• What is the stimulus for eating ice cream, even when you are not hungry?
  • Sweet and fatty foods: humans developed a taste for these in our evolutionary past, when these foods provided precious energy.

This type of behavior is innate (instinctive) and is determined by genes.

An egg outside the nest serves as a sign stimulus that triggers an FAP for this goose: roll the egg back into the nest with her bill.

An FAP follows simple pre-programmed rules - sometimes too simple.

• Is there an evolutionary explanation for such innate behavior?
  • Care and protection of eggs increases reproductive success.

Monkeys are not born with a fear of snakes, but easily acquire that fear by observing fear to snakes in other monkeys.

• Is there an evolutionary explanation for learning to fear snakes?
  • Snakes can kill monkeys; such fears may provide reproductive fitness.

This Australian social spider lets her spiderlings eat her alive.

This altruistic behavior is not explained if she were selfish and concerned with her own survival.

Evolution explains it if this leads to better reproductive success: leaving more offspring.

• Are you selfless?
  • Sometimes? You are likely to be altruistic toward close relatives, and most altruistic toward offspring.

Belding's ground squirrels live underground in colonies where adult females are closely related (kin).

A squirrel may give a loud call to alarm its nest mates when it sees a predator.

Making alarm calls is dangerous: about half the time the caller is killed by the predator, but this may save its kin.

Close relatives share many genes, so kin selection may explain such altruistic behavior:

• Self-sacrifice enhances the reproductive success of some of her genes.

ground squirrel alarm call

Competition within the same sex for the opposite sex is intrasexual selection.

Often this competition occurs within males.

• A male's reproductive success increases with each mating.

• Males have an incentive to mate with as many females as possible.

Females have less reason to mate multiple times: her egg output is unaffected by how many times she mates.

Mate guarding can minimize paternity uncertainty.

There are big differences in how much males and females invest in reproductive investment.

Females produce larger, fewer gametes, while males produce smaller, more numerous gametes.

A female mammal has much more invested in reproduction than males: long gestation and nursing of young.

Some examples:

• prolonged mating in some frogs.

• holding the female while she lays eggs

• self castration in the black widow spider

Females usually choose their mates; this is intersexual selection.

• Elaborate courtship rituals among the Western Grebe ensure his commitment.

• Marmot males demonstrate reproductive fitness by controlling resources.

• A male may bring an item of value - a "nuptial gift" - as a sign of his reproductive fitness.

• Fancy and bright plumage demonstrates a male's fitness; females are usually less colorful.
  

Exceptions occur when the male has a heavy reproductive investment.

In these species the male's nuptial gift is a nutritious spermatophore for the female to eat.

• It can represent up to 1/10 of her lifetime calorie intake.

With such heavy reproductive investment, the males are picky about females.

• Monogamy: one male mates with one female; many birds bond for life.
  The sexes tend to be monomorphic.

• Polygamy can be subdivided into
  • Polygyny: one male with many females: an alpha elephant seal controls a harem of many females.
  • Polyandry: one female with many males: a honeybee hive has one queen who mates with many males.
  The sexes tend to be dimorphic.

• Promiscuity: many male with many females: a chimpanzee may mate with several members of the opposite sex, and vice versa.

• What other system can you think of?
  • Self-fertilization: many plants are hermaphrodites and can self-fertilize; but hermaphrodite animals such as slugs still need to find a mate.

Reproductive behavior is usually:

• both sexes care for young.
• both sexes equally choosy about the opposite sex.

Many birds are monomorphic.

Reproductive behavior is usually:

• one sex (mostly female) cares for young.
• the other sex (mostly male) compete with each other by intrasexual selection for the opposite sex.

For example, male moose are much bigger than females due to intrasexual selection, and do not take part in rearing pups.

• Chemical signals such as pheromones released by female moths, detected by bushy antennae of males, up to 3 miles away.
  
  
  
  
• Auditory signals such as a lion's roar can also be detected far away.
  
  
  
  
• Visual signals, such as spines of a balloonfish, can advertise defense traits.

More complex behaviors are also possible.

• The angle between the the sun and food is represented as the angle between the straight part of the dance and vertical.

• If the straight part of the dance is 20° to the right of vertical, the food is 20° to the right of the sun.

• The distance to the food source is indicated by the speed of waggles.