Mating Systems

Is there Romance left in nature, or is nature governed by so many instinctual selective devices that animals act more or less like machines run on a computer program?

The study of mating as a whole has been revolutionized in the last few decades. Let me start the discussion of mating systems in birds with a quick review of relevant terminology.

Monagamy (90 percent of birds): Birds have one mate.

Polygamy: mating system where an individual may associate with many different members of the other sex.

Polygyny: males pair with two or more females (2 percent).

Polyandry: Females pairing with several males. (1 percent) Found predominantly in shore-birds. Often occurs in sexual reversal (one sex is larger than another).

Polygynandry: Males pair with many females and females pair with several males. Basically a mixed mating system. It's common if fishes but not with birds. Its characteristic of tinamous, and flightless ratites.

Promiscuity: Indiscriminate sexual relations (its doubtfull that they are indiscriminate, even though this is how it's defined). Males though may be willing to copulate with as many males as possible.

Sexual Selection: Form of natural selection in which variation in fitness among individuals is generated by different mating success.

Sexual selection can arise from two different methods

  • 1) male rivalry: One male could deny access by other males to a female. For instance, territoriality would be a form of acquiring beneficial resources that would allow them better success in mating.
  • 2) Female preference: if females prefer some character of males, as a form of better fitness, they can greatly effect breeding traits.

Darwin's notion of sexual selection

"Before we discuss further the question whether females select the more attractive males or accept the first whom they may encounter, it will be advisable briefly to consider the mental powers of birds. Their reason is generaly, and perhaps justly, ranked as low; yet some facts could be given leading to an opposite conclusion. Low powers of reasoning, however, are compatible, as we see with mankind, with strong affections, acute perception, and a taste for the beautifull and it is with these latter qualities that we are here concerned."

Charles Darwin, 1871

The Descent of Man and Selection in Relation to Sex.

Wallaces response:

A young man, when courting, brushes or curls his hair, and has his moustache, beard or whiskers in perfect order, and no doubt his sweetheart admires them; ….

‘the permanence of female taste was ‘scarcely verifiable in human experience’

Goddes, P., and Thomson, J.A. 1889

The Evolution of Sex

When the concepts of sexual selection and mate preference were coming to the forefront on scientific thought, they were hotly contested. One of the main reasons for the debate was that the men in that day and age could not agree that human females had any preference for other males. If human females had no preference, how could birds have a preference?

Some of the evidence came from extra-pair copulations, also called extra-pair paternity.

Other evidence came from females of whom have a certain reperoire size that would choose males with a larger repertoire size (length of their call). Thus it is believed that the size of the song is a trait of male birds that has been shapped by female presence.

Its also important to show that males and sons have some sort of genetic linkage. One is that bright-stripped males were linked to bright-stripped sons.

Long tailed Widows in Africa: Males have very long tails. It’s a classic study to see if females prefer longer tails. Thus, he had several birds that were control. He also took some males and cut their tale, and took other males and glued their tales. Turns out that Female long-tailed widows greatly preferred males with longer tails.

Anders Molar did a study with Barn Swallows. He showed that female barn swallows preferred to breed with males that had long streamers.

Polygyny threshold model. (Orians 1969): Why would a female ever choose to be a secondary female in a males territory that already has a female, instead of being the only female in another males territory. You can basically look at fitness curves in relation to certain territories. There is a cost to being the secondary female would have a curve shifted down. Thus, how much better does a territory have to be to allow a female to choose to be a secondary female on that territory.

If ‘A’ is the decrease in fitness induced on a female by being the secondary female on a territory. ‘B’ is then the change in territory quality of two curves.

EPP: environmental potential for polygyny: A high potential for polygeny occurs if there is patchy distributions of resources so that one male can obtain more than his fair share of resources.

LEKS: Aggregations of males that come together for no other reason than to attract females. Its in leks that males can reaches extremes. The most dominant male will sometimes get more than 50 percent of matings.

European RUFFS: Some males in a lek look very different. For example the European Ruff an have a satellite male that looks almost albino. The terminal males are darker. The satellite males exist only to get the females that are inbetween male territories. (Photo by C.C. Doncaster).

Swallow-tail manikins: Engage in cooperative displays in leks. One group in a single lek can have a dominant male. There are three helpers in the group with the dominant. They will then engage in a marvelous leapfrog dance. The idea is that maybe the helper males are just younger males that need to learn the ropes from the older males. They are not closely related though. Or maybe the next helper may become the next dominant.

New Guinea Birds of Paradise exibit the extremes of gaudiness.

Cotingas of central and south America. In addition to colored feathers, birds have skin accessories. For instance, blood engourgment in the mouth area become erect when blood enters the area.

By Rob Nelson


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