Reproductive Behaviors

mating, courtship, & parental care

“Colorful Peacock Head” by Daniel (CC by 2.0)

Males are vibrant, bigger, more vocal on average…why?

Bowerbird species: males build elaborate nests,  and the observed behavior appears to be ritualized. Females come and inspect nests to determine if she will mate with the male. Variation in reproductive success is greater for males than females in the satin bowerbird. Males typically have more mating partners than females

Why is the bower attractive to a female? Some possible suggestions are:

  • Demonstrates the scavenging ability of the male
  • Indicates fitness
  • Bower building is correlated with brain size
  • Positive correlation found with bowerbird IQ and mating success

Sexual selection: Traits influence the fitness of the individual, traits that lower survival but enhance the ability to mate are sexually selected traits.

“Male Peacock Spider” by Bron (CC by 2.0)

For Example: Colorful patterns make the male more visible to predators….but it attracts mates more often so his genes are passed onto the subsequent generations.

****tension between natural selection and sexual selection****

It is different from mate choice

  • Male and female gametes differ greatly in size
    • Female eggs are energetically costly, fewer are made than male counterpart, spend a lot of time nurturing and raising young ~ protect the investment.
      • In order to ensure survival of her offspring she will mate with males that are of higher quality so that her time and energy investment ultimately increases her personal fitness.
    • Males can mate repeatedly and often due to a less energetically costly gamete that is rapidly made.
  • Operational Sex Ratio: Male biased. There are more males available/ready to mate but only a few females are sexually receptive (some are already pregnant/raising young). With a greater proportion of males the females are able to be more selective.
    • Actual sex ratio that is seen in nature  is 50/50 for male/female
      • Frequency dependent

Parental investment influences sexual behavior due to gamete size, form of parental care, and the resources given to mate.

  • Low parental investment: is seen when there are low numbers/quality of donations, higher sexual selectivity, competition for mates, the goal is to have more mates to create an increase in fitness.
  • Higher parental investment: more donations of better quality are given to the mate, less sexually active, selection among mates, better mate quality leads to an increase in fitness.

Conventional roles: males fight for mates as females are less available due to gestation/offspring rearing. Males tend to be more elaborate.

“Dad to be” by Owen Evans (CC BY-SA 2.0)

Sex Role Reversal: 

  • For example: males with a larger parental investment, in populations that have more sexually receptive females than males, competition among females instead of between males is seen, male mate choice predominates.
    • Seahorses brood pouch: females lay eggs directly into the pouch of the male.
      • Predator protection
      • Females can mate with many males
      • Males are able to be more selective about mate choice
      • the females tend to me more elaborate and colorful

“Female Seahorse” by Peter C (CC by 2.0)

Nuptial gifts: males give an edible gift to females to attract her and mate.

  • The some crickets and grasshoppers produce spermatophore which provide protein to the female and thus the fertilized eggs. This is an energetically expensive process for the males and so the payoff of mate choice must outweigh the cost of developing the gift. After it is deposited into the female, she etas the edible part. Bigger spermatophores will attract the females.

“Shield-back Katydid with spermatophore” by Judy Gallagher (CC by 2.0)

Food availability affects sex roles 

  • Low food abundance: not many males can produce the spermatophore. Creates a sex role reversal because only a few males produce the gift. Men get to choose and females compete for access.
  • High food abundance: all males are able to make the spermatophore. Females get to choose because there is an abundance of males that produce the gift.

Convergent evolution in male weaponry: many different species have horns

“Pronghorn Buck” by USFWS Mountain-Prairie (CC by 2.0)

“Southern White Rhino” by William Murphey (CC BY-SA 2.0)


Pecking order: a dominance hierarchy among males

  • Ritualized aggression
  • Options for subordinate males:
    • Female friendships
    • Banding together with other subordinate males to fight off a dominate male

Alternative Mating Tactics

Conditional Strategy: low ranking males can make the best of a bad situation (form alliance), higher fitness payoffs by adopting alternative tactic rather than fighting dominant male.

  • A male Panorpa scorpionfly: will defend dead insects to entice females, secretion of saliva will provide nutrients to female but is costly to the male, or forcing copulations.
    • If you remove the top ranking males that have killed an insect, will the saliva secretors switch to insect killers? Studies have shown that they either stand by the dead insect or kill their own.
      • A genetically based strategy: If one strategy has more fitness then the behaviors will change to the dominant strategy. Since all three of these strategies still exist in the population, they are equivalent in fitness benfits.
  • Satellite males: low ranking males can get some female access by attaching to female like in horseshoe crabs. The stronger males will get about 90% of the fertilization, the satelites do get to fertilize about 10% of the female’s eggs. “Sneaker males”

“Horshoe Crab” by U.S Fish and Wildlife Services (CC by 1.0)

Genetically based straegies or Conditional strategies:

  • Genetic differences create morphological differnces that cannot change becuase that are genotypically influenced.
  • Conditional strategies are alternative tactics that have the same mean reproductive success such as multiple types of mating behaviors.
  • Competition for access to mates has developed interesting adaptations to aide in mating.
    • The male damslefly has an apendage that can remove the sperm of another male so that he can mate with her instead.
    • Mate guarding behaviors occur in baboons and some fish species. Evolved in response to female promiscuity where females had more choice in mate selection.
    • Mating plug of male spiders. A pedipalp is used to transfer sperm to females and this detaches so that she cannot mate again. Females are less frequent and losing the apendage is less costly than never finding another female or other males removing his genetic material from the few females around.

“Male spider with black pedipalps” by John Flannery (CC BY-SA 2.0)

Sexual Cannibalism:  in some species, one sex (usually the female) consumes the other during sexual reproduction.

  • Is the cannibalism an offer to the female?
  • Is it the female choosing to eat the male?
  • Will she eat the male before a successful mating?
  • Examples: Praying mantis, scorpions, Austalian redback spiders

Sexual Suicide: the ultimate nuptial gift, the male will most likely have no other mating opportunites. The male Austalian redback spider will somersault into the mouth of the female to provide nutrients to offspring and ensure mating success. The male is controlling this decision.

Male Ornamentation and Courtship Displays

  • Good Parent Theory: females prefer to mate with males that provide more paternal care. Courtship is linked to his parental abilities.
  • Healthy Mate or Good Genes: bright coloration can indicate good foraging ability, good health, stronger immune system because they are able to sustain themselves and are free of disease and parasites. The male has inherited these traits from his offsrping and can therefor pass these traits onto his own offspring.
  • Do male ornamentations actually signal good genes?
    • The male peacock tail: the mean area of the eye spots indicates the size of his tail which has a postive correlation with the survivorship of his offspring.
  • Runaway Selection: ornamentation that is very excessive, it is a sexually selected for trait. Female mate choice is inherited and the genes for male attribute is inherited together. Daughters inherit trait preference, males inherit preference. The preference for the trait is selected for, not the gene/ornamentation/fitness benefits. More elaborate traits have been shown to lead to decreased fitness.
    • Bird of Paradise
    • Stalk-eyed flies
    • Natural selection will outweigh the sexual selection over time.

“Stalk-eyed fly” by Bernard Dupont (CC BY-SA 2.0)

  • Chase-away selection theory: sensory exploitation. Preexisting bias in females leads to a mutuation in male display traits (explotation). This can lead to a decrease in fitness because the male may or may not benefit her fitness. The threshold for female mate choice will increase, the males will have to continue to exagerate the trait to overcome the sensory bias.
  • Sexual Conflict: females resist matings, often have control over which mates get to mate, males develop traits that enhance his ability to even if it lowers female fitness.
    • Such as: forced copulations, infanticide, tranfer toxic substances during mating.




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