Gonadal hormones modulate sex differences in judgments of relative numerousness in meadow voles, Microtus pennsylvanicus

Abstract

Animals in a variety of taxa discriminate between a greater quantity and a lesser quantity of the same object, an ability that is referred to as relative numerousness judgment. For example, meadow voles can distinguish between areas containing more over-marks by one opposite-sex scent donor and fewer over-marks by another opposite-sex scent donor. Females appear to be able to make better discriminations between more or less over-marks than do males. In that gonadal hormones have been implicated in modulating cognitive function associated with spatial tasks, we tested the hypothesis that high titers of testosterone and estradiol are necessary for male and female voles, respectively, to distinguish between the top- and bottom-scent donors in an area containing mixed over-marks. We gonadectomized voles, giving them either gonadal hormone replacement (testosterone for males and estradiol for females) or no hormone replacement, and tested their spontaneous judgments of distinguishing between the top- and bottom-scent donors in an area containing mixed over-marks; a task involving judgments of relative numerousness. Female voles given replacement estradiol performed better than did female voles not given replacement estradiol in determining the top-scent and bottom-scent males in areas containing mixed over-marks. In contrast, males not treated with replacement testosterone performed better than did males treated with testosterone in determining the top-scent and bottom-scent males in areas containing mixed over-marks. Thus, high titers of estradiol and low titers of testosterone are associated with better performance on tasks involving relative numerousness in female and male voles, respectively. The results of this task on relative numerousness judgments are discussed in relation to the effects of gonadal steroid hormone on spatial ability, a closely related cognitive domain, and the social biology of meadow voles.

Figure 1

Mean (± SEM) amount of time (seconds) that (A) gonadectomized + testosterone-treated (GX + T) male and (gonadectomized + blank-treated male (GX + BL) voles and (B) ovariectomized + estradiol-treated (OVX + E) female and ovariectomized + blank-treated (OVX + BL) female voles exposed to 7 over-marks by donor A and to 0 over-marks by donor B later spent investigating the odor of these two scent donors. Histograms capped with different letters are statistically different at p < 0.05.

Figure 2

Mean (± SEM) amount of time (seconds) that (A) gonadectomized + testosterone-treated (GX + T) male and (gonadectomized + blank-treated male (GX + BL) voles and (B) ovariectomized + estradiol-treated (OVX + E) female and ovariectomized + blank-treated (OVX + BL) female voles exposed to 6 over-marks by donor A and to 1 over-marks by donor B later spent investigating the odor of these two scent donors. Histograms capped with different letters are statistically different at p < 0.05.

Figure 3

Mean (± SEM) amount of time (seconds) that (A) gonadectomized + testosterone-treated (GX + T) male and (gonadectomized + blank-treated male (GX + BL) voles and (B) ovariectomized + estradiol-treated (OVX + E) female and ovariectomized + blank-treated (OVX + BL) female voles exposed to 5 over-marks by donor A and to 2 over-marks by donor B later spent investigating the odor of these two scent donors. Histograms capped with different letters are statistically different at p < 0.05.

Figure 4

Mean (± SEM) amount of time (seconds) that (A) gonadectomized + testosterone-treated (GX + T) male and (gonadectomized + blank-treated male (GX + BL) voles and (B) ovariectomized + estradiol-treated (OVX + E) female and ovariectomized + blank-treated (OVX + BL) female voles exposed to 4 over-marks by donor A and to 3 over-marks by donor B later spent investigating the odor of these two scent donors. Histograms capped with different letters are statistically different at p < 0.05.

Figure 5

Mean (± SEM) amount of time (seconds) that (A) gonadectomized + testosterone-treated (GX + T) male and (gonadectomized + blank-treated male (GX + BL) voles and (B) ovariectomized + estradiol-treated (OVX + E) female and ovariectomized + blank-treated (OVX + BL) female voles exposed to 4 over-marks by donor A and to 4 over-marks by donor B later spent investigating the odor of these two scent donors. Histograms capped with same letters are not statistically different at p > 0.05.