2.4 Sex and behaviour

•  When it comes to parental investment, in most species, it is the female who invests the most.​

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  Females also have an investment in the egg  structure in non-mammals or in the uterus and during gestation in mammals. 
• Parental behaviour can be classified in one of two groups: r-selected  or  K-selected.  
• r-selection tends to occur in unstable environments where the species has not reached its reproductive capacity, whereas Kselection tends to occur in stable environments
• External fertilisation

  • benefits: very large numbers of offspring can be produced
  • costs: many gametes predated or not fertilised; no or limited parental care; few offspring survive
• Internal fertilisation

  • benefits: increased chance of successful fertilisation; fewer eggs needed; offspring can be retained internally for protection and/or development; higher offspring survival rate
  • costs: a mate must be located, which requires energy expenditure; requires direct transfer of gametes from one partner to another
• Mating systems are based on how many mates an individual has during one breeding season .These range from polygamy (polygyny and polyandry) to monogamy
• Monogamy: the mating of a pair of animals to the exclusion of all others.
• Polygyny: one male mates exclusively with a group of females.
• Polyandry: one female mates with a number of males in the same breeding season.
• Many animals have mate-selection courtship rituals
• Successful courtship behaviour in birds and fish can be a result of species-specific  sign  stimuli and fixed  action  pattern  responses . 
• Many species exhibit sexual dimorphism as a product of sexual selection .  ​

  Females are generally inconspicuous​

  Males usually have more conspicuous  markings , structures  and behaviours . ​

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  Reversed sexual dimorphism can occur in some species such as vultures
• Female choice involves females assessing honest signals of the fitness of males
• Honest signals can indicate favourable alleles that increase the chances of survival of offspring (fitness) or a low parasite burden suggesting a healthy individual.
• Some bird species exhibit lekking behaviour. Dominant males occupy the centre of the lek, with subordinates and juveniles at the fringes as ‘satellite’ males. During the display, female choice occurs.
• Success in male-male rivalry through conflict (real or ritualised), increases access to females for mating