But, after 50, many great singers do increasingly resemble Dame Edna. Some don't retire because they say their fans won't let them. Thus Joan Sutherland - a hefty, down-to-earth, compulsive knitter - continued to sing mad, vulnerable Donizetti heroines, her audience relishing the absurdity. Montserrat Caballe stretches credibility even further, her voice now in tatters, her stage presence always maternal.
Wagnerian sopranos tend to go on shrieking forever, perhaps because there are so few anyway. Birgit Nilsson was an interesting example; her voice retained its quality but her intonation failed. It's much easier to age as a mezzo; you can move gracefully down the cast to a large number of maid or companion roles - Bertha in The Barber of Seville, Suzuki in Butterfly. Someone should write an opera for 'sopranos of a certain age' - a libretto by Fay Weldon would do nicely.
On the other hand, you can give up, gradually or in full glory. Most resolute of all was Dame Janet Baker, who published an autobiography and retired to Harrow on the Hill. Good examples of gradual retirement are Elisabeth Soderstrom and Brigitte Fassbaender, who turned producer; also Elisabeth Schwarzkopf, whose boudoir stage presence gave way to a virago in masterclasses. Dame Nellie Melba celebrated 50 by founding her own opera company, which set a new standard for opera in Australia. But was it worth the trouble? After she had slogged her way through Mimi in La boheme, a second-rate tenor singing Rodolfo greeted her during the curtain call with 'Madame, you still sing beautiful]'.
It's easy to understand why few dancers go on after 50; for singers, the ageing process is more mysterious. Josephine Veasey, the distinguished mezzo who now works with young singers at English National Opera, is refreshingly frank. 'The physical change in me was enormous,' she says. 'I had the most dreadful lethargy, and a feeling of instability. The menopause causes stress, loneliness.' For Veasey, a broken marriage and the collapse of a nine-year romance didn't help. 'I sang Rosina for the first time at 50, at the Royal Opera House. A mad thing to do. There I was on stage, pretending to be 16 years old. My feet wouldn't move. I had frozen shoulders. My bones ached. And if you're not happy, you don't want to sing.'
Hormone replacement can dispel the depression, but Veasey says there's no avoiding the fatigue suffered by the muscles which support the vocal cords. 'The strength needed for breathing seems to let you down. Many singers feel something going wrong, and try to force the sound. Singers should have many interests. Life's so short.'
Dame Kiri Te Kanawa has husbanded her resources more prudently than most. 'The quality of her voice is as good as it was 20 years ago,' says Sir John Tooley, who presided over the Royal Opera House when Kiri became a star. 'She's been immensely careful about what roles she does, concentrating on those which display the beauty of her voice.' She's also chosen roles that suit her natural innocence and placidity on stage - Maria in Simone Boccanegra, Desdemona in Otello. The more dramatic and strenuous Puccini roles - Mimi, Butterfly and especially Tosca - have been tried but discarded. She has avoided bold producers like Gotz Friedrich, Johannes Schaaf and Harry Kupfer, preferring to work with someone like Elijah Moshinsky, whose productions allow singers to look at the conductor and can be revived with minimal rehearsal. Dame Kiri recently admitted that all she and Placido Domingo need to rehearse for another Otello is how he will strangle her. Critics might say this is precisely what's wrong with their performances these days: too cool, too knowing.
'Kiri doesn't look her age on stage,' protests record producer Christopher Raeburn. 'But she's never done the young girl roles anyway. Kiri does the 34-year-olds.' And what wonderful women they are, these discreetly tragic heroines. Mozart's Countess in Figaro, neglected by her husband, and Elvira in Don Giovanni, abandoned after three days. Strauss's Arabella, the elder sister awaiting the right husband, and the Countess in Capriccio, toying with two younger men. Best of all, for sopranos over 50, the Marschallin in Rosenkavalier, only 32 but already listening to time ticking at night.
Now that Mirella Freni is finally giving up, Raeburn would like to hear Dame Kiri in more Verdi roles, especially Elisabeth de Valois in Don Carlo. 'And what about Adriana Lecouvrer,' he says. 'And Strauss's Ariadne?- but that's a hell of a sing, and you've got to compete with the tenor.'
Veasey considers the challenge of new roles essential to keeping going, but Kiri has a reputation as a slow learner, lazy even. Certainly not one to spend up to three months learning a new role in Czech or Russian. To celebrate her birthday this month, EMI is issuing Eugene Onegin, conducted by Charles Mackerras, sung in English with Dame Kiri as a coy Tatyana.
Raeburn thinks Dame Kiri's laziness exaggerated. 'When she gets down to something, she really works hard to get it right. She gets irritated with other singers who have the flair to get by, especially in their own language.' A lot depends on the conductor, and much of her best work has been with Georg Solti, whose outrageous charm seems to fire her on all cylinders as the Marschallin.
In private life, Kiri has also made choices that enhanced her professional assets. Her fans might be disillusioned to see the diva in her dressing room, not winding up the passion but planning a new plumbing system with her husband and manager, Desmond Park, a former mining engineer. Their two teenage children were adopted - a rewarding choice for Dame Kiri, adopted herself, but also one which prevented the voice becoming darker after pregnancy. Golf keeps her fit. She sees herself as a tomboy for whom frocks, earrings and Rolex watches are only for public consumption.
Who will replace her? The singers' agent Tom Graham suggests Carol Vaness for Desdemona and the Mozart repertoire. 'Kiri had the whole package - the voice, looks, charisma, and her intriguing half-Maori background.' Vaness is a larger-than-life personality but she, and the talented Rene Fleming, may suffer from being American.
Graham rattles down the soprano stakes as only an agent can. 'Felicity Lott, the thinking man's artist. Barbara Bonney, all the Mozarts, plus the lyric roles like L'elisir and Nanetta in Falstaff. Barbara will move on to Eva in Meistersinger, the Countess in Figaro, but probably stay within the German lyric rep. Then there's Sylvia McNair - first choice for both the vegetarian and grown-up conductors. And Nancy Gustafson for Slavic and German rep, but like all sopranos she wants to die on stage in an Italian heap.' The leading British contender is Amanda Roocroft. 'Very pingy', say other sopranos, but maybe moving too fast.
'You've got to sing with the voice you've got,' says Graham, 'not with the voice you'll have in five years' time.'
At 50, Dame Kiri Te Kanawa has a voice other singers would kill for. It's too soon to retreat to concert medleys (two at Hampton Court in June) and the occasional Marschallin. As Dame Edna might say, 'if you've still got it, don't sit on it'.
Dame Kiri Te Kanawa at the Royal Albert Hall, 9 March. (Returns only: 071-589 8212)
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