Chapter 9. The evolution of communication.
Hyena social behavior • Hyenas live in social groups called clans. • Clan members defend a territory and hunt in groups. • Females are the dominant members of the clan and have a clear dominance hierarchy.
Dominant females have higher reproductive success than other females.
Major feature of social interaction in hyenas is penis sniffing. Both males and females possess “penises”
Female penis really a pseudopenis, which is an enlarged clitoris. Pseudopenis Picture here
Enlarged pseudopenis is costly. . Birth canal passes through pseudopenis
Pseudopenis is costly • 10-20% of females die giving birth first time and 60% of first-born pups die. • Pseudopenis must provide big selective advantage to balance this. What is benefit to female?
Advantages of pseudopenis • There has been considerable debate on the issue. • Initial research focused on possible role of male hormones in masculinizing clitoris. Speculated exposure to hormones increased aggression in females, which increased their social status and access to food, and enlarged pseudopenis was a byproduct of that.
Advantages of pseudopenis • Hormonal side-effect hypothesis does not explain why females are dominant over males or why selection would not have favored a reduction in pseudopenis size.
Advantages of pseudopenis • More recently attention has focused on usefulness of pseudopenis in communication. Sniffing appears to enhance cooperation among hyenas. • Hyenas presumably communicate information during sniffing events and these may affect dominance interactions between females.
Advantages of pseudopenis • Sniffing may enable dominants to monitor hormonal status of other females. • Dominants’ benefit: know if challenge likely. • Subordinates’ benefit: allowed to remain in pack.
Evolution of complex traits • All complex traits evolve from less complex ones as a result of a long sequence of small developmental changes. • This sequence of changes is an evolutionary pathway.
Things to remember about evolutionary pathways. (1) Natural selection can only operate on the material available to it.
Panda’s strip bamboo with “thumb.” Thumb is modified wrist bone (radial sesamoid). True thumb committed as part of foot. Natural selection forced to use available material.
(2) Evolutionary intermediates must be improvements over what preceded them. An organism cannot get worse to ultimately get better.
(3) Intermediate structures can have different functions to their current ones, but should be useful in some way. Human ear bones were once jaw bones. Had to work effectively at both jobs.
Evolution of hearing in Noctuid moths • Whistling moths signal to females by banging “castanets” on their wings together. Sounds are ultrasonic (approx 30KHz). castanet
Evolution of hearing in Noctuid moths • Most moths cannot hear. • How did ability to hear ultrasound evolve in these Noctuid moths?
Ears on side of thorax. Ear: thin tympanic membrane covering an air sac. Vibration of air sac stimulates sensory receptors. Ear
Evolution of hearing in Noctuid moths • Non-hearing Sphingid moths have sensory cells attached to the cuticle as in Noctuids. • These supply positional information when moth vibrates wings.
Ear Sensory cells 9.7
Evolution of hearing in Noctuid moths • In ancestral Noctuid, sensory cells could have provided ability to hear those sounds loud enough to move cuticle.
Ear evolution pathway • (1) Thin cuticle to enhance vibrations. • (2) Enlarge air space. • (3) Tune sensory cells to the desired frequency.
What was likely selection pressure on noctuids to hear ultrasound?
Noctuids evolved hearing to avoid bats. Later evolved signaling ability.
Evolution of insect flight • What were the precursors of wings? • Gill plates of extinct immature aquatic insects are plausible pre-wing structures (function to move water over the gills).
Evolution of insect flight • Gill plates appear to have evolved into a wide variety of structures in arthropods including wings, gills and lungs.
Evolution of insect flight • Gill plates, if retained in the adult, could act as sails allowing the insect to skim over the surface of the water. • Increasing “wing” size would increase skimming speed. • Beating wings would increase speed still further so adding musculature would be favored by selection.
Evolution of insect flight • Modern stoneflies include a variety of species that use different ways of moving over the water surface that allow the insects to move progressively faster. • Stonelfies include species that sail, row skim and fly. There are also species that differ in the number of legs they keep in contact with the water while skimming.
Evolution of insect flight • The fewer legs in contact with the water the faster the stonefly can move. • From hind-leg skimming, it is only a short step to true flight.
Exploitation of preexisting biases in evolution of communication • In whistling moths evolution exploited existing sensory system to develop communication system. • Remember: Complex structures not evolved from nothing (e.g. Panda’s thumb)
Exploitation of preexisting biases in evolution of communication • Sensory biases/preferences may precede evolution of many signals. • E.g. Many sex pheromones of insects have floral odor (exploits sensory bias towards food finding). Examples:
Males give whining call sometimes followed by one or more “chucks.” Females prefer males who give chucks.
If females prefer males who chuck, why don’t all males chuck all the time?
Because bats prey on calling males. Fig 9.30 a
Tungara frogs • Close relatives of tungara frog don’t chuck. • However, females of these species prefer the calls of males to which chucks have been added • Females have an innate preference for chucks.
Example 2. Female swordtail fish prefer males with long tails.
Platyfishes close relatives of swordtails. Males have short tails. Female Platyfish prefer males with artificially elongated tails.
Elongated tail in swordtails evolved after preference in place.
What is an adaptation? • Characteristic of an organism that is maintained or spread by natural selection. • Adaptationists try to figure out the value of traits. E.g. what is the value of signalling?