When Nicolas Fasel, from the University of Bern, Switzerland, talks about bat reproduction, it sounds more like war than a love story. Having established their supremacy over a territory, dominant ‘harem’ males then have the right to mate with the females that shelter under their protection; but their battle to father offspring is far from done. Fasel explains that males that do not guard a harem creep around in a bid to mate sneakily with females while their guardian's back is turned. ‘So males still have to continue to struggle after copulation to obtain offspring as females mate with several males when they are fertile’, says Fasel. And it appears that these sneaky males are better prepared to vanquish their more successful roost-mates as they have more vigorous sperm. Intrigued by the subordinate males’ apparently superior fertility, Fasel and his colleagues Charlotte Wesseling, Heinz Richner and Fabrice Helfenstein wondered whether frequent mating could compromise the fertility of harem males, providing sneaker males with an opportunity to outmanoeuvre more successful males at the final hurdle.
Fortunately, Fasel had access to a captive colony of Seba's short-tailed bats at the nearby Papiliorama zoo where he could investigate the question; ‘Visitors can enter a dome, the Nocturama, where you have the bats flying freely’, he explains. Initially, Fasel, Wesseling and their colleagues Ahana Fernandez, Felizia Koch, Alvaro Sobrino and Laura Panchione gently trapped males as they emerged from their cave roost before carefully collecting blood and semen samples. Investigating the condition of the sperm as soon as it was collected, the scientists could clearly see that the sneaker males' sperm moved faster than the harem males' sperm, potentially beating it to the egg. Then, having individually identified each bat with a unique set of rings attached to the bats’ forearms, the team released the bats before monitoring their behaviour over a 13 month period to determine which males were defending a harem and which animals were sneaking around behind their backs.
Having established each male's role, Fasel and Wesseling returned to impose a period of sexual abstinence on the harem males. ‘We wanted to control the copulation rate and monitor the sperm quality afterward’, says Fasel, who recalls that the males were surprisingly cooperative: ‘Inside the cage, they rapidly settled down, and in the absence of females, all of their competitive behaviours disappeared and they grouped together like old friends’. Collecting new blood and semen samples from the bats at the end of 3 days of seclusion, the team was impressed to see that the sperm from the abstinent harem males was as agile and healthy as that of the sneaker males.
But what had facilitated the recovery of the harem males’ sperm? Fasel and his colleagues wondered whether the period of abstinence had allowed the quality of the bats’ seminal fluid to improve and produce well-nourished sperm that could compete better with that of the sneaker males. However, when they tested for signs of an increase in a component (superoxide dismutase) that could protect sperm from damage in the abstaining males, there was none, so the males are using other mechanisms to improve sperm quality.
Fasel suspects that the sneaker males do not invest more in semen to increase their odds of producing offspring; instead, he suggests that harem males may be the victims of their own success. He says, ‘The high copulation rate of harem males may reduce their sperm quality and actually offer sneaker males the opportunity to reproduce’.
