Superfetation in cute, cuddly, badgers

I used to work on Eurasian badgers, Meles meles, a fascinating mustelid carnivore that is relatively easy to observe in the wild. My work was in cranial morphometrics - measuring skulls and detecting differences - and I was more interested in variation in sexual dimorphism than anything else (though I wrote two papers on the possible menas by which the species colonized Ireland). The following press release by the University of Chicago Press caught my eye.

In a fascinating new study forthcoming from The Quarterly Review of Biology, biologists from the University of Oxford explore a rare tactic employed by females badgers to maximize their reproductive success. The authors argue that conception during pregnancy, known as superfetation, benefits female reproductive fitness by reducing the risk of infanticide, extending the female's window of opportunity for conception, and increasing the genetic diversity of the litter.

"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, whose discrete mating tactics may have masked the extent of reproductive conflict between the sexes," write Nobuyuki Yamaguchi, Hannah L. Dugdale, and David W. Macdonald, all of the Wildlife Conservation Unit at the University of Oxford.

One of only two known species that exhibit or are presumed to exhibit both superfetation and embryonic diapause - during which the newly fertilized egg temporarily ceases development and remains free in the uterus cavity instead of implanting directly into the uterus - the female European badger first ovulates and is fertilized in late winter-early spring (January-March). However, implantation does not occur until December or January of the following year, a gestation period of nearly eleven months.

"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]," write the authors.

The European badger (Meles meles) is unique among badgers in exhibiting large variation in social organization, from large, multi-male, multi-female groups in southern England to small group and paired coexistence. All other species of badgers are primarily solitary.

The paper is Yamaguchi et al. (2006) "Female Receptivity, Embryonic Diapause, and Superfetation in the European Badger (Meles meles): Implications for the Reproductive Tactics of Males and Females." Quarterly Review of Biology, 81(1), p. 33-48. It's currently available online at the QRB site.

For completeness sake, the abstract is below the fold.

The European badger Meles meles is thought to mate throughout the year, with two mating peaks occurring in late winter/spring and summer/autumn. After mating, fertilized ova enter embryonic diapause (delayed implantation) at the blastocyst stage, which lasts up to eleven months. Even if mating is successful, however, the estrous cycle may continue during embryonic diapause, which suggests that female badgers are capable of superfetation (conception during pregnancy). This may increase female fitness by facilitating polyandry, and reduce the risk of infanticide by resident males through paternity confusion. Detailed understanding of female receptivity, specifically the association of superfetation with embryonic diapause, may explain field observations of seemingly inconsistent reproductive tactics of male badgers with regard to, for instance, whether or not they guard mates or defend territories. The combination of embryonic diapause and superfetation may occur in other mustelids; if so, the sociobiology of mustelids will need re-evaluating, and the Mustelidae may prove to be a good model taxon for studies of sexual conflict in the reproduction of eutherian mammals.