05 May 2012

Spring Conditions Influences Gender of Bats

Golden crowned fruit bat
(Acerodon jubatus)
Bats are mammals that have forelimbs that form wings which allows them to fly. Although there are other mammals that are believed to fly, these, such as the flying squirrels, merely glide through the air. Bats are the only mammals capable of true and sustained flight.

Bats are part of the order Chiroptera and are subdivided into two orders, Megachiroptera and Microchiroptera. These represent how bats are ordered; by size. Megachiroptera means large bat while microchiroptera means small bat. The largest bats have wingspans that reach six feet while there are bats that are just one inch long.

There are about 1,240 species of bat. In fact, they make up about 20% of all classified mammal species worldwide.

70% of bats live on a diet of insects. Some consume fruit and nectar. The most popular bat due to pop culture is the vampire bat. These bats hunt living prey and suck its blood.

Bats are known for their radar like ability to hunt. This is called echolocation. As they have poor eyesight, bats emit sound that bounces off of objects in their path, the resulting echo that that the bats picks up determines the size, location, distance, speed and even texture of the object in an instant. Another behavior, albeit less popular is torpor. Torpor is regulated hypothermia that allows the bat to survive the winter. Other species migrate or hibernate to get through the cold.

Bats are slow reproducers. Gestation of a bat is about 40 days for small bats and up to six months for bigger bats. An average litter is just one pup. The pups are taken care of by the colony (females congregate to bear and raise young).

Early spring means more bat girls

There must be something in the warm breeze. A study on bats by a University of Calgary researcher suggests that bats produce twice as many female babies as male ones in years when spring comes early.

The earlier in the spring the births occur, the more likely the females are to survive and then reproduce a year later, as one-year olds, compared to later-born pups, according to Dr. Robert Barclay's research published in PLoS ONE.

Video: Bat Sense

"The early-born females are able to reproduce as one year olds, whereas male pups can't," explains Barclay, professor in the Department of Biological Sciences at the University of Calgary.

"Thus, natural selection has favoured internal mechanisms that result in a skewed sex ratio because mothers that produce a daughter leave more offspring in the next generation than mothers who produce a son."

The length of the growing season has an impact on the ratio of female to male offspring and the time available for female pups to reach sexual maturity, the study found. This suggests that not only does sex-ratio vary seasonally and among years, but it also likely varies geographically due to differences in season length.

Big brown bat (Eptesicus fuscus)
Barclay analyzed long-term data on the variation in offspring sex-ratio of the big brown bat, Eptesicus fuscus, a common North-American species that consumes insects.

"In this species, more eggs are fertilized than eventually result in babies, so there is some mechanism by which a female embryo is preferentially kept and male embryos are resorbed (Remove by gradual breakdown into its component materials and dispersed in the circulation; reabsorbed) early in pregnancy," says Barclay. But, he adds, the biochemistry behind the skewed sex ratio is unknown.

"Some other mammals and some birds have the ability to adjust the sex ratio of their offspring," says Barclay. "Even human-baby ratios vary—there is a study showing that billionaires produce more sons than daughters, for example."

This is the first long-term study on sex ratios in bats, says Barclay and it "suggests some pretty interesting physiology."


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