Real males that lactate: a batty story.

How's this for a gender-bender? Scientists have discovered the first case of a male mammal that produces milk in the wild.

Charles M. Francis, a research associate affiliated with the Wildlife Conservation Society in New York City, stumbled upon this strange case while on a field trip to Malaysia. After capturing a number of fruit bats in nets stretched across the rain forest canopy, he realized that the males, although perfectly normal in every other way, had noticeable mammary glands.

"My initial thought was that they were freak bats," recalls Francis. After netting more of the animals, he realized that the males of this species, Dyacopterus spadiceus, or Dayak for short, expressed a small amount of milk.

Francis sent tissue samples from three male and two female bats to Thomas H. Kunz at Boston University. Kunz examined the tissue under a microscope and found that male mammary tissue looked very similar to female breast sections.

The researchers noted that tissue obtained from the testes of two male bats appeared normal. Such findings leave open the possibility that these bats sire and suckle their own offspring, Francis says. They describe their findings in the Feb. 24 NATURE.

There have been a few reports of lactation in domesticated male animals. However, those cases appear to result from severe inbreeding, Kunz says. Certain pathological conditions may cause human males to produce milk, the authors add.

Not much is known about these fruit bats. The authors wonder whether the bats practice monogamy. in which a male and female pair up - generally for life - and raise a family together. Most mammals are polygamous, Kunz notes. If these bats are monogamous, the male might gain an evolutionary advantage by helping its partner with wet-nursing duties, he speculates.

There is an alternative explanation for the milk production, however. These fruit bats may eat some plants that contain high concentrations of phytoestrogens, natural hormonelike substances that may spur the growth of male breast tissue and even trigger a small amount of milk production, the authors add.