Reliability of placental scar counts in the Red fox {Vulpes vulpes L.) with special reference to fading of the scars

The use of placental scar counts in determining litter size and pregnancy rate among Red foxes (Vulpes vulpes L.) is reviewed. There is no experimental material to test the validity of the method, although many authors have discussed it. Recent workers have (1) included uteri from vixens killed later in the season and, (2) become more concerned about effects of resorptions and/or abortions on scar counts, as well as the possibility of scars persisting from earlier pregnancies. In the present study, scar counts overestimated numbers of persisting corpora lutea to a higher degree in old vixens than in yearlings; and scars were shown to fade over winter. Estimates of frequency of breeding among females, based on various combinations of placental scars of different shades, and on date of death, are discussed. The relation between number of scars of different shades, date of death and known minimum litter size in three wild and one captive vixen is compared with the foregoing. Counts of all scars evidently overestimate litter size at birth and breeding frequency, probably because some scars originate at resorption sites and others persist from earlier pregnancies. Only dark scars should be counted early in the season, but those of increasingly lighter shade should be included as winter progresses. A guide to counting placental scars over winter is presented.