SHANGHAI - The food safety watchdog in Shanghai has launched an investigation into eggs which have hardened yolks and can bounce like rubber balls when boiled, officials said Friday.

Small quantities of the so-called "rubber eggs" are reported to have appeared on the market in dozens of regions across China in recent weeks. The investigation is designed to appease consumers' concerns, after some suspected they bought artificial eggs made by unconscientious traders seeking profits.

But food experts say the eggs are likely to be natural, with their unusual characteristics explained by high levels of a compound called gossypol. When egg-laying hens eat gossypol-enriched feed, it binds to protein in egg yolks.

While gossypol normally exists in the residue of cotton seeds added to chicken feed as an extra protein source, large doses of the compound will suppress sperm activity as gossypol had been tested to be used in male contraceptive pills in a couple of countries, the experts say.