Lecture 4 Sexual mimicry in Nature and the strange case of the hyena Flashcards
occurence of sexual mimicry male - female, female - male
rare for males to mimic females.
PLANET EARTH THING THO
Some male cuttlefish are too small to fight for a mate, so have to come up with a sneaky plan if they are to get their way with the girls. Approaching a cuttlefish couple a smaller male assumes the role of a female, tucking up his tentacles and changing colour in an attempt to fool the large male who stands guard over the female. The large male thinks his luck is in with two females after his attention and is completed fooled by the small newcomer. As long as he avoids being grabbed in the mating embrace the cross-dresser is safe. Surreptitiously the female who’s not too picky, mates with the smaller, sneaky male right in front of the large male. The eggs she now lays will contain a mixture of sperm from both fathers, giving her eggs the best possible chance of success.
what is cliteromegaly?
Mimicry of primary sexual characteristics (penis and scrotum) by females.
Development of penis-like structure from the clitoris (the female homologue of the penis).
animals that exhibit cliteromegaly
moles, spotted hyenas, lemurs, bonobo, spider monkeys.
how does the false penis in spotted hyenas occur?
false penis from the clitorus, and false scrotum from the vuvla (filled with fatty tissue).
what are the consequences for the spotted hyena?
Females mate and give birth through the “peniform” clitoris.
In pre-pubertal females, the urinogenital sinus is only slightly larger than the penis.
At puberty it enlarges and retractor muscles develop
super hard to mate, Male has to flip the semi-erect penis against the abdomen of the female as he searches for the clitoral opening.
social aspects of sexual mimicry?
Genital sniffing and inspection is part of social greeting display. Hyenas have ferocious teeth, so a good deal of trust is required. Females re-enforce their position in hyena society by developing the most delicate appendages.
What are the theories for sexual mimicry?
Competition aggression hypothesis:
Intense competition for food – selection for increased female size and aggression in order to survive.
This hypothesis requires that female rank, body size, aggression and reproductive success are closely linked and that variance in reproductive success is greater in females than males.
spotted hyenas ecology?
Female rank is inherited – from their mothers
Females dominate males at the kill and in social encounters.
Females 10% larger. Other hyenids (striped, brown, aardwolf) have normal anatomy and lack dimorphism
High ranker produce more offspring, breed earlier, neonates survive better.
Spotted hyenas are the only true carnivores of the group.
Female status is a key predictor of offspring survival.
the only hyenas to show dimorphism.
describe spotted hyenas offspring
Neonates are very aggressive.
Born with fully functional teeth
Twins are often produced
Fighting commences within 1-5 minutes of birth
Often one of the cubs dies
Survival is in favour of females – ie the male cub often killed.
Juvenile females show male-like play patterns
describe problems at birth for the spotted hyenas.
In a normal mammal of this size, distance from cervix to external vagina is about 30 cm.
In hyenas, the distance is ca 60 cm.
Umbilical cord is only 12-18 cm long
Cord breaks before birth
In primiparous females, the neonate can lodge in clitoris.
Birth can only occur after urinary meatus has torn. A large proportion of first cubs die of anoxia.
Female cubs are larger than male cubs at birth and in adult hood.
9-18% of females die at the attempt of first birth.
how is the spotted hyena placenta different?
The hyena placenta converts ovarian androstenedione to testosterone.
During pregnancy, ovary produces
High levels of androstenedione
(AD; weak androgen).
Normal mammal placenta convert to oestrogen via aromatase.
Early studies suggested that hyena placenta is deficient in aromatase.
In hyenas, placental 17β
hydroxysteroid dehydrogenase is
high, and converts AD to testosterone.
sex steroids in spotted hyenas?
Measures of umbilical cord steroid levels shows females exposed to similar “T” concentrations than males
Neonatal androgen blockers (AB) do not prevent peniform clitoris from forming
But, such clitorises are thicker and cannot be stretched as far in AB-treated females.
Males are unable to mate as adults – so neonatal androgens are essential. Penis is too small to penetrate the female clitoris.
what happens when females are treated with androgen blockers?
for females:
reduces the length of clitorus, and increases circumference (more male like).
For males:
reduces length of penis, increases circumference.
neonatal mortality rates in primiparous females:
- Controls: 12 pregnancies, 20 cubs, 11 stillborn
- Blocker treated: 4 pregnancies, 7 cubs, 0 mortality
therefore neonatal mortality rates may be related to the steroid dependent development of the clitoral structures.
primiparous?
bearing young for the first time
female and males mounting behaviour?
Fenstemaker et al 1999 Nature Neuroscience Vol 2, p 943.
females mount less frequently and wrongly orientated.
