The lack of desirable sex is never ending story in the world of breeders. Everybody sometimes looks for a male, sometimes for a female to set up a good breeding pair. However, researchers have worked intensively on artificial insemination techniques recently, getting of desirable sex can be sometimes an enormous trouble. What if we could just influence the sex of hatched chicks? Can we manage such conditions for our parrots that they will breed the desirable sex?
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The question of sex ratio manipulation interests me since the high school. I guess that the first impuls was when I was purchasing Purple-bellied lories (Lorius hypoinochrous). I got these birds from german birdpark Walsrode and they were two males. It’s already been clear before that acquiring of females will be a very tough mission. It was and actually I have never got them but that’s not the point of this article. At the moment when I start chasing all breeders of this species and ask them to sell me one female, I put myself a question: „Where are all the females? How is that there are so rare?“ Practically every chick hatched wherever was a male. Male of lories are not so agressive as for example cockatoos so the male aggression could be the cause. Imports are a history in EU for a long time already so it could happen that more males were brought in here because by the time the sex ratio in captive population would fix to 50:50.
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Purple-bellied Lory, (c) Quartl. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
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The only reasonable explanation is that the skewed sex ratio exists on level of embryos. Simply females of some species produce more male eggs than female eggs or reversely. We can ask – can we find the same phenomena in the wild? That’s what we actually don’t know and it’s very difficult to find out. Within recording of sex ratio in natural population of adult birds (which lack sexual dimorphism) it’s necessary to catch every bird and take DNA samples. Consequently, we have to count more populations so the results have sufficient statistical value. However, we can say that in general, there are balanced sex ratio in natural populations and an extreme imbalance is very rare. What is more, lories are monogamic birds with equal mortality in both males and females so dominance of one sex wouldn’t make the sence.
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Now we touch the point – birds probably manipulate with the sex of their offspring only sometimes when it can be „advantageous“ for them. Under certain circumstances it can be better to have only males or females. And how they do that? A very short insertion from the field of genetic is necessary. In comparison to mammals, birds don’t have sex chromosomes X and Y but sex is determined by Z and W chromozomes. The heterogametic sex (ZW combination) is a female. Male has ZZ chromosomes. According to our basic biologal knowledge we know that gametes arise during meiotic cell division. We are interested mostly in the segregation of chromosomes which determine the sex. The result of meiosis in males are 4 sperms, in females only 1 egg while a half of chromosome is extruded to polar bodies. In the past, scientists thought that the destiny of every chromosome is random (50:50). According to the latest research, it however seems that exactly at this moment the female can influence if the chromosome Z is going to the egg or chromosome W. So simply she can decide about the sex of her chick.
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The first example is about the Eclectus Parrot (Eclectus polychloros). Robert Heinsohn – australian ornithologist and herpetologist – occupies himself with the study of these birds for a long time. He found out many interesting things including the sex ratio manipulation. His experiment proved that eclectus females can produce very long sequences of one sex in a row. Sometimes even 20 males and no females. The probability that this could be caused by chance is zero. In the wild there are more males which are helpers like in case of Seychelles Warblers. The species is probably polyandric as well.
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Eclectus Parrot, male, (c) Peter Békési. This file is licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license.
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The most popular study about sex ratio manipulation in birds is probably the experiment with the Seychelles Warbler (Acrocephalus sechellensis). In this species we can observe helpers. These are older youngers which stay with parents and help with feeding their brothers and sisters. This role in this case of a warbler is hold by young females (helpers are females only). Males shortly after fledging leave parents and migrate to other teritorries. For parents it could be advantageous to produce males only in seasons when there is a lack of food because afterwards they don’t waste their poor sources and immediately leave. On the contrary, in rich seasons when there is enough food it’s better to have more females which will help with feeding youngers so they can produce more of them. The mentioned experiment really confirmed this theory and the study of sex ratio manipulation in birds has begun.
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Seychelles Warbler, (c) Christian Hauzar. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
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Very extreme case represents sex ratio in Australian Brushturkey (Alectura lathami) which doesn’t incubate its eggs but buries them into the sand molds. These molds acumulate heat. Birds have special thermoreceptors on their beaks so they find optimal places for incubation. Researchers found out that under lower temperatures there are more females hatched and under higher more males. However, it’s not caused by sex chromosome segregation but by sex specific embryo mortality. Simply female embryos are sensitive to higher temperatures and males to lower.
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Alectura lathami, (c) Fritz Geller-Grimm. This file is licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license.
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The last example is about a Gouldian Finch (Chloebia gouldiae). To my mind, it’s probably the best proof of sex ratio manipulation in birds. The whole study was based on two experiments. The first has asked a question – why they influence the sex of their offspring? and the second – how they do that? Australian researchers established the facility for breeding of these birds and tested the impact of male phenotype on the sex ratio of their offspring. Surprisingly, they found out that imcompatible pairs (in colors) suffer by higher mortality of their female chicks then pairs of the same colored pairs. They also proved that imcompatible pairs produce more males than females. How is that? It’s obvious, why they should have more females in their nests if they wouldn’t survive anyway? The next experiment tested the mechanism of the sex ratio manipulation and authors found the answer – by hormone corticosterone.
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Gouldian finch, mutation, (c) Marcos Andr. This file is licensed under the Creative Commons Attribution 2.0 Generic license.
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There are many similar studies and the analysis of these experiments represented the content of my bachelor thesis. Now I still work on this project within my MSc. studies at Charles University in Prague. At home I established special facility for this purpose and I test the impact of several conditions (age of parents, food quality, epigenetics, …) on the sex ratio manipulation. Hopefully, I will get some useful results soon.
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My facility for 30 pairs of zebra finches.
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If anybody among breeders has some experience with the sex ratio manipulation in his breeding or he or she would like to join this project, contact me, please: lubos.tomiska@seznam.cz. All comments are appreciated.
Author: Lubos Tomiska
Title photo: (c) Dick Daniels. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
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