The market effect in the wood mouse, Apodemus sylvaticus: Selling information on reproductive status

Grooming in the wood mouse is a means by which males obtain information abo ut the reproductive state of females, as grooming creates a situation which allows the male to smell the groomed female's anogenital area to ascertain her phase of oestrus. Although grooming is reciprocal in this species, it is asymmetrical in that males groom females more often than vice versa. Thi s grooming asymmetry was studied using Markov chain analysis for grooming s equences in two captive wood mouse colonies, and transition rates were used to represent motivation in both sexes. Grooming sessions were often initia ted by a male's attempt to sniff an immobile female's anogenital region, wh ile the female would immediately react by avoiding or biting the male. In o rder to entice the female to remain, the male would begin grooming the fema le's head and shoulder area, surreptitiously and consistently grooming down wards towards the female's anogenital region, until she would again termina te such contact either by avoiding or biting the male. While, therefore, th e male's tendency to sniff the female's anogenital region was stronger than his tendency to groom her, the female's tendency to terminate the male's n aso-anal contact was much stronger than her tendency to terminate his groom ing bouts. If the male did not initiate grooming after the female terminate d naso-anal contact, she avoided further contacts and escaped. In mice, as in most mating systems, the demand for matings by males is far larger than the number of matings females offer. The mating market, therefore, is highl y skewed, which gives females the opportunity to demand 'commodities' in re turn for allowing males to mate. This system allows females to 'bargain' wi th males to obtain grooming in return for anogenital contact. Females asses s the length of time they receive grooming and will only allow males to att ain anogenital contact after a certain threshold value has been passed. If anogenital contact provides the male with information about the female's re productive state and/or with sexual stimulation, then this process represen ts the first quantified example in short-lived mammals of females 'selling sex' in terms of the market effect. This paper therefore provides a new vie w of the regulation of grooming: grooming is not simply reciprocal with bot h participants concerned that the other does not 'cheat' (e.g. tit-for-tat (TFT)-like strategy), rather grooming is a commodity which can be bartered against female reproductive information or matings.