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A very large African gander, he is fit and strong now, but in a few years he may be too big to mate and too heavy to run around!

I quite like watching Crufts on the television. I am not a dog owner, but there are enough parallels with the poultry and waterfowl fancy to set into action a train of thought.

Pet dogs are hugely diverse in size, shape, colour, hair length and performance. Heights at the withers range from just a few inches in the Chihuahua to roughly three feet in the Irish Wolfhound. The ancestor species, by comparison, is quite conservative. The wolf has developed over a long period of time for the prime reason of survival. Natural selection has moulded the development, a process called 'evolution', in a way that the animals most suited to their particular environment, life-style, social grouping, diet, foraging, hunting, etc. survive. Those unsuited . . . don't survive. That's what it really means by 'survival of the fittest'. Not fit only as strong, fast, athletic, etc but 'fit' as in 'fitted to the life-style' . . . suited.

Pet, or even working, dogs, have evolved under domestication. There are theories that domestic dogs have crossed back to wolves many times in their development (Carles Vilà et al, Science, 1997), yet primarily they have been developed by human beings over time for many purposes, hence the multitude of shapes, sizes, colours, etc.

Whether all of them are healthy, or desirable, is another issue. Much has been said and written about breed deformities. Most people agree, however, that breeding (especially continual inbreeding) determined largely by arbitrary aesthetic criteria is a major cause of such deformities.

Let me explain. If we say such and such a characteristic is desirable simply because we 'like' it, and we breed for it generation after generation, things have a habit of going wrong. Take for example size: if we breed dogs (ducks or geese) simply to get them bigger and bigger and bigger, we are likely to encounter problems. They found this with captive Rainbow Trout. A four pound fish will have a heart roughly the same size as a twenty pound trout. Hence a four-pounder may be fit, fast and strong whilst the giant twenty-pounder might be slow, lethargic and very likely to die of a heart attack. Very big African, Toulouse and Embden geese seem to reach a maximum size before succumbing to arthritis, heart problems, inability to mate and lack of fertility, whilst very slightly smaller birds may go on into their teens and look (and perform) as sprightly as a four-year-old. Just ask breeders of St Bernards and similar dog breeds; you will probably find similar problems.

All right, if we choose such breeds as Call Ducks or Indian Runners, how safe is it to follow the aesthetic line ('that's what I like'), and how can we avoid the inbreeding problems?

Let's face it, originally most duck breeds were produced from the single mallard species for specific purposes: Aylesbury and Rouen ducks were bred for the table. They weren't expected to live very long! Indian Runners were bred to produce lots of eggs and to walk from one part of an Indonesian island to another, or just run out into the paddy fields. Call Ducks were developed for calling in wild birds - literally 'decoy' ducks. Nowadays most of these breeds are bred for the show pen.

We have lots of very fine written standards to define the better 'aesthetic' qualities of the breeds. How can we be sure that we are retaining the other essential characteristics? I can remember being brought to a halt by a reminder that Khaki Campbells were defined as much by their egg-laying as by their appearance. That is perhaps why Mrs Campbell was so reluctant for many years to allow any standard description of her birds. She wanted them to retain their utility characteristics. Are there limitations on choosing breeding stock merely by what the birds look like?

Hybrid vigour
Companies like Cherry Valley look on their web-site spend a lot of time and money making sure that their birds are 'fit' for the job. These largely Pekin type birds are strictly for the table and their practical characteristics are salient. However, Cherry Valley know that things do not stand still. They cannot keep the same old strain and expect it to generate quality offspring year in and year out.

The aim of Cherry Valley's breeding programme is to improve the growth efficiency and carcass quality of Pekin ducks without compromising reproduction, liveability, mobility, or identifiable metabolic adequacies, such as the pulmonary system. This programme ensures a pragmatic balance of profitable genetic change in both cost and quality production traits, whilst ensuring the health and welfare of the stock.

Ducks are selected after a performance test including measurements of growth rate, feed conversion ratio and breast muscle depth.

Without going into the scientific and statistical work (BLUP1 and EBV2), it is interesting to note one method used: Cherry Valley's commercial products are hybrids produced from various crosses of the elite strains, ensuring high level of heterosis1 in the parents and final product. This process produces a range of birds that are fecund, robust and profitable.

'Heterosis' is the interesting bit, and it is not new. I suspect 'hybrid vigour' has been recognized for centuries. The theory is that offspring from a cross between two very inbred lines or breeds are likely to be healthier and more 'vigorous' than the two original parent groups. It is a way of countering what is known as 'inbreeding depression'. You can get 'outbreeding depression' as well, but let's leave that to another time.

'Heterosis can be classified into mid-parent heterosis, in which the hybrid shows increased strength which is greater than the average of both parents, and best-parent heterosis, in which the hybrid's increased strength is greater than that of the strongest parent. Mid-parent heterosis is more common in nature, and it is easier to explain' Wikipedia. It is all to do with homozygotes and heterozygotes.

If the two chromosomes, one from the mother and one from the father, are identical at a particular gene, they are said to be 'homozygous'. If they are different (alleles), then they are said to be 'heterozygous'. Birds inbred for many generations are likely to be homozygous at many sites on the chromosomes. This is what is thought to be at the heart of the problem. By out-crossing, and increasing the number of heterozygotes, a certain amount of extra 'strength' is put into the offspring. I think the work 'strength' here is used in a wide, evaluative sense.

Wikipedia summarizes two theories for this quite neatly: (a) The overdominance hypothesis implies that the combination of divergent alleles at a particular locus will result in a higher fitness in the heterozygote than in the homozygote. Take the example of parasite resistance controlled by gene A, with two alleles A and a. The heterozygous individual will then be able to express a broader array of parasite resistance alleles and thus resist a broader array of parasites. The homozygous individual, on the other hand, will only express one allele of gene A (either A or a) and therefore will not resist as many parasites as the heterozygote.

(b) The second hypothesis involves avoidance of deleterious recessive genes (also called the general dominance hypothesis), such that heterozygous individuals will express . . . fewer deleterious recessive alleles than its homozygous counterpart.

In other words (a) the wider the gene pool, the more heterozygotes, the better; (b) the greater variety of alleles reduces the concentration of possibly disadvantageous, recessive genes.

What has all this to do with keeping domestic ducks as pets or for showing? Quite a lot really, but not always in an obvious way. How can you cross an Indian Runner and not lose its characteristic shape, carriage and appearance? How can you keep a perfect Call 'type' and yet bring in some hybrid vigour? The simple answer is to cross 'strains' or 'varieties' within the breed.

Knowing how to breed between colours, without introducing more problems than you have cured, is an art in itself.

Avoiding extremes of size or shape can also avoid breeding and development problems. The late Wim Biallosterski, one of the great Dutch Call experts, once showed me how not to use Call females with very small pelvic bones. Small is not always beautiful.

Monitoring healthy parents needs to be combined with aesthetic show criteria. It is no good breeding Calls with ultra short bills or legs if they are prone to sinus problems or difficulty in walking. It is no good risking a very tall Indian Runner in the breeding pen if it has a kinked neck or a serious colour fault. The problems may always come back to haunt you, or the people you sell birds to.

Although it is vital to retain as many of the original 'pure breeds' as possible, simply for the sake of genetic diversity, it is also important to realise that 'nothing stands still'. Khaki Campbells (the miracle breed of the early twentieth century) were initially a deliberate cross between an Indian Runner and a Rouen drake. Buff Orpingtons, Saxonies, Magpies, Welsh Harlequins, Abacot Rangers were all developed from outcrosses. Silver Bantams and Silver Appleyards were similar products of out-breeding.

I think it may have been Benjamin Disraeli who once defined conservatism as keeping the best of the old alongside the best of the new.

My O level history is a bit rusty, but this contains more than an ounce of truth for duck breeders. Perhaps Edmund Burke is just as apposite: A state without the means of change is without the means of its conservation.' Just alter state' for duck'!

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