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”

Picture here Pseudopenis Female penis really a pseudopenis, which is an enlarged clitoris.

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.

9.7 Ear Sensory cells

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).

Fig 9.9

Evolution of insect flight Gill plates appear to have evolved into a wide variety of structures in arthropods including wings, gills and lungs.

Fig 9.8

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.

Stonefly skimming.

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.

Fig 9.11

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:

Example 1: Tungara Frog

Tungara frogs attract mates by calling.

Males give whining call sometimes followed by one or more “chucks.” Females prefer males who give chucks.

Fig 9.30b

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?

Risk avoidance by signalers Signaling can be risky (recall Tungara frog). Illegitimate receivers use signal to detriment of signaler. Many signalers try to reduce risk. High pitched alarm calls of birds are very similar in sound and hard to locate.

Risk avoidance by signalers In contrast, mobbing calls have evolved to attract other birds and are easy to locate.

9.31a

Risk avoidance by signalers Selection to avoid attracting predators has led to the evolution of very similar alarm calls in different species of bird.

9.33

Adaptationist approach to signal receivers Ritualized fighting is widespread. Why do receivers believe the signals they are receiving? Why do animals not fight harder for resources? What is the benefit to giving up a fight?

Adaptationist approach to signal receivers By giving up, organism avoids engaging in fights it probably will lose.

Toad mating behavior In mating toads male sits on females back and grips her.He fertilizes eggs as she releases them. Other males may attempt to displace male. Male croaks when touched. Upon hearing croak usurping male may cease attack.

Toad mating behavior Hypothesis: croak is signal of defending males size. Deeper croaks indicate larger males. Smaller males deterred by deep croaks.

Toad mating behavior Tested by playing taped call when silenced defender touched. Pitch of call significantly affected amount of time attacker spent attacking.

9.34

Toad mating behavior Toad call appears to be an honest signal. Small toads cannot fake a deep croak.

Honest signals Expect signals need to be unfakeable because otherwise cheaters would invade the system. Honest signals provide useful information and benefit both the signaler and the receiver.

Honest signals Expect threat display signals to be expensive and difficult for smaller/weaker individuals to imitate. Should be good indicators of fighting ability.

Male red deer control groups of females.

Males compete to control groups, but rarely resort to pushing matches.

Instead, first engage in roaring matches. Roaring is energetically expensive. Only males in top condition can roar for a long time. Only if roaring match is indecisive does escalation to fighting take place.

Antlered flies display antlers to rival males. Males with smaller antlers usually retreat.

Antler size closely correlated with body size and so it is an honest signal.

Honest signals As one might expect not all signaling is honest and some organisms exploit receivers.

Dishonest signals Male and female fireflies signal each other by flashing in distinctive patterns. Female Photuris fireflies mimic signal of female Photinus to attract males, whom they catch and eat.

Responding to a deceptive signal maladaptive. Why respond?

Dishonest signals If responding to a dishonest signal is maladaptive, why respond?

Dishonest signals Possible explanations include: (1) Novel environment hypothesis. Mistake made because environment has changed. Response once adaptive, is now maladaptive.

Maladaptive mate choice in an Australian beetle

Dishonest signals (2) Exploitation hypothesis. Response adaptive on average, but sometimes exploited. Risk outweighed by benefits.

Dishonest signals On average, male Photinus flies do better by responding to light flashes. Males who respond mate. Males who don’t respond aren’t eaten, but don’t mate either.

Dishonest signals Successful illegitimate signalers should exploit responses that are usually adaptive. E.g. predatory spiders lure out other spiders by producing prey-like vibration on web.

Anglerfish attract small fish by waving a fish-shaped lure. Benefit of attacking likely prey outweighs low risk of predation for small fish.

Dishonest signals White-winged tanager-shrike forages in mixed-species flocks with other birds. Scans for prey from perch. Often first to spot approaching predatory hawks. Gives alarm call that other birds respond to.

Dishonest signals Sometimes tanager-shrike gives call to distract another bird from prey item it is chasing. Usually, other bird aborts chase and tanager-shrike gets item.

Dishonest signals For other bird cost of ignoring alarm call potentially is very high (i.e. death). Usually best to give up chase.