February 26, 2021

Bats self-harm to attract females


An adult male with a scab on the dorsal region of the forearms.

An adult male with a scab on the dorsal region of the forearms.
Sinc Agency

Males of many species display striking and showy characteristics for females to notice. This is the case of the peacock and many other birds that flaunt their colorful and exuberant plumage. But, What happens to bats that come out at night to breed?

In their case, males have to resort to another sense: smell. The more scent they give off, the more attracted the females will be. That is why fringed-lipped bats (Trachops cirrhosus) have developed a technique that allows them to be the center of attention. In their forearms they cause wounds that form fetid scabs that they exhibit during reproduction.

Males scratch their entire body with the claws on their hind legs, bite them, and then spit a sticky yellow substance on their forearms.

This was discovered three years ago by scientist Victoria Flores, a predoctoral fellow at the Smithsonian Tropical Research Institute (STRI) in Panama, that speculated that scabs might play an important role in mating. Scientists now prove it in the journal Behavioral Ecology and Sociobiology.

To provoke the injuries and the subsequent scab, the males scratch their entire body with the claws of the hind legs, bite into them and then spit a sticky yellow substance onto their forearms, a behavior that only they do, and not the females. Once formed, the size of the scab depends on the male’s testosterone level, as the researchers have confirmed.

“When we measure the size of the smelly scabs on the male’s forearms, we quantify their testosterone levels and the size of their testicles. We find that all these factors are related,” says Mariana Muñoz-Romo, first author of the work and postdoctoral fellow at STRI.

Thus, as the research team shows for the first time, males with the highest levels of testosterone – a hormone associated with natural body odors in humans – and the largest testicles had the largest scabs on their forearms. This feature “makes us pretty sure that this trait is associated with reproduction and courtship “, adds Muñoz-Romo.

Females in heat, males with scabs

To measure these hormone levels in males, the scientists took blood samples from wild bats at the Institute of Science and Technology in Panama. They also observed the time of year when the males have the most scabs and checked if it corresponded with the time of greatest fertility of the females.

“Actually, there are very few studies that have measured testosterone levels, female fertility, and this unique trait of males“, emphasizes Muñoz-Romo

“Actually, there are very few studies that have measured testosterone levels, female fertility, and this unique trait of males in mammals. As far as we know, this is the first study of its kind in bats,” emphasizes Muñoz-Romo .

One of the reasons that studies of this type are so rare is that it can be difficult to know if females are in heat. In this case, the researchers took vaginal cell samples to find out if the females were fertile. Most of the young of this species of bats are born at the end of Panama’s dry season in May.. Most of the males had large scabs about five months earlier, during the mating season.

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