Research shows female badgers conceive during pregnancy

27 March 2006

Picture: European badgers Meles meles.

By re-examining extensive existing data on the badgers' mating behaviour, they found the male badgers exhibited seemingly inconsistent reproductive tactics, for instance, whether or not they guarded their mates or defended their territories. The research team pieced together a picture from the data, in particular the post-mortem analysis of dead female badgers, which strongly suggests that the females exhibit superfetation - when another foetus forms in the uterus before the delivery of the first one. The badgers then have just one litter that year. The researchers believe superfetation benefits female reproductive fitness - by increasing the gene pool of the litter, lessening the risk of infanticide, and extending the female’s window of opportunity for conceiving.

It's now thought that female badgers may come into heat recurrently peaking twice, in spring and autumn. They usually mate with several different males throughout the year. The researchers concluded that this could be an adaptation designed to confuse the males about the paternity of cubs to reduce infanticide. Some mammals are known to kill any offspring they suspect as not being their own.

One of the authors, Professor David Macdonald, from the Wildlife Conservation Research Unit said: 'This opportunity for additional fathers to join in during the span of one gestation period may also enable the female to adapt her mate selection to changing circumstances - a male who seemed a good choice as a mate during the spring oestrous period might not seem such a good choice six months later!'

Professor Macdonald added: 'Natural selection and sexual selection act on both sexes. However, emphasis on sexual selection as a direct evolutionary force acting on males has diverted attention away from the selective process acting on females. Adaptations whereby females manipulate the reproductive options for males are becoming more widely known, and we suspect that the very unusual sex lives of European badgers are an example.'

The female European badger first ovulates and is fertilized some time between January and March, but the egg doesn't become implanted in the womb until December or January - a gestation period of nearly eleven months. It is one of only two mammalian species known to conceive during pregnancy and also to exhibit embryonic diapause - where the newly fertilized egg is suspended in development and remains free in the uterus cavity, instead of implanting directly into the womb.

The other species displaying this quirky private life, the American mink, is now widespread as an introduced species in Britain. The female mink exhibits an accelerated version of the process - she comes into heat repeatedly during a window of just a few weeks, and may mate with several different males - the result being that a single litter can have several fathers.

The researchers believe the combination of embryonic diapause and superfetation may benefit females, regardless of their social system, by enabling cryptic polyandry - mating with more than one male. They conclude that this study could prove that the badger - part of the Mustelid family - could be a good model for future studies of sexual conflict in the reproduction of mammals.

The European badger (Meles meles) is unique among badgers in varying its social grouping patterns. Some choose to be part of large single sex groups; others are in small groups or pair up. All other species of badger are primarily solitary creatures.

Source / Credit: University of Oxford