A Study in Skew Sex Allocation Bias in Birds

Mary Nogare


You have a funny feeling And you think your mind's infirm.

You check your breeder records And these results confirm

The patterns male and female Of the chicks your pairs have raised.

For chance they "re quite unlikely, And frankly, you 're amazed.

You shake your head and wonder How it really could be true - Relax Dear Aviculturist!

What you've just seen is SKEW!

S kew is a term used to describe something that is "more developed on one side than the other" or is distorted "from a true value or symmetrical form" [ 1]. For the purposes of this brief discussion, skew refers to sex allocation bias or sex ratio adjustment, which is variance of a population or its progeny from the evolutionarily stable 50:50 (male:female) sex ratio [28]. Skew is much more than individual nests or groups having more males or females by chance.

A Little Background Gender Biology

Before beginning a discussion of skew in birds, it may be helpful to review some basic gender biology.

Animals such as mammals and birds are "chromosome dependent" [28) species, which means gender results from sex chromosomes. The cells of most animals contain a complete paired set of chromosomes, one set inherited from each parent. These cells are called "diploid" ("di" meaning "two" and "ploid" from "ploidy" which refers to the basic number of an organism's chromosomes). In a two-phased process called meiosis, which occurs in the gonads, reproductive cells (gametes) are formed. Each gamete contains half of a complete set of chromosomes and is "haploid" ("ha" meaning "half'). Fertilization occurs when a gamete from the male organism combines with a mature gamete from the female, called an oocyte, to form an offspring with a complete paired set of chromosomes (diploid).

Chromosomes contain genes. The combinations of the genes of the sets of chromosomes comprise the genetic "blueprint" for the organism. Genes even determine how an organism may be able to react to its environment.

The set of all of an organism's genes, regardless of whether a gene is expressed, dominant or recessive is called its genotype. The observable outward characteristics of the organism, including attributes such as color patterns and 

Mary Nogare, Snoqualmie, WA

behavior resulting from the interaction of the individual's genotype with its environment are called its phenotype.

In animals such as mammals and birds, where both males and females are normally diploid, gender results from combining two sex chromosomes. Because of their general shape, mammalian sex chromosomes are designated "X" and "Y". In mammals, the father, who has X and Y sex chromosomes, determines the gender of the offspring. He is the heterogametic parent ("hetero" meaning "different"), which means he can form gametes with either an X or Y sex chromosome. The mother is the homogametic parent as both of her sex chromosomes are X ("homo" meaning "same"). Therefore, unfertilized mammal ova can only contain X chromosomes, and gender cannot be determined until fertilization. If an X gamete of the father fertilizes an egg, the resulting offspring will be female (XX). If a Y gamete fertilizes an egg, the resulting offspring (XY) will be male.

In birds, the chromosomes are designated Z and W and the heterogametic parent is the mother [13]. A female bird has ZW sex chromosomes, and a male has ZZ. This means that the female bird determines the gender of her offspnng.

Biological Mechanism for Avian Sex Determination (21, 22]

Avian research indicates that maternal sex steroids such as testosterone, androstenedione, 5-alpha-dihydrotestosterone and beta-oestradiol [21] influence the segregation of the sex chromosomes in the oocytes. The female appears to adjust the amounts of these sex steroids delivered to the yolk of the forming gametes, determining gender during the first of the two phases of meiosis [2, 12, 22], just a few hours before ovulation and prior to fertilization [8, 13, 21, 24]. This appears to be accomplished in part via two layers of steroidogenic cells that make up the follicular walls; these layers are known as the theca (outer layer, in direct contact with the female's bloodstream) and granulosa (inner layer, closer to the yolk of the gamete) [22]. Research indicates that this adjustment occurs in response to factors in the mother's current environment [22]. As such, response of the mother bird to her environment appears to be part of the mechanism for determining the sex of her young.

This implies two things important to skew. First, it implies that avian sex ratios are not constrained by fixed numbers of male or female oocytes within the ovary.

Second, avian mothers may be able to adjust the sex ratio of their progeny based on their genetic ability to respond to factors in the environment (facultative adjust- 

ment). These factors, which may include territory and food quality and availability, adequate appropriate nesting sites, availability of suitable mates, actions of weather, predators, parasites and diseases, etc., are called selection pressures [28, 29).

Brief Discussion of

Sex Allocation Theory [15, 28, 29)

Sex allocation, or sex ratio bias is theorized to be a result of an organism's response to various selection pressures that suggest greater success for passing on its genetic material if the offspring are predominantly one gender versus the other [28). Further, "facultative sex ratio variation will only be favored when the fitness benefits of this behavior are greater than its costs [29]".[15]

If the factors influencing the success of an individual are relatively equal to its needs, then selection favors the environmentally stable sex ratio of 50:50. However, the more the pressures vary from parity, and the more the success of offspring of different gender may be influenced by these pressures, the more selection favors parents who can skew the sex of their progeny to accommodate it [28).

Models of sex allocation theory attempt to identify these factors and predict their influence on the gender ratio of offspring. Conversely, sex ratio bias may be observed, and a model developed in an attempt to determine the factors influencing it.

Mechanisms For Sex Ratio Adjustment in Birds

There are two basic ways that parent birds might control the sex ratio of their offspring. In the first, one or both parents adjust the sex ratio after the young have hatched. This might be accomplished by allowing chicks of the unwanted gender to die through such means as neglect, direct destruction, or even tossing them out of the nest [7, 8, 12]. Methods of this type are called Secondary sex ratio adjustment. 




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