However, we do sometimes see shifting gender ratio, which can be as skew as 47/53. When this happens, we normally see more females than males hatching, at least in broilers – and to understand why, we need to look more closely at the determination of sex.

In birds, the female determines the gender of her offspring. At the moment the yolk is formed, the sex of the resulting chick is already decided. In mammals, gender is determined by the male, and fixed only after fertilisation.

Reptiles – specifically crocodiles - are even more sophisticated, determining gender according to incubation temperature. In some types of crocodile, incubation at a certain temperature gives 100 per cent males. But if that temperature is reduced by just 2°C, those same eggs will produce 100 per cent females, with a linear distribution of males to females over this 2ºC range. In other types of crocodile, a specific incubation temperature will produce 100 per cent females, while temperatures higher or lower than this will result in more males.

If chickens and crocodiles shared that temperature-related gender influencing mechanism, the ability to influence a shift from male to female would certainly be of enormous value. However as the chicken’s sex is determined even before fertilisation of the blastoderm - this is unlikely.

When we do see a shift in sex ratio, this can sometimes be explained by sexing errors, especially if cloaca sexing cannot easily determine gender. In breeders especially, chicks that cannot be decisively sexed will be considered off-sex, reducing the number of sexing errors for the customer - and at the same time influencing the observed male/female ratio.

It does seem, however, that incubation temperature does have some impact. For example, overheating during late-stage incubation can produce a greater number of females. But further investigation of the unhatched eggs will normally reveal the majority of late deads as male. So if we add the numbers of hatched chicks and late deads together, we usually find that 50/50 ratio between males and females restored.

The gender shift in late deads is much more extreme than in hatched chicks, simply because even a shift from 50/50 to 49/51 males/females will produce three times more late dead males than females (based on late deads at 4-5 per cent).

So why do more males than females die late in incubation? One possibility is that males are weaker and more susceptible to sub-optimal conditions than females. This may be true, as we see virtually the same trend in day-old chicks.

Or perhaps males produce more heat during incubation than females? Although there is currently no scientific data to support this, we do have some practical evidence.

Per set, males normally hatch several hours earlier than females, which may be due to higher metabolic heat production during incubation, resulting in higher temperatures in-ovo and accelerated growth, compared with the females.

If we experiment by setting eggs of exactly the same weight, the hatched males do show more signs of overheating - for example, more yolk sac residue and less development – than females. This could explain the shift in sex ratio under high temperatures. With increased heat production, there is more likelihood of exceeding maximum temperature. As a result, more males will die late in incubation, while the females are less affected and therefore hatch.

Take it one step further, and we could even speculate that the males are perhaps not weaker by nature, but because they are overheated during incubation. This being the case, sexing embryos in the egg - currently in development with in-ovo techniques - could greatly improve the incubation process, by enabling us to create optimum incubation conditions each for males and females.

Right now, we do not have scientific evidence that males do produce more heat than females, or what mechanism may be responsible for that. Further investigation will teach us more about the differences between males and females - and how those differences can help improve processes and productivity in the future.

For the time being, if we see a shift from male to female ratios that cannot be explained by sexing errors, the key may lie in overheating. But unlike the crocodile, our chicks are not shifting gender: we are merely seeing a similar effect through the survival of a greater number of females, not in the ratio of male to female overall.

May 2007