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Sexual dimorphism
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Female (left) and male Common Pheasant, illustrating the dramatic difference in both color and size, between the sexes
Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. Examples include size, color, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers or tusks.
Examples
Peacock courting peahen
In many species, including most mammals, the male is larger than the female. In others, such as most spiders, birds, reptiles and amphibians, many insects and fish, and certain mammals such as the spotted hyena, the female is larger than the male. Other sex-specific differences include differences in colouration (sexual dichromatism), presence vs. absence of certain body parts such as horns, antlers, tusks or display feathers; size of the eyes (some insects); possession of stings (various kinds of Hymenoptera), and different thresholds for certain behaviors (aggression, infant care, etc).
Female (left) and male Greater Painted Snipe, displaying reverse sexual dimorphism
Sexual dimorphism is particularly apparent in ducks, and most gamefowl perhaps most dramatically including peafowl. Male pheasants are notably larger than females and possess bright plumage; females are usually a drab brown irrespective of the particular species. In some birds (most of which are waders such as the phalaropes and painted snipes), females have brighter colors than males. As this is the opposite of the usual sexual dichromatism, it is termed reverse sexual dimorphism. In many predatory birds females are larger than males, often considerably so. This seems to reduce competition between members of a pair, as they have different optimal prey sizes. Some cases of sexual dimorphism in birds are so striking that males and females of the same species were originally taken to be members of entirely different species, as in the case of the Eclectus Parrot (Eclectus roratus), where the male is predominantly green with an orange beak and the female scarlet and deep blue with a black beak.
The Huia (Heteralocha acutirostris), a New Zealand bird species (now extinct), was another striking example of sexual dimorphism. The male’s bill was short, sharp and stout while the female’s was long, thin and crescent shaped. This beak dimorphism allowed mated pairs of Huia to avoid competing for the same food source, with males chiseling into and breaking apart rotting logs, while females were adept at probing into fresher wood for grubs.
Male (front) and female mallards. The male mallard has an unmistakable green head.
Certain sexual dimorphisms have obvious utility beyond mate attraction, such as the Blue Wildebeest (and many other biungulates). The horns of the male are much larger, allowing the male to engage in combat more effectively as he competes with other bucks for mating privileges.
An extreme example of sexual dimorphism is found the genus Osedax of polychaete worms, which lives on whale falls. The females feed on the bones of the dead whale, but the males live inside the females and do not develop past their larval stage except to produce large amounts of sperm. In the echiuran Bonellia viridis, females force larvae which encounter them to develop into the tiny, semi-parasitic males. The argonauts also have males which are tiny compared to the female. In the parasitic barnacles Sacculina, the males are tiny, free-ranging animals, whereas the females only exist as a web-like tissue inside their hosts.
Female Triplewart seadevil, an anglerfish, with male attached near vent (arrow)
Some species of anglerfish also display extreme sexual dimorphism. Females are typical anglerfish, while males are tiny rudimentary creatures with no digestive systems. The males must find a female and fuse with her – he then lives parasitically, becoming little more than a sperm-producing body. A similar situation is found in the Zeus water bug Phoreticovelia disparata where the female has a cavity on her back where males live permanently attached.[1]
[edit] Psychological and behavioral differentiation
Sex steroid-induced differentiation of adult reproductive and other behavior has been demonstrated experimentally in many animals. In some mammals, adult sex-dimorphic reproductive behavior (e.g., mounting or receptive lordosis) can be shifted to that of the other sex by supplementation or deprivation of androgens in fetal life or early infancy, even if adult levels are normal.
[edit] Evolution of sexual dimorphism
[edit] Handicap principle
Main article: Handicap principle
The handicap principle is the evolutional force that gives males of some species traits that by the first glance seem to place the organism at a disadvantage.
For example, the bright colouration of male game birds makes them highly visible targets for predators, while the drab females are better camouflaged. Other examples are bird of paradise and lyrebird, whose males have such large plumes that their flight is inhibited. Strong smells, loud cries and singing can also attract predators.
The answer to this apparent paradox is that, at a biological level, the reproductive success of an organism is often more important than duration of life. This is particularly apparent in the case of game birds: a male Common Pheasant in the wild often lives no more than 10 months, with females living twice as long. However, a male pheasant’s ability to reproduce depends not on how long he lives but whether females will select him to be their mate.
A brightly coloured and heavily plumed male demonstrates to the female that he is fit in evolutionary terms – he has been able to survive in spite of impediments and must therefore be a good choice to father her chicks (especially her daughters, who will have his "fit" genes, but will not be hampered by male plumage). This explanation was first proposed by Amotz Zahavi.
Development of such characters could not at first be explained in terms of simple natural selection. In 1871 Darwin advanced the theory of sexual selection, which related sexual dimorphism with sexual selection.
[edit] Polygamy
Comparison of sexual dimorphism in birds and their mating habits shows that the time spent in search for mates, staking territories and mating competes with the demands of taking care of young. For birds and in general, it can be stated that the stronger the dimorphism in a species, the more likely is it to be polygamous and the less is the task of caring for offspring shared among the sexes. This theory is developed by R. L. Trivers’ in the parental investment theory. It applies to all ecology.
[edit] Sexual dimorphism in humans
Top: Stylised illustration of humans on the Pioneer plaque, showing both male (left) and female (right).Above: Comparison between a male (left) and a female pelvis (right).
Main article: Sex differences in humans
Sexual dimorphism in humans is the subject of much controversy, especially relating to mental ability and psychological gender. (For a discussion, see biology of gender, sex and intelligence, gender, and transgender.) Obvious differences between men and women include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physiological and behavioural effects.
Such undisputed sexual dimorphism include gonadal differentiation, internal genital differentiation, external genital differentiation, breast differentiation and hair differentiation.
Some biologists theorise that a species’ degree of sexual dimorphism is inversely related to the degree of paternal investment in parenting. Species with the highest sexual dimorphism, such as the pheasant, tend to be those species in which the care and raising of offspring is done only by the mother, with no involvement of the father (low degree of paternal investment). This would also explain the comparatively smaller degree of sexual dimorphism in humans, who have a greater proportion of paternal investment than most other primates.
Among the proposed differences between the human sexes are sexually dimorphic behaviors, most especially dealing with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies (David M Buss, 2007). However, there is a great degree of overlap between the sexes with regard to these behaviours.[Full citation needed]
The basal metabolic rate is about 6 percent higher in adolescent boys than girls and increases to about 10 per cent higher after puberty. Women tend to convert more food into fat, while men convert more into muscle and expendable circulating energy reserves. At age eighteen, men (on average) have about 50 percent more muscle mass than women in the upper body, 10 to 15 percent more in the lower. Men, on average, have denser, stronger bones, tendons, and ligaments. This allows for heavier work.[2]
Men dissipate heat faster than women through their sweat glands. Women have a greater insulation and energy reserves stored in subcutaneous fat, withstanding cold better, and performing better in activities requiring extraordinary endurance. Sex differences in endurance events are less significant than for sprinting events.
Men typically have larger tracheae and branching bronchi, with about 30 percent greater lung volume per body mass. They have larger hearts, 10 percent higher red blood cell count, higher hemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and higher peripheral pain tolerance.[2]
Women typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than males. Hence they develop fewer infectious diseases and succumb for shorter periods.[2] Ethologists argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage.[3]
CRAIG DAVID LYRICS
"Don’t Love You No More (I’m Sorry)"
[VERSE 1]
For all the years that I’ve known you baby
I can’t figure out the reason why lately you’ve been acting so cold
(didn’t you say)
If there’s a problem we should work it out
So why you giving me the cold shoulder now
Like you don’t even wanna talk to me girl
(tell me)
Ok I know I was late again
I made you mad and dinners thrown in (the bin)
But why are you making this thing drag on so long
(I wanna know)
I’m sick and tired of this silly game
(silly games)
Don’t think that I’m the only one here to blame
It’s not me here who’s been going round slamming doors
That’s when you turned and said to me
I don’t care babe who’s right or wrong
I just don’t love you no more.
[CHORUS]
Rain outside my window pouring down
What now, your gone, my fault, I’m sorry
Feeling like a fool cause I let you down
Now it’s, too late, to turn it around
I’m sorry for the tears I made you cry
I guess this time it really is goodbye
You made it clear when you said
I just don’t love you no more
[VERSE 2]
I know that I made a few mistakes
But never thought that things would turn out this way
Cause I’m missing something now that your gone
(I see it all so clearly)
Me at the door with you in a state
(in a state)
Giving my reasons but as you look away
I can see a tear roll down your face
That’s when you turned and said to me
I don’t care babe who’s right or wrong
I just don’t love you no more.
[CHORUS]
Rain outside my window pouring down
What now, your gone, my fault, I’m sorry
Feeling like a fool cause I let you down
Now it’s, too late, to turn it around
I’m sorry for the tears I made you cry
I guess this time it really is goodbye
You made it clear when you said
I just don’t love you no more
[BRIDGE]
Don’t say those words it’s so hard
They turn my whole world upside down
Girl you caught me completely off guard
On the night you said to me
I just don’t love you more.
[CHORUS 2X]
Rain outside my window pouring down
What now, your gone, my fault, I’m sorry
Feeling like a fool cause I let you down
Now it’s, too late, to turn it around
I’m sorry for the tears I made you cry
I guess this time it really is goodbye
You made it clear when you said
I just don’t love you no more
Craig David Unbelievable Lyrics
*** Complimentary Unbelievable Ringtone ***
Always said I would know where to find love,
Always thought I’d be ready and strong enough,
But some times I just felt I could give up.
But you came and you changed my whole world now,
I’m somewhere I’ve never been before.
Now I see, what love means.
[Chorus]
It’s so unbelievable,
And I don’t want to let it go,
Something so beautiful,
Flowing down like a waterfall.
I feel like you’ve always been,
Forever a part of me.
And it’s so unbelievable to finally be in love,
Somewhere I’d never thought I’d be.
[Unbelievable lyrics on http://www.metrolyrics.com]
In my heart, in my head, it’s so clear now,
Hold my hand you’ve got nothing to fear now,
I was lost and you’ve rescued me some how-.
I’m alive, I’m in love you complete me,
And I’ve never been here before.
Now I see, what love means.
[Chorus]
When I think of what I have, and this chance I nearly lost,
I cant help but break down, and cry.
Ohh yeah, break down and cry.
[Chorus]
Now I see, what love means
Craig David’s on the rise (uh-uh)
Check it out
I wouldn’t ever try to hurt you
I just needed someone to hold me
To fill this void while you were gone
to fill this space of emptiness (oh lady)
Chorus
I’m only human
Of flesh and blood I’m made
I’m only human (what what?)
Born to make mistakes (tell me whatcha gonna do?)
if you wanna say goodbye
That’s your deal (whatcha gonna do)
I’ll just have to settle my time
Till you’re near (lady)
I wouldn’t ever try to hurt you (nah-nah, nah-nah)
I just needed someone to hold me (to hold me, yeah, yeah)
To fill this void while you were gone (whatcha gonna do?)
To fill this space of emptiness (oh lady)
Check it out and we don’t stop
I’m only human (to the, to the)
Of flesh and blood I’m made (to the heart,
tick tock you don’t stop,
stop)
I’m only human
Born to make mistakes
To the heart, tick tock you don’t stop, stop X 9
Craig David’s on the rise (uh-uh, check it out)
Check it out
I’m only human
What am I suppose to feel
I’m only human (lady, lady)
I’m only human what am I suppose to do
Born to make mistakes (I’m born to make mistakes)
I’m only human (i’m only human)
I’m only human
What am I suppose to do now baby (what am I suppose to do?)
I’m only human (I’m only human)
What am I supposed to do (lady)
Human lady, human, human baby
I’m only human hey lady, hey lady
What am I supposed to do?
I’m only human (I’m only human)
Born to make mistakes
Chorus to fade
Well I wonder,
Could it be?
When I was dreaming ’bout you baby
You were dreaming of me
Call me crazy,
Call me blind,
To still be suffering is stupid after all of this time
Did I lose my love to someone better
And does she love you like I do, I do
You know I really really do
Well hey… so much I need to say
Been lonely since the day
The day you went away
So sad but true
For lonely since the day
The day you went away.
Go On, Yay… Oh…
I remember - date and time
September 22nd Sunday twenty-five after nine
In the doorway
With your case
No longer shouting at each other
There were tears on our faces
And we were letting go of something special
Something we’ll never have again, I know
I guess I really really know
Well hey, so much i need to say
Been lonely since the day
The day you went away
So sad but true, for me there’s only you
Been crying since the day
The day you went away
(The Day you went away)
The Day you went away
Go on, Yay…
Oh oh oh
Did I lose my love to someone better
And does she love you like I do, I do
You know i really really do
Well hey, so much I need to say
Been lonely since the day
The day you went away
So sad but true, for me there’s only you
Been crying since the day
The day you went away
Why do we never know what we’ve got till it’s gone
How could I carry on
The day you went away
Cause I’ve been missing you so much I have to say
Been crying since the day
The day you went away
The day you went away
The Day you went away…
Go on, Yay… Oh…
