The Challenge of Growing Weapons

Question 1

What are the sexual dimorphisms in antlers?

Answer 1

• With intense male:male competition and a polygynous mating system, there is strong selection fo males to grow large horns
• Females also have selection for horns – not due to access to mates, but access to resources. They are used to guard a food resource so she can look after her young.
− Eg, Reindeer in the artic
− Both males and females have antlers, but seasonal pattern of shedding different
− In males, antler is cast and there is gap of a few months before it grows again
− In females, the antler is held right throughout winter, when she is looking after the young. She digs craters in the snow for a feeding resource, and she needs to defend these.

Question 2

How do weapons change with age?

Answer 2

• Red deer – only species that produces a weapon of bone. The dead bone is shed each year and the annual replacement allows for increase in size with age
• Mouflon – horns are living throughout. Made from a keratinised base, and growth occurs in annual increments from the base.

Question 3

Why did weapons evolve?

Answer 3

• We can see the evolution of antlers as weapons if we look at the nearest extant relatives of the deer – giraffes and ocapis
• We can see the evolution of a mechanism to grow a regenerative deciduous structure from a bony pedicle.
• Weapons evolved to interlock – the strategy is to turn the opponent to the side and stab him.
− Mortality is high – there is nothing rituatlised about male-male aggression in polygynous deer species, it is merely an attempt at murder.

Important Weapons of Reproductive Success
• Experiment done where the antlers were cut off a high socially ranked male.
• When this happened, he was unable to hold a harem, and he dropped down in social rank
• The deer cast the old antler, grew new antlers and moved up the social status – regaining the harem.
→ strong selective pressure for large weapons for reproductive success

Question 4

Summarise how testosterone controls antler formation

Answer 4

• Low T in non-breeding season promotes antler growth from living pedicle
• Rising testosterone drives transformation to bone, shedding of velvet and induces cellular re-modelling of antler
• Declining T at the end of the breeding season causes osteoclastic erosion of pedicle and shedding
• Implants of T prevents antler shedding and causes die-back into the frontal bone
• Castration leads to continued living growth, ultimately leading to a mass of necrotic tissue and death
− Antlergenic tissue therefore needs to be exposed to periodic testosterone cycles
− Therefore, the male deer cannot remain reproductively active throughout life, they must cycle

Question 5

What is the irritable male syndrome

Answer 5

• Acute declines may occur in circulating T concentrations in man as a result of hormone manipulations, surgery, trauma, psychological stress and ageing, and in animals after castratino or at the end of the breeding season in seasonally breeding species.
• The ‘irritable male syndrome’ is a behavioural state of nervousness, irritability, low libido, lethargy and depression that occurs in male mammals following withdrawal of T.
• It has been described in men following withdrawal of androgens and is a striking feature of seasonally breeding animals.

Soay ram as a model:
• At the peak of the cycle, rams show increases in ritualised male aggressive behaviours and expression of sexual behaviours
• At a later stage, male behaviour becomes more fractious and unpredictable
• The fractious behaviour dissipates spontaneous in association with return of the normal appetite, which is markedly reduced throughout the sexual phases of the cycle.

Working model:
• Activity of GnRH neurons closely coupled with the expression of sexual and aggressive behaviour
• T modules GnRH and behaviour
• Dopaminergic and seratoneric pathways relay the effects of sex steroids
• IMS is a behavioural state expressed when dopaminergic and serotonergic pathway turnover is low, and the T-dependent opiodogenic tone declines.
• IMS can thus be viewed as an opiod withdrawal symptom.

Conclusion:
• This review challenges the dogma that male mammals are constantly reproductively active following sexual maturity at puberty
• Males of many long-lived species express periodic changes in testicular activity and behaviour during their normal life cycle
• In the most extreme examples, such as the Soay ram, males continually cycle between a sexually active fertile state, and the sexually inactive infertile state.
• Testicular activity may vary with changes in social status, nutrition, health, age and other factors