Aggressive mothers: the strong weak sex of Hyenas

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Aggressive mothers the strong weak sex of
Hyenas
1/6/03 seminar in biology course
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The spotted Hyena (Crocuta crocuta)

• Order Carnivora
• Family Hyaenidae
• Habitat African savannas
• The largest in the family
• Hunts and lives in clans
• Social hierarchy among individuals
• Females dominate over males,

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Genital masculinization
Frank et al. (1995) Masculinization costs in
Hyenas.
60 mortality of first borne cub 10 mortality of
mothers at first birth A 180o turn in the birth
canal The umbilical cord is only 18 cm long
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The questions

• Is masculinization of genitals related to female
  dominance ?
• What was the driving force to develop a female
  dominated society ?
• How did natural selection allow the existence of
  such costly organ ?

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Hormones affect our brains and hence our
behavior. Sex determination, and sexual behavior
is also mediated by hormones. Can the female
hyenas be the product of some hormonal disorder ?

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Development of external genitals
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Testosterone affects the brain at specific
periods during development
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Congential Adrenal Hyperplasia (CAH) or (AGS) 1
out of 10,000 births, Females with developed
external genitalia due to exposure to androgens.
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The Guevedoces of the Dominican republic
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Aromatase
Maternal ovary
Androstenedione
Estradiol
17-ß hydroxysteroid dehydrogenase
Aromatase
Testosterone
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A possible Epigenetic mechanism
Yalcinkaya et al. (1993).
Testosterone causes a reduction in the number of
cells of the ovary responsible for turning
androstenedione to estrogen but the cell
producing androstenedione are maintained
unaffected. Parental testosterone turns the
ovary of the fetus to hayena tipical ovary which
will masculine the next generation etc
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• Problems in the explanation
• Why are male hyenas less aggressive and smaller
• Androgen-blocking drugs administered to pregnant
  female do not result in females with feminine
  genitalia (but do result in feminine males)
• Is hormonal levels in the adult female really
  that high ? Can androstenedione levels elevate
  aggression of females ?
• Males joining a new clan have high androgen
  levels but are subordinate to the native males of
  the clan

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Fenstemaker et al. (1999)
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Alternative suggestions
Sexual mimicry (usually reserved to males)
Siblicide ( females win 82 of the cases)
Maternal rank inheritance for females
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The questions
Is masculinization of genitals related to female
dominance ? What was the driving force to
develop a female dominated society ? How did
natural selection allow the existence of such
costly organ ?
Are androgenic females more aggressive ?
Reproductive success and maternal rank inheritance
Not clear. Is it really that costly ? Is there an
advantage ?
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Something to think about
Perhaps the hyena society is an intermittent form
of society between the very organized group
(wolves) and the weakly cooperating group (lions)