Primates

About Gender. Notice: You can find the original article here

Once we recovered from the shock of Darwin's suggestion of a common evolutionary path, parallels have been drawn between humans and apes. For a while, we were compared with such species as gorillas or baboons, where a dominant male controls a harem of females. There was also a forlorn attempt to compare humans with the monogamous gibbons, ignoring the fact that gibbons, though forming permanent pair bonds, are solitary creatures. Most recently, genetic evidence has shown a close relationship with the chimpanzee, and confirmed the existence of some common ancestor of both apes and humans.

We will therefore concentrate on primates, which comprise some 200 species, and are extremely varied in their size and behaviour. In general, primates, including humans, are tree dwelling, tropical mammals, quadrupedal, with all four feet adapted for grasping. They have good eyesight, and generally have acute hearing, with a brain relative to their body size larger than most other species. Their sense of smell however, apart from the prosimians, seems to be less acute. Nearly all are vegetarians, most species mainly relying on fruit, although sometimes supplemented by insects or meat.

The Prosimians are thought to be representative of the earliest primates of some 60 million years ago, very small in size, nocturnal, insect eating creatures. A short while later, the anthropoids appeared - independently, it is thought, in what was to become America and Africa. These were the ancestors of the new world and old world monkeys - an interesting example of parallel evolution.

The first hominoids appeared about 35 million years ago, in Africa, leading to the gibbon and orangutan, plus an ancestor which gave rise about five million years ago to gorillas, chimpanzees and humans.

Primates range in size from the tiny mouse lemur, weighing less than a hundred grammes, to the gorilla, which may weigh over a thousand times as much.

There are relationships between such factors as diet and body size, brain size relative to body size and time taken to reach maturity.

We have suggested that early prosimians were insectivores, a diet that is nutritionally rich. The smaller a creature is, the more energy per gramme of body weight it expends, and needs to obtain through its food. It used to be thought that primate vision evolved to cope with the three dimensional world of an arboreal existence. It is now thought that binocular vision, in particular, along with grasping hands, evolved to capture prey.

Small creatures tend to live very 'fast' lives, usually breeding quickly with a short lifespan. However, the trade off in developing a larger brain to process the information from acute vision, and sometimes hearing, is that gestation times are lengthened. Primates tended, therefore, to have very few young, and live through several breeding cycles, and probably could not have survived in temperate latitudes, where the opportunities are limited by the seasons.

The benefit of a larger body size includes greater reserves to withstand adverse conditions, particularly changes in temperature. It also allows for a large digestive system which can process less nutritious food. Thus monkeys and apes generally eat vegetable matter, especially fruit, while the largest, such as the gorilla, exist mostly on leaves.

A larger body size allows a slower pace of life. It also increases the length of time need to bear young. Most primate young are precocial. While not, initially, able to support themselves fully, they can cling to their mother's fur, and can soon move around on the ground or in the trees. While most group living species have hierarchies of dominance, primates, especially some of the apes, have taken social organisation to new levels of complexity, further extending the period before offspring mature. Thus while lemurs may be adult within a few months, chimpanzees stay with their mothers for up to eight years.

Acknowledgments

Primate Info Net http://www.primate.wisc.edu/pin/

Singapore Zoological Gardens http://www.szgdocent.org/pp/p-main.htm

Citation

Bland, J.,(2002) About Gender: Primates

http://www.gender.org.uk/about/10ethol/a5_prim.htm

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Dominance and Male Behaviour

About Gender. Notice: You can find the original article here

The concept of dominance hierarchies has anthropomorphic overtones, particularly when writers refer to male dominance over females or vice versa.

The usual definition of dominance is an individual's preferential access to resources over another. Clearly, if males dominate females, it is to restrict mating opportunities for other males.

A species in which the males consistently deprived the females, and hence their young, of food resources would be at a disadvantage in evolutionary terms.

Dominance is measured by the number of times individuals defer to one another in social or other interactions. Since it is measured in terms of a hierarchy, it is relevant only within groups not across groups. In particular, the idea of male dominance (over females) or vice versa is entirely artificial, since each has different needs and agendas.

A female may defer to a male, but only because he is bigger and stronger, but that comes about because of male competition. To be anthropomorphic for a moment, animals are total pragmatists and she has nothing to gain by opposing him. However, the dominant male of the moment is likely to be healthy and in his prime. She may, therefore, mate with him in preference to others.

Similarly, a recent television programme on ring-tailed lemurs, highlighted the aggressiveness of females and virtually portrayed the males as 'wimps'. But ring-tailed lemur males play no part in the lemur social group, and had nothing to gain from opposing the females. There was nothing passive in their behaviour towards one another when a female came into estrus.

While individual females may only nurture a few young, most males have the capability to produce vast numbers. In an environment where females were able to nurture as many young as they were able to, the behaviour of males would have little effect.

In such species, the sexes may forage separately, males perhaps joining groups of females when the latter are coming into season.

What tends to be obscured, however, in many evolutionary accounts, is that, even for males, it is not simply the number of offspring that are produced that matters, but the number that survive and, moreover, long enough to breed themselves. Thus, in a more difficult environment, if a behaviour appeared among males that assisted the success of their offspring, the behaviour would be represented in an increasing proportion of future generations.

Where the sexes forage separately, there is less direct competition for food resources. A male lemur may range over a large area, covering the territories of several females, and areas in between. Orang utan males compete aggressively to defend a territory and are much larger than females. However, their larger size also means that some of their diet is of the tougher vegetation that females cannot utilise.

Going one step further, among ring-tailed lemurs, the males play no part in the group. Whereas the females usually rely on threat displays, in one instance in a television documentary, a territorial dispute was extremely violent. Two of the mothers were carrying youngsters; both were injured, one fatally. It follows that, where males prevent access to females by other males, they may incidentally protect against predators - or other females.

Citation

Bland J. (2002) About Gender: Dominance and Male Behaviour

http://www.gender.org.uk/about/10ethol/a4_mldom.htm

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Living in Groups

About Gender. Notice: You can find the original article here

Different species vary in their behaviour from living a fairly solitary existence, through clustering into groups, to closely related individuals with a more less complex social organisation. Living in a group increases visibility to predators. On the other hand there are more eyes and ears to sense danger. If a predator takes only one individual at a time, being part of the group reduces the chance of being that individual and, often there is competition to be at the centre. It may help with hunting, as with a pack of wolves, which may kill much larger prey, or be able to surround it. It may assist in defence as when a flock of birds mobs a Sparrowhawk. Living closer together also increases the risk of an infection spreading. Also, a given food source has to support more individuals, and often there is competition within the group.

One might predict that, where resources are sparse or scattered, the young will tend to disperse, indeed they may be driven away by the mother as they become adult. Possibly for reasons given above or because there is some particular focus of attention, they may gather in loosely in often transitory groups.

When considering a resource, one has to take into account the diet. A species with a specialised diet might find resources sparse even in, say a jungle, which would appear at first sight to be rich.

This may be encouraged if their resources are bounded by a more sparse environment, or if there are localised patches that are rich in a particular resource, particularly if their diet is restricted in some way, related females may stay close together in kin alliances capable of exploiting the resource and defending it.

It is the behaviour of the female in her requirements for nutrition and safety for nurturing her young that determines the behaviour of any given species, and hence that of the males. This is entirely our interpretation about how group living may appear, but it is mirrored by Wrangham, who writes "it is selection pressures on female behaviour which ultimately determines the effect of ecological variables on social systems."⁽¹⁾

The theory here is based on the idea of kin altruism. An individual co-operates with others who are more or less genetically related to her. Nevertheless, any individual has her own needs to meet, therefore each member of the group makes a delicate trade-off between competition and co-operation. There inevitably arises a balance in the group between more or less powerful individuals, typically referred to as a dominance hierarchy. In such groups, disagreements and conflicts between individuals are assuaged by placatory behaviours such as grooming, with complex and shifting alliances between individuals within the group. Animal studies have, until recently, concentrated on the behaviour of the dominant males, and ignored the social interactions within the group which determine its continued cohesion.

For males, who have only themselves to feed and potentially unlimited reproductive potential it tends to be the availability of mates which is the limited resource. Either males may stay within the group and compete with other males, or they may range around a number of groups. In practice there are more complex issues which will be discussed later.

The tendency of one sex to remain with the group is referred to philopatry. Alternatively the tendency of one sex to move out in search of mates is referred to as exogamy. One effect is to maintain genetic diversity and prevent in-breeding.

In most cases, where males stay within the group, and compete aggressively, the biggest and strongest will tend to monopolise the supply of mates, which leads to a selection pressure towards sexual dimorphism. It is a general rule of thumb, which we will meet when we discuss human ancestry, that in species where the males are significantly larger than females, inter-male aggression is implied. As a rider, Martin has noted that in a less favourable environment, small females breed earlier.⁽²⁾ Other suggestions that have been made is that larger males are more able to protect against predators, and that they are able to utilise different food resources, thus not competing directly with the females. However, these may be incidental consequences, rather than an evolutionary cause.

Footnote:

It might be as well to add some more about altruism. One of the revolutions in thinking which came about with new ideas on evolution, typified by such books as Dawkins The Selfish Gene,3 is that no individual acts "for the good of the species", thus signalling the death knell of what was known as Group Selectionism.

The individual that succeeds through its behaviour in passing on its genes, or more specifically, in producing young, will be more likely to pass that behaviour to future generations. Thus behaviours that enhance individual success are evolutionarily favoured - hence selfish genes. Yet it is quite clear that animals do at times co-operate, the theory suggesting that such co-operation will be between relatives - kin altruism.

There is a phenomenon known as allo-mothering, whereby a daughter will assist in the nurturing of her siblings, rather than having young of her own, where the likelihood is not good that she will be able to do so. The theory suggests that her siblings will be as closely related genetically to her as her offspring would be (about one half)

There are other issues, such as reciprocal altruism, that we won't go into here. Yet, though an individual behaves to benefit itself, its behaviour may benefit others. It may benefit by being part of a group, for instance, but the group may benefit by being of a certain size. Allomothering sometimes occurs among non-related females, particularly in primate groups. However, when assessing behaviours such as alarm or feeding calls, one has to be sure that there isn't a simpler explanation.

Bibliography

1.  Wrangham, R.W., (1980) An ecological model of female-bonded primate groups, Behaviour, 75:262-300
2. Martin, in Lewin, R., (1998) Human Evolution: An Illustrated Introduction, (Fourth ed.) Massachusetts: Blackwell Science.(p69) (bookshelf)
3. Dawkins, R., (1989) The Selfish Gene, London, Oxford

Citation

Bland, J.,(2002) About Gender: Living in Groups

http://www.gender.org.uk/about/10ethol/a3_group.htm

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Maternal Strategies

About Gender. Notice: You can find the original article here

The use of word "strategy" by ethologists and socio-biologists to describe a complex of behaviours which achieve a certain end, seems less than satisfactory. To the uncritical it implies something that is consciously worked out, rather than a behaviour which has emerged as the most successful for a given species in a given environment.

Many species produce large numbers of eggs or young, most of whom will not survive until adulthood and provide no care whatsoever.

Among mammals, some provide a degree of care, but nevertheless produce large litters, after a short gestation period and early weaning, and are said to be r-selected. Usually they are shortlived and may live in a difficult environment either because of predators, or more usually because the time periods when conditions are favourable for young are short and unpredictable.

At the opposite extreme are species which are said to be K-selected. Generally the species is larger, gestation may be longer, as may be the period of development and parental care. Usually the environment is predictable. Parents may have more than one batch of young through successive seasons, often having developed ways of surviving the worst conditions.

Each species has a balance between an optimal length of gestation and the period during which care will be necessary. Among the so-called altricial species, shorter gestation produces relatively helpless young. The drain of gestation on the mother's energies will be reduced, but extended postnatal care will be needed. Usually such young are protected in burrows or nests. Other species produce precocial young. Gestation may last longer, but the young are more able to fend for themselves, as for instance young cattle or deer that have to be able to run and walk as soon as possible.

Every mother faces the problem of whether the investment of energy in bearing young will result in success. In evolutionary terms, a mother who died in the attempt, or whose offspring was not able to grow and themselves breed would not pass on her genes to succeeding generations.

Unless she has built up the necessary energy reserves and, as zoo-keepers put it, come into "breeding condition", she is not likely to become estrous. Environmental stress may equally have an effect. Similarly for mothers who provide an extended period of care, it may be better to abandon the young and conserve energy for a possible future brood.

Aquarium keepers are well aware of the need to separate some species of female fish from their eggs. Probably, in the wild, they would swim away after laying. In the aquarium they remain in close proximity. It may be that they do not recognise their eggs for what they are, but simply as food.

However, with species where after-birth care is needed, if a threat is sufficiently serious, such as from a predator, mothers may abandon and even eat their young, on the basis that they represent an investment of energy which can be reabsorbed to provide for a future litter under more propitious circumstances.

Citation

Bland, J., (2002) About Gender: Maternal strategies

http://www.gender.org.uk/about/10ethol/a2_mtnl.htm

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Reproduction

About Gender. Notice: You can find the original article here

The basis of sex is that two individuals exchange genetic material and produce offspring that have some from each, but different, through recombination, from either of them. How sex came to be is still the subject of much scientific speculation.

Single cell organisms exist in their billions, reproducing by division. Clearly they also evolve. Any mutation may enable them to bypass their host's defences, such as the flu virus, or the new antibiotic resistant strains of tuberculosis. In recent years, however, it has been found that, in fact, bacteria do sometimes join and exchange genetic material.

Given that the more complex species multiply by sexual means, one may assume that the appearance of sex facilitated their evolution. One theory is that mutated alleles are usually recessive. If individuals that are homozygous for the mutation are successful and go on to breed, there will be an increase in those heterozygous for the allele. If these are, in turn, especially successful, perhaps with changing circumstances, its frequency will increase in the population. If on the other hand, it is harmful, it will affect only a quarter of the population and will gradually disappear.

Most plants can multiply either sexually or asexually - so-called vegetative reproduction. The offspring in the latter case grow very close to the parent plant which may produce overcrowding. Sexual reproduction, by producing masses of pollen is wasteful and chancy, since only a small amount of the material is likely to grow. However, fertilised seeds containing the genetic material may be, for instance, transported in the gut of an animal which eats the fruit. It does, therefore, allow plants to spread over a wider area.

The simplest multicellular organisms are clusters of identical cells which congregate together, like the slime moulds. In more complex organisms different cells become specialised for various purposes and thus become interdependent on each other. The successful way for such organisms to propagate themselves, is by producing a single undifferentiated cell which contains the information for the cells produced at each division to specialise as necessary. In plants, these are known as meristems.

Some animal species reproduce in this way, a process known as parthenogenesis. Parthenogenesis is well known in such species as aphids, bees, and Daphnia but is rare in vertebrates. The offspring are clones, genetically identical to the mother. In many social insects, such as the honeybee and the ant, the unfertilized eggs give rise to the male drones and the fertilized eggs to the female workers and queens.

There has been much in the news lately about fishes being affected by estrogens in our rivers. Given these were largely press reports, one wonders whether they actually "changed sex," but simply didn't develop properly. However, different species of fish use a wide range of sexual processes. Parthenogenesis is relatively common, while in many shoaling species, the sex of an individual depends on the sex ratio of the shoal, where females may change to a male role. In other species, individuals change roles at certain stages of their lives.

Crocodiles and turtles do not rely on sex chromosomes, but on the temperature at which the eggs are incubated. It appears to be related to the production of hormones within the egg. Attempts to incubate alligator eggs for reintroduction to the wild invariably produced males, although it has been recently found that females could be induced by the use of estrogens. Some writers have speculated that this contributed to the extinction of the dinosaurs. Changing global temperatures, they suggest, skewed the sex ratio, adding to the problems that all species were facing at that time.

Insects such as fruit flies seem to use genetic sex on a cell by cell basis. Mosaics occur relatively frequently, as in the fruit fly which, with their short lifespan and high reproductive rate, have been models for much genetic research.

In mammals, though many cells are sexually dimorphic, their development is controlled largely by the circulation of hormones on a whole body basis.

While therefore, sex is varied and flexible among many species, it is a flexibility which mammals, with the complex physiology of placental nurture, have sacrificed

The sex chromosomes would appear to be a relatively recent evolutionary "bolt-on goody" which, in mammals, might explain why, though serious autosomal anomalies are almost always lethal to the organism, many anomalies of the sex chromosomes are not.

Whereas the sperm produced by the male contains little else but the germ cell, the egg produced by the female, in almost all cases, also contains the nutrients required for initial development. In mammals there is also the energy investment involved in gestation and after-birth care. A central principle in ethology, then, is that the energy investment by the female means that relatively few eggs are produced, while a male can produce vast numbers of sperm. Further that, while a male is potentially able to mate with many females, the female has constraints on when and with which individuals she will mate.

Citation

Bland, J., (2004) About Gender: Reproduction.

http://www.gender.org.uk/about/10ethol/a1_repro.htm

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Ethology: Introduction

About Gender. Notice: You can find the original article here

Ethology is the study of animal behaviour. Its emphasis is the interaction of different species with each other and their environment. Consequently it has close links with ecology.

It is of interest to us for two reasons:

•  Psychologists frequently use animal experiments to draw parallels with human behaviour.
• Sociobiology has drawn heavily on the behaviour of other species as models for human behaviour, often in a highly speculative way.

Ethologists themselves are very wary about using the behaviour of one species to predict that of another, even a closely related, species. One of the pioneers of ethology was Niko Tinbergen. He pointed out that there are four fundamental questions to answer:

Causation: Why does a particular animal behave in a certain way? What prompts the behaviour?

Function: What purpose does the behaviour serve?

Development: What has happened during its life, to cause or enable the animal to perform the behaviour. This might be inherent in its physical development or something in its past experience.

Phylogeny: What is the history of the species, that has led to this behaviour arising, and aided its survival?

David McFarland (1) has expressed this in another way by asking the following four questions:

Why do birds sit on eggs?
Why do birds sit on eggs?
Why do birds sit on eggs?
Why do birds sit on eggs?

We may paraphrase these questions by asking why humans have gender.

What is it that people recognise about gender, what does it consist of, in terms of attitudes and behaviours?

Why do people interact with each other in gender biased ways and why do they feel it necessary to do so?

Why is it that humans have gender? Do other species have it and, if so, what are the differences and why?

What is the function of gender? Why is it useful to humans?

How have gender systems appeared in different ways through history?

That could express the aim of this website though we don't guarantee to give any final answers.

It will be impossible to cover the whole topic of ethology in one website, so we intend to limit ourselves to the primates, and look at certain general considerations.

One outcome of sociobiology has affected the study more than any other; the realisation that there is an inherent difference between male and female mating strategies.

We intend to go even further and suggest that male behaviour in probably all species is constrained by the female's requirements in producing and nurturing their young.

Since we will be introducing certain hypotheses, the section is inevitably speculative. We hope, however, it will raise questions and provide food for thought.

Reference

1. McFarland, D., uncited reference in Hall, M., Halliday, T., (1992) Biology Brain and Behaviour: Book 1, Behaviour and Evolution, Milton Keynes: Open University Press

Citation:
Bland, J., (2002) About Gender: Ethology Introduction
http://www.gender.org.uk/about/a0_intro.htm

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