How did this unique trait and behaviour evolve? What is its adaptive significance?
What do other hyenas do? Spotted Hyena Striped Hyena Brown Hyena Aardwolf All species of Hyena engage in chemical communication – generally using anal scent glands to mark their home habitat. Males and females also inspect the anogenital regions of conspecifics At some point, must have added the inspection of erect penises – but where did the female “penis” come from?
Because the female clitoris and male penis develop from the same embryonic tissue, one proposal is that the psuedopenis could be a developmental effect of high levels of male sex hormones in female hyenas. When females of most mammals are exposed to high levels of testosterone, the clitoris enlarges, looking like a penis However, turns out that females have lower levels of circulating androgens than males (though pregnant females do have higher androgen levels than lactating females)
What happens if we administer anti-androgenic chemicals? Turns out that the females retain their enlarged pseudopenis (though slightly altered) Thus, the proximate, developmental causes are unclear. Perhaps the relationship between androgens and pseudopenis development has become decoupled with a less costly mechanism in place.
Ultimate explanations: Adaptive value Costs: Increased androgens can reduce fertility; birth through the clitoris can be costly (in one study 10% of females died giving birth, in another field study, no effects were found) Benefits: Social Interactions?
Females are socially dominant over males Female social status strongly influences reproductive success Hans Kruuk (1972) suggested a function to reduce tension in highly aggressive females Or….mimic subordinate males
1)Female dominance evolved before the pseudopenis 2)Sexually motivated males presented an erect penis to females 3)Presentation of the erect penis would signal male identity and subordinate status, encouraging females to accept male’s presenance rather than reject 4)Females with a pseudopenis would have been able to tap into this system that was already established for signaling subordination 5)Benefit to signaling subordination would be remaining with the clan, rather than being tossed out An Adaptationist Hypothesis
Tapping into established signals and co-opting them for another purpose is commonplace In many bowerbirds, the harsh “skrraa” call is used in aggressive interactions However, in one clade, that signal has been co-opted and is used in courting females. In other words, the signal already existed, but has been put to another purpose – females can use the signal to make adaptive choices about mates
Signals can be exploited Sensory exploitation occurs when an animal produces a novel signal that taps into the preexisting perceptual mechanisms in a signal receiver.
Males exploit the predatory behavior of females waiting to ambush small copepods 1)With females in the attack position, males approach and vibrate forelegs in front of her 2)Female grabs the male (like a copepod), but releases him unharmed 3)Male turns around and deposits spermatophores near the female that she picks up if receptive
Perhaps males are mimicing the stimuli produced by copepod prey; the reaction identified her as a potential mate and showed the male where to position his spermatophores 1)Hungry, unfed females should be most responsive to males (they are) 2)Male behaviour effectively activated a preexisting detection mechanism in females 3)However, hungry females may just want to acquire a nutrient rich spermatophore
The body of this giant wood spider is extremely colourful. When the spider is present, more prey are captured (same goes for the spider being painted black). What relevance does this research have for those interested in the sensory exploitation hypothesis for the evolution for courtship signals in animals?
If sensory exploitation is a major driver of effective signals, then we should be able to elicit responses from animals by providing novel experimental signals they’ve never seen before.
Species may respond to signals that existed in the past. If so, descendants may retain that sensory preference, even though they no longer exhibit those signals. Ancestr al state
Least auklets normally do not have crests, but females show a strong preference for males with crests.
So, how might this trait have evolved in the first place? Why was it favoured by selection?
Sometimes it can be costly to evolve defenses against exploitative signals Male garter snakes “court” females by slithering on top of them and moving their bodies against them in a way that makes it difficult to breathe. This behaviour activates a cloacal gaping response
Natural selection works with what it’s got. Parthenogenic whiptail lizards (right) continue to engage in similar courtship and pre- copulatory behaviour as their sexual counterparts. Females that engage in this behaviour are much more likely to produce a clutch of eggs than those that don’t receive sexual stimulation.
Male tungara frogs often give whining calls without chucks, despite the fact that females prefer males who add chuck elements to their calls. Why not add the chucks? The fringe-lipped back is an illegitimate receive that responds the tungara frog chuck calls – the more chucks, the more likely an attack. Frogs only give chucks when predation risk is lower (like in large groups).
Receivers can lose fitness by responding to signals generated by illegitimate signalers (deceivers) Femme fatale predatory Photinus moths respond to the signals given by Photirus moths. In addition, after approaching the male Photirus moths (whose signals they illegitimately received) Photinus females answer the males’ mating signals by mimicking females of their own species. Some Photinus females can mimic 3 species of Photirus moths Once close enough, they’ve got a meal.
How do we explain these maladaptive responses? 1)Novel environment theory: Maladaptive response is caused by a proximate mechanism that was once adaptive, but no longer is. Modern conditions are different than those that shaped the condition in the past. Not enough time to “fix the problem”. 2)Exploitation theory: Maladaptive response is caused by a mechanism that results in fitness losses that reduce, but do not eliminate the net fitness gain associated with reacting to a given signal.