Dioecious plants have 2 separate sexes

Perfect flowers contain both male and female sexual organs

Separate Sexes versus Hermaphroditism

When does adding a second sexual function (becoming hermaphroditic) make sense? gains from adding a second sexual function

must not bring about even greater losses in the original sexual function

this seems to be the case in plants, where basic floral structures are in place

for many animals, adding a second sexual function entails a net loss in overall sexual function

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Sequential hermaphroditism

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Check it out: more on the web

Some organisms are male first and then become female later in their lives

Some organisms are female first and then become male later in their lives

From ‘more on the web’

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In most organisms, including humans, sex is determined by genetic factors. Nature is endlessly inventive, however, and in some species of reptiles and fish the temperature at which the egg develops determines sex. In the case of reptiles, females lay their eggs in sand or dirt and the temperature of the soil at the depth at which the clutch is laid determines the temperature of the eggs. The mechanism of temperature-dependent sex determination (TSD) has not been worked out fully, but it involves an effect of temperature on gene expression in the developing gonads of the embryo. Each species with TSD has a critical temperature, usually around 28°-30°C, below which offspring become one sex and above which offspring become the other sex. Whether high temperatures produce males or females depends on the species, although in most cases higher temperatures produce females. Why do you think that some groups of animals would adopt TSD? How is temperature likely to affect the length of the embryo growth period and the relative size of the hatchling, and how would these factors differentially affect the fitness of male and female offspring? Would it be possible for an egg-laying female to control the sex of her offspring? What factors might constrain the temperature regime that a female can provide for her clutch of eggs? How would a string of unusually hot or cold seasons affect sex ratio in a population, and how should females respond to this in determining where to lay their eggs? A recent discussion of these issues can be found in R. Shine (1999).

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Whether a sequential hermaphrodite is first male or first female depends on how reproductive success through male or female function potentially increases with increasing body size. Larger females have larger reproductive organs and can lay more eggs. Small males can have high reproductive success where fertilization is internal and males do not contest social status, as in the case of the slipper shell. Where males compete for territories, as in the wrasse, large size is prerequisite to successful reproduction.

For others…

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Sequential hermaphroditism reflects changes in the costs and benefits of male and female sexual function as an organism grows. In some marine gastropods having internal fertilization, such as the slipper shell Crepidula, insemination requires the production of only small amounts of sperm. Hence male function consumes few resources and has little effect on growth. Consequently, individuals of many such species are male when they are small and become female when they are large and thus able to produce correspondingly large clutches of eggs

Check it out on the web for more explanation…

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Mechanisms of sex determination

Inheritance of sex-specific chromosomes (eg: humans, birds…)

Other factors…

Competition among X- and Y-bearing sperm to fertilize eggs or selection abortion of male or female embryos Birds: control is so precise that the chance of an

offspring being male changes predictably from the first to the last laid egg in the clutch – as a way of controlling (maybe) competitive interactions between male and female siblings

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Mechanisms of sex determination

Determined by the physical environment Several species of turtles, lizards, alligators: sex determined

by the temperature at which it develops in the egg Embryos that develop at lower temp produce males; higher

temp: females; for turtles. Opposite for alligators and lizards. Hmm… ?

Determined by the social environment The wrasse (discussed earlier) is a sequential hermaphrodite Raised in isolation females; raised in small groups, at least

one develops initially into a male w/o passing through a female phase

Females may become males later when they grow large enough to compete for territories ; primary males never change their sex

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Sex ratio of offspring is modified by evolution.

When sexes are separate, sex ratio may be defined for progeny of an individual or for the population as a whole. Sex ratio: number of males relative to the number of

females

Humans have 1:1 male:female sex ratios, but there are many deviations from this in the natural world.

Despite deviations, 1:1 sex ratios are common. Why?

Every product of sexual reproduction has one father and one mother if the sex ratio is not 1:1, individuals belonging to the rarer sex

will experience greater reproductive success: such individuals compete for matings with fewer individuals of

the same sex such individuals, on average, have greater fitness (contribute

to more offspring) than individuals of the other sex12

1:1 Sex Ratios: An Explanation

Consider a population with an unequal sex ratio...

individuals of the rare sex have greater fitness

mutations that result in production of more offspring of the rare sex will increase in the population

when sex ratio approaches 1:1, selective advantage of producing more offspring of one sex or another disappears, stabilizing the sex ratio at 1:1

this process is under the control of frequency-dependent selection

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Why do sex ratios deviate from 1:1?

One scenario involves inbreeding: inbreeding may occur when individuals do not

disperse far from their place of birth a high proportion of sib matings leads to local

mate competition among males

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Female condition and offspring

In some situations, a parent may benefit from a skewed sex ratio among its progeny, meaning that it should produce a preponderance of either male or female offspring.

competition for matings among individuals of one sex (usually males) can create variation in reproductive success: when competition is keen, some males may achieve many matings, others none.

it is often the largest males that win the lion's share of contests over access to females.

In mammals, a mother cares directly for her offspring, and her condition is likely to influence the fitness of her offspring. Therefore, females in poor condition should invest more in female offspring, which are likely to mate successfully regardless of the parental care they receive. Females in good condition ideally should produce male offspring, which will grow large and fare well in male-male competition for mates.

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Female wood rats…

a laboratory study of female wood rats (Neotoma floridana).

when investigators restricted food intake during the first 3 weeks of lactation to below the maintenance level of a nonreproductive female, mothers actively rejected the attempts of male offspring to nurse.

As a result, males starved, and the sex ratio of the offspring at 3 weeks shifted to about one male for every two females.

Faced with the likelihood that their young would be poorly nourished and that some of them would probably die before they were weaned, the mothers favored their female offspring.

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Sex ratio and pollution Recent study: “Lower oxygen levels in polluted waters could

lead to a higher ratio of male fish that may threaten certain species with extinction”

hypoxia (O2 depletion) can affect sex development, sex differentiation and the sex ratio in fish species. hypoxia can inhibit the activities of certain genes that control the production of sex hormones and sexual differentiation in embryonic zebra fish.

In his study, Wu found that 61 % of zebra fish - a universal freshwater fish widely used in scientific and pollution research - spawned into males under regular oxygen conditions. Under hypoxia conditions, the ratio of males increased to 75 %.

Hypoxia can be a naturally occurring phenomenon, particularly in areas where salt and fresh waters meet in estuaries such as the Pearl River Delta. It can also be caused by pollution. 17

Human sex ratio and pollution: PCBs…

PCBs were banned in the 1970s, … they are linked to problems with the brain, nervous and hormone systems, and although average levels in the human body have dropped, human exposure continues. Why? PCBs are persistent contaminants, which means they build up in the environment and in us.

Evidence continues to build that PCBs also affect birth sex. A recent study of blood serum from women who were pregnant in San Francisco in the '60s found that those with higher PCB levels were more likely to give birth to boys than those with low PBC levels.

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Is it PCBs?

Dr. Pete Myers brings up an important point in his summary of the report: The exposure levels observed in the study are high compared to today. Thus if these results are indicative of a causal relationship (never possible to confirm with epidemiological studies) then the simplest prediction would be that the chances of having a boy baby should be increasing because PCBs have been decreasing. That is not the case, at least as of the most recent analysis from Canada and the US.

Evidence from a large-scale study of four industrialized nations indicates that the sex ratio is skewed, and fewer boys are being born – But PCB levels have dropped…

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So? What do we know?

in-utero exposure to pollutants can affect a child's sex.

There are more than 80,000 chemicals in production today, many of which are known to be persistent or to disrupt hormone systems, and most of which haven't really tested for their impact on human health.

A 2007 study from the University of Pittsburgh found that during the past thirty years, the number of male births has steadily decreased in the U.S. and Japan. The study found a decline of 17 males per 10,000 births in the U.S. and a decline of 37 males per 10,000 births in Japan.

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Human sex ratio and pollution

The steepest sex ratio declines observed in the world have occurred on the 3,000-acre Aamjiwnaang (pronounced AH-jih-nahng) First Nation reservation in Canada.

The ratio of boys to girls there began dropping in the early 1990s. Between 1999 and 2003, researchers found, only 46 boys were born out of 132 recorded births. (35%)

Dozens of petrochemical, polymer and chemical plants border the reservation on three sides. Mercury and PCBs contaminate the creek that runs through the land, and air-quality studies show the highest toxic releases in Canada, said Jim Brophy, executive director of Occupational Health Clinics for Ontario Workers, based in Sarnia, the nearest city.

Boys made up only 42 % of the 171 babies born from 2001 to 2005 to Aamjiwnaang living on the reserve or nearby.

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Mating Systems: Rules for Pairing

There is a basic asymmetry in sexually reproducing organisms:

a female’s reproductive success depends on her ability to make eggs: large female gametes require considerable

resources the female’s ability to gather resources determines

her fecundity

a male’s reproductive success depends on the number of eggs he can fertilize: small male gametes require few resources the male’s ability to mate with many females

determines his fecundity

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Promiscuity: is a mating system for which the following are true

males mate with as many females as they can locate and induce to mate

males provide their offspring with no more than a set of genes

no lasting pair bond is formed it is by far the most common mating

system in animals

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Promiscuity 2 …

it is universal among outcrossing plants

there is a high degree of variation in mating success among males as compared to females: especially true where mating success

depends on body size and quality of courtship displays

less true when sperm and eggs are shed into water or pollen into wind currents

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Polygamy: occurs when a single individual of one sex forms long-term bonds with more than one individual of opposite sex

a common situation involves one male that mates with multiple females, called polygyny: (eg: elephant seals) polygyny may arise

when one male controls mating access to many females in a harem

polygyny may also arise when one male controls resources (territory) to which multiple females are attracted

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polyandry

Rare cases of a single female having more than one male mate Some human communities 1% of birds.. A common example of this can be found in the Field

Cricket Gryllus bimaculatus of the invertebrate order Orthoptera

Widely shown in frogs (Agile frogs, Rana dalmatina), polyandry was also documented in polecat (Mustela putorius) and other mustelids Why?

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Monogamy: the formation of a lasting pair bond betw one male &one female:

the pair bond persists through period required to rear offspring

the pair bond may last until one of the pair dies

monogamy is favored when males can contribute substantially to care of young

monogamy is uncommon in mammals (why?), relatively common among birds (but recent studies provide evidence for extra-pair copulations in as many as a 1/3 of the broods leading to mate-guarding)

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Real monogamy?

Extra-pair copulations (EPC) 1/3 or more of the broods produced by some

monogamous species contain 1 or more offspring sired by a different male

Mate guarding behavior on the part of males during their mates’ periods of fertility

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The Polygyny Threshold

When should polygyny replace monogamy?

For territorial animals: a female increases her fecundity by choosing a

territory with abundant resources polygyny arises when a female has greater

reproductive success on a male’s territory shared with other females than on a territory in which she is the sole female

the polygyny threshold occurs when females are equally successful in monogamous and polygynous territories polygyny should only arise when the quality of male

territories varies considerably

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Sexual Selection

In promiscuous and polygynous mating systems, females choose among potential mates:if differences among males that influence

female choice are under genetic control, the stage is set for sexual selection: there is strong competition among

males for mates result is evolution of male attributes

evolved for use in combat with other males or in attracting females

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Consequences of Sexual Selection

The typical result is sexual dimorphism, a difference in the outward appearances of males and females of the same species. Charles Darwin first proposed in

1871 that sexual dimorphism could be explained by sexual selection

Females of many spider species are larger than males

Traits which distinguish sex above primary sexual organs are called secondary sexual characteristics.

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Pathways to Sexual Dimorphism

Sexual dimorphism may arise from: (1) life history considerations and

ecological relationships: females of certain species (e.g., spiders) are

larger than males because the number of offspring produced varies with size

(2) combats among males: weapons of combat (horns or antlers) and

larger size may confer advantages to males in competition for mates

(3) direct effects of female choice: elaborate male plumage and/or courtship

displays may result

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Female ChoiceEvolution of

secondary sexual characteristics in males may be under selection by female choice:

in the sparrow-sized male widowbird, the tail is a half-meter long: males with artificially elongated tails experienced more breeding success than males with normal or shortened tails

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Runaway Sexual SelectionWhen a secondary sexual

trait confers greater fitness, the stage is set for runaway sexual selection:regardless of the original

reason for female preference, female choice exaggerates fitness differences among males: leads to evolution of

spectacular plumage (e.g., peacock) and other seemingly outlandish plumage and/or displays

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The Handicap Principle

Can elaborate male secondary sexual characteristics actually signal male quality to females? Zahavi’s handicap principle suggests

that secondary characteristics act as handicaps -- only superior males could survive with such burdens

Hamilton and Zuk have also proposed that showy plumage (in good condition) signals genetic factors conferring resistance to parasites or diseases

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