individual behaving territorially may be defending the resources present within, the potential of the territory for their future, or indeed other individuals found in their territory.

In the wild turkey amongst others, multiple strategies for mating are employed by males who gather together in a showy competitive display - lekking. Dominant males lead the show, with satellite males present on the outskirts, trying to influence female mate choice.

Satellite males benefit from the dominant male-led lek by either inheriting the mating ground after they die, or by congregating with other satellite males to recreate the impression of the optimal size lek in the absence of the dominant males.

Imprinting is a type of learning that is thought to occur at a specific, sensitive time and have irreversible, lasting consequences regardless of its outcome. Imprinting can take place with regards to parentingmating and other types of situations such as human-computer interactions.

It is defined as a period during which a particular set of lessons is established rigidly. It was classically represented by Konrad Lorenz and the baby ducks that imprinted on him as their parent and followed him around. This type of parenting-related imprinting is called filial imprinting.

They do not just follow the parent around, but full-blown fly with them too! If you think this is a bit crazy, you ain't seen nothin' yet because we are about to cover sexual imprinting.

Sexual imprinting refers to imprinting related to what sexual partner is found attractive. Many birds raised from hatching by humans sexually imprint on them rather than a fellow bird. This gets to a point where the human keeper has to be a mediator between the birds during mating. This involves wearing a special hat that catches bird semen as the bird mates with the human head, and then deliver the semen to the other bird via a pipette or syringe while pressing its back with a hand to simulate bird mating.

Sexual imprinting onto human keepers has also happened with a panda bear who rejected a fellow panda when they were brought together for mating, but approached the human instead. Within the same species, male zebra finches sexually imprint on the bird who raised them even when it's not biologically related, so later are attracted to birds that look alike.

human-computer interaction imprinting scenario is called baby duck syndrome and refers to people having a hard time programming in an environment different to the one they were originally taught in. The original system is seen as the benchmark against which others are judged, and it can interfere with the learning process.

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