Flow Channel Preference Is a Reliable Precursor of a Female Stickleback’s Ultimate Spawning Decision. In previous experiments, odor-based preference for males by gravid female sticklebacks was determined in a flow chamber that was fed by two water columns under conditions of laminar flow (1, 2). Under these conditions, female fish appear to compare their own set of MHC alleles with those of the prospective mating partners and show preference for the scent of males providing as optimal a complementation of alleles as possible (2). In the present study, we first tested whether the preference for males as determined by the flow channel reflects a female’s ultimate mate choice during spawning.

Female sticklebacks that were ripe for spawning were placed in the flow chamber. Fish were able to freely investigate the composition of water in the two halves of the chamber for two periods of 300 s each, with spatial reversal of water sources after the first period of 300 s. If the two sources were equally attractive, the fish should spend an equal period (i.e., 300 s) with each source of stimulus. To compare odor preference in the flow channel with final spawning decisions, gravid females were offered the choice between water from the tank of a male that allowed for an almost optimal combined (female plus male) number of MHC class IIB alleles and water from the tank of a male that allowed for sub- or superoptimal numbers of MHC class IIB alleles (see below). The female was then transferred from the flow chamber to a choice tank (20 × 38 cm, 20 cm water level) that was partitioned along the midline by an opaque divider 24 cm high and 20 cm long, which allowed the female to switch between sides in the undivided back part of the tank (Fig. 4). On either side of the divider, one artificial nest each made by us from green threads was randomly allocated to either side. The two stimulus tanks (same size as choice tank), each containing one of the males from the preceding flow-channel test and its nest, were placed either side of the choice tank (long sides together) with opaque partitions in between. Close to one of the short walls of each of the males’ tanks an overflow (U-shaped glass pipe) fed water into one partition of the female’s choice tank, close to an artificial nest, so that water from one male was fed into one side of the choice tank, and water from the second male was fed into the other side of the choice tank. Male tanks were continuously supplied with water at a rate of 6 liter/h. The choice tank had two water outlets located symmetrically in the second short wall, opposite the wall with the overflows from the males’ tanks. The males’ tanks had no additional outlets so that their water could leave only through the overflows and through the choice tank (Fig. 4). The female’s behavior was time-lapse video-recorded until she spawned. Gravid female sticklebacks usually have to spawn their eggs within 1 day of becoming gravid (3). From the video record, we determined the artificial nest into, or close to which, the female had spawned her eggs. Each stickleback was used only once for this experiment.

As determined by odor-based female preference for male sticklebacks, the optimum number of MHC class IIB alleles corresponds to 5.2, on average (2). Because each parent provides a haploid set per gamete, the combined number of different alleles of a pair should be ≈10 alleles to yield the apparent optimum of ≈5.2 alleles in an offspring (2). Optimal complementation was therefore considered to be present in pairs with a combined total of 10 or 11 different MHC class IIB alleles, nonoptimal complementation is assumed in pairs with a suboptimal number of alleles [5.9 ± 0.5 (mean ± SEM)] or a superoptimal number (13.1 ± 0.6 alleles) [seven triplets (each consisting of one female and two males) in each group]. Females preferred males providing optimal complementation of MHC class IIB status in the flow channel (Fig. 5A; n = 14 females plus 28 males in total, t = 2.14, paired t test, directed, P = 0.03) as expected (1, 2) and also preferred the predicted male when they actually spawned in the subsequent spawning test (Fig. 5B; 79%, n = 14 females plus 28 males, P = 0.04, binomial test, directed). The significant preference for the optimal male in her spawning decision shows that a female’s preference exerted for male odor in the flow channel is a reliable precursor of her mate choice.

1. Reusch, T. B. H., Häberli, M. A., Aeschlimann, P. B. & Milinski, M. (2001) Nature 414, 300-302.

2. Aeschlimann, P. B., Häberli, M. A., Reusch, T. B. H., Boehm, T. & Milinski, M. (2003) Behav. Ecol. Sociobiol. 54, 119-126.

3. Wootton, R.J.A. (1985) Functional Biology of Sticklebacks (Croom Helm, London).