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Social preference of female gerbils (Meriones unguiculatus) as influenced by coat color of males
BEHAVIORAL AND NEURAL BIOLOGY
54, 184-190 (1990)
BRIEF REPORT Social Preference of Female Gerbils (Meriones unguiculatus) as Influenced by Coat Color of Males RODERICK WONG, PENELOPE GRAY-ALLAN, CATHERINE CHIBA, AND BRAXTON ALFRED *'1
Department of Psychology, and *Department of Anthropology, University of British Columbia, British Columbia, Canada V6T 1 Y7 Female gerbils (Meriones unguiculatus) of three distinct coat colors (agouti, black, and sandy or pink-eyed dilution) were tested in a Y-maze whose arms led to compartments containing unfamiliar male gerbils of varying coat colors. The stimulus animals were separated from the females by a Plexiglas door. The trials lasted for 2 min and each female was exposed to the following four combinations: two males of the same coloring as the female; one male of the same color and another of a different color from the female; both males of different color from the female. The number of crossings to the left and right arms was relayed by photocells to an IBM PC computer. The results indicate that agouti females preferred visiting the arm occupied by agouti males while those of the other coat colors showed no preference for the "wild-type" males. Instead, sandy and black female gerbils preferred to be in proximity with those of nonwild types. © 1990 Academic Press, Inc.
When considering the bases of mate choice among females in a variety of species, there is substantial evidence for the influence of factors such as the quality of the territory held by males, the vigor and persistence of male courtship behavior, and phenotypic characteristics of the males (Bateson, 1983). An excellent example of the latter can be found in snow geese which occur in two color morphs, white and dark grey ("blue"). Because more blue than white males were observed among mixed pairs, Cooke and Davies (1982) argued that females appear to be using plumage color as a criterion of mate choice. In rodents the influence of phenotype on mate choice has been illustrated by research on the effects of olfactory i Reprints should be addressed to R. Wong. This research was supported by a NSERC of Canada grant to R.W. We thank Nancy Burley for her suggestion that artificial selection might affect female preference. This paper is dedicated to the memory o f Rob Gray who collected the data> 184
0163-1047/90 $3.00 Copyright © 1990 by Academic Press, Inc. All fights of reproduction in any form reserved.
GERBIL SOCIAL PREFERENCE
185
cues on mate preference. Recently, Beauchamp, Yamazaki, and Boyse (1985) reported that mice (Mus musculus) differing only in the MHC complex would preferentially mate with others of a dissimilar H-2 type. Mice can distinguish between H-2 types and this results in a mating bias. Although the influence of olfactory cues on mate choice in rodents has been established, it has not been established whether other phenotypic characteristics such as coat color play a role. For this reason we studied the effects of the latter variable on social preference of gerbils. The present experiment involved Mongolian gerbils (Meriones unguiculatus) that exhibit three distinct coat colors. The most common color among wild and domesticated gerbils is agouti (wild-type). Its dorsal hairs are dark with a yellow band, and the belly is light in color. The second coat color is black or non-agouti (first reported in gerbils by Waring and Poole (1980)). These animals' hairs are all black with small patches of white hairs on the wrists and chin, and the color is manifested when the agouti gene is homozygous recessive. The third coat color is sandy or pink-eyed dilution (Henley & Robinson, 1981). These gerbils' hairs have a gray base (lighter than in agouti), a yellow center band, and no coloring in the tips. The belly color is similar to that of the agoutis and their eyes are pink rather than black as seen in agoutis and blacks. The sandy color is the result of recessive homozygosity at the pink-eyed dilution locus (Silvers, 1979). Using these phenotypes we studied the possible effects of coat color on the social preferences of female gerbils. The females were placed in a Y-maze whose arms led to compartments containing unfamiliar males with different coat colors. The stimulus animals were separated from the females by a Plexiglas door. Because there have not been any reported observations of the black and sandy phenotypes in the field, we hypothesized that coat color may be related to success in social competition and that there may be a strong selection for the agouti wild-type coat color or selection against any variants. On that basis we predicted that there would be a tendency for females to be more responsive to agouti males and thus "visit" them more often than males of another coat color. There are five possible outcomes when females are tested under the conditions of the present experiment. (1) They could show a preference for the side of the maze occupied by a male of the same coat color (familiar) or (2) they could show a preference for a male of a different phenotype (novel). (3) They may prefer to visit the side housing the agouti male as hypothesized or (4) not show any preference and thus respond at a chance level in the choice test situation. (5) It is also possible that they may show a position preference in the Y-maze independent of the type of males in the end-arms. A total of 30 female and 60 male gerbils (M. unguiculatus) were used with 10 females and 20 males of each of the three coat colors (agouti,
186
WONG ET AL.
sandy, or black). The sandy and black animals were bred for recessive coloring over a 2-year period. All females came from litters where all members were of the same color, and none of the males were littermates of the females to whom they were presented. All gerbils were weaned at 25 days and were sexually naive when tested at age 70 days. All were housed individually in 12 x 17 x 27-cm plastic cages (Carworth) and maintained on a 12-h light-dark cycle. The animals were tested in a clear Plexiglas Y-maze which had alleyways 15 cm wide and 15 cm deep. The stem of the maze was 30 cm long and each arm was 60 cm long. In order to confine the males to the ends of their respective arms, a clear Plexiglas partition was inserted into slots located 15 cm from the ends of each arm. Olfactory cues were sealed off in the end arms by a plastic sealant that was sprayed along the edges of the partition and the top of the maze was enclosed. Photocells were used to record each female's m o v e m e n t in the maze. Three pairs of photocells were placed 2, 19, and 36 cm from the partition of each arm. Crossings were relayed to an IBM PC computer which calculated the number of visits and time spent in each arm. During any given trial the female gerbil was placed in the start box of the maze and the test did not begin until after she had adapted to the conditions; i.e., at the beginning, many females experienced seizures just after placement in the start arm of the maze. By Day 3, 20% (6 of 30) of the females showed seizures and after that period only 1 out of 30 continued to do so. The trials lasted for 2 min and each female was exposed once to each of the following four combinations: two males of the same coloring as the female (one trial); one male of the same color and another of a different color from that of the female (two trials); both males of different color from the female (one trial). The left-right positions of the stimulus animals are indicated in Table 1. The estrous of the female was not assessed because vaginal smears of gerbils disturb their cycle and thus could affect the results. After each trial the floor of the maze was wiped with a mixture of vinegar and water to remove odor cues.
Comparisons were made of the number of times that females crossed the photocells positioned 2 cm in front of the ends of right and left arms of the maze in which the males were confined. The number of crossings made by females to the end arms housing the males varying in coat color was analyzed b y computing the ratio of the number of times the female chose the left arm to the number of times she chose the right arm. This ratio was then transformed to its natural logarithm in order to linearize the expression. The statistical significance of the transformed value was determined by a t test (two-tailed) for departure from zero, this value indicating no preference (Alfred, 1987). A negative t value that reached significance indicated a preference for the male in the right arm. Similar
* p < .05.
Black (Blk)
Sandy (San)
Agouti (Agt)
Female
Blk Agt Agt San
San Agt Agt Blk
Agt San Agt Agt
(18.2 (11.6 (11.5 (11.5
(29.7 (14.7 (15.0 (12.6
(18.2 (17.7 (20.9 (16.4
2.2) 2.3) 2.2) 1.5)
± ± ± ±
2.6) 1.9) 2.2) 3.3)
± 3.0) ± 1.5) ±_ 3.5) ± 2.0)
± ± ± ±
Male (L)
(2.80 (2.20 (2.32 (2.07
(3.33 (2.65 (2.48 (2.40
(2.83 (2.80 (2.97 (2.75
± ± ± ±
± ± --_ ±
± ± ± ±
.15) .30) .15) .31)
.13) .09) .23) .18)
.13) .13) .13) .10)
Log value
Blk San Blk Blk
San Blk San San
Agt Blk San Blk
(14.8 (18.2 (15.9 (17.8
(26.7 (21.1 (16.0 (19.7
(14.9 (22.7 (14.0 (11.1
± ± ± ±
± ± ± ±
± _+ ± ±
Male (R)
1.7) 2.4) 2.0) 2.4)
3.1) 1.8) 1.1) 1.6)
1.7) 2.2) 1.6) 1.3)
(2.63 (2.81 (2.70 (2.78
(3.21 (3.02 (2.75 (2.95
(2.63 (3.05 (2.59 (2.34
± ± ± ±
± --_ ± ±
± ± ± ±
.11) .15) .11) .16)
.13) .08) .08) .09)
.14) .13) .11) .13)
Log value
TABLE 1 Mean Number and Standard Error of Crossings by Female Gerbils to Region of Y-Maze Occupied by Male Gerbils
+1.29 -2.57* -1.81 -2.40*
+0.96 -2.83* -1.51 -2.75*
1.65 -1.90 3.26* 2.24
t value
Z
©
F
7z
188
WONG ET AL.
analyses of the data on time spent in the sectors adjacent to the end arms did not reveal any significant effects (p > .05). The results on number of crossings are summarized in Table 1. The above results indicate that agouti females show a tendency to visit the side occupied by an agouti male. There was a significant difference (p < .01) when they were given a choice between agouti and sandy males and the result approached significance (p < .053) when they were tested with agouti and black males. There were no left-right position arm preferences when the females were tested with agouti males in both arms. The analysis of the data on sandy females indicated their tendency to prefer a black male when they were given a choice between it and an agouti male (p < .05) but did not show a preference when given a choice between an agouti or a sandy male (p > .05). However, when given a choice between a black or a sandy male, they made more visits to the side occupied by a sandy male (p < .05). No position preferences were evident when the sandy females were tested with sandy males in both arms (p > .05). The black females showed a preference for the side housing the sandy male over that occupied by the agouti male (p < .05) but no preference when given a choice between either a black or an agouti male (p > .05). When given a choice between a black or a sandy male, the black females preferred the side occupied by the black male (p < .05). No position biases were revealed when these females were tested with black males in both arms (p > .05). Different patterns of preference seem to be reflected in the Y-maze behavior of the female gerbils as a function of their coat color. Agouti females definitely preferred agouti over sandy males and tended to prefer agouti males over black males. This finding is consistent with the prediction that females should show a preference for wild-type males. The natural environment of Mongolian gerbils is sandy, semi-arid terrain (Thiessen & Yahr, 1977) for which the agouti coloring provides camouflage against predation. Consequently, if coat color is a factor for survival, there is a basis for the preference of wild-type females for agouti males. The fact that black and sandy gerbils did not show such a preference may explain why such animals are not seen in the wild, since their offspring would be less likely to evade predation. The agouti females' choice might also be interpreted as evidence of phenotype-matching, an effect where females direct preferential behavior toward those conspecifics that most closely resemble their littermates (Holmes & Sherman, 1982), i.e., ones that are familiar. Males with sandy and black coat colors were novel to the agouti females, who had only agouti littermates. Black and sandy females, when given a choice between two novel males, chose the male exhibiting the other recessive coat
GERBIL SOCIAL PREFERENCE
189
color rather than males with the dominant agouti pelage. When given a choice between males carrying the familiar recessive coat and a novel recessive one, they chose the familiar, but showed no preference when given a choice between males of familiar and agouti color. It is interesting that both sandy and black females showed the same patterns of social preference and neither seemed to prefer males with the dominant agouti coloring. However, an explanation for this pattern is not clear. It could be the result of artificial selection produced when breeding for the recessive coat colors. Sandy and black females who were reluctant to mate when "forced pair" with males of the same coat color may have been selected against. However, we found that all females would eventually mate with any male if he was the only one available when they were in estrus. The only criterion for choosing females for mating pairs was coat color, and willingness to mate was not a factor. Bateson (1978) noted that male Japanese quails (Coturnix colturnbc japonica) preferred mates that were slightly different in color from their littermates. He did, however, find limits to their preference for novelty and stated that these quails preferred familiar and slightly novel colored females to those with grossly unfamiliar plumage. While our agouti female gerbils always showed preference for familiar males, the sandy and black females did not. Perhaps like Bateson's quails, they preferred slightly novel coat colors. However, this hypothesis of optimal discrepancy implies that black and sandy gerbils look more like each other than they do agouti. This implication is based on the premise that in terms of color, agouti is intermediate to sandy (light) and black (dark). However, from the phenomenology of the animals, the novel recessive coat color may be viewed as being less novel than the agouti. Alternatively, these females may not be choosing males on the basis of coat color, but on some other factor which we have not identified. REFERENCES Alfred, B. (1987). Elements of statistics for the life and social sciences. New York: Springer Verlag. Bateson, P. P. G. (1978). Sexual imprinting and optimal outbreeding. Nature (London), 273, 659-660. Bateson, P. P. G. (1983). Mate Choice. Cambridge, England: Cambridge Univ. Press. Beanchamp, G. K., Yamazaki, K., & Boyse, E. A. (1985). The chemosensory recognition of genetic individuality. Scientific American, 253(1), 86-92. Cooke, F., & Davies, J. C. (1982). Assortative mating, mate choice and reproductive fitness in Snow Geese. In P. Bateson (Ed.), Mate choice. Cambridge, England: Cambridge Univ, Press. Henley, M., & Robinson, R. (1981). Non-agouti and pink-eyed dilution in the Mongolian gerbil. Journal of Heredity, 72, 60--61. Holmes, W. G., & Sherman, P. W. (1982). The ontogeny of kin recognition in two species of ground squirrels. American Zoologist, 22, 491-517.
190
WONG ET AL.
Silvers, W. K. (197%/. The coat colors o f mice: A model for mammalian gene action and interaction. New York: Springer-Verlag. Thiessen, D. D., & Yahr, P. (1977). The gerbil in behavioral investigation. Austin: Univ. of Texas Press. Waring, A. D., & Poole, T. W. (1980). Genetic analysis of the black pigment mutation in the Mongolian gerbil. Journal o f Heredity, 71, 428-429.
54, 184-190 (1990)
BRIEF REPORT Social Preference of Female Gerbils (Meriones unguiculatus) as Influenced by Coat Color of Males RODERICK WONG, PENELOPE GRAY-ALLAN, CATHERINE CHIBA, AND BRAXTON ALFRED *'1
Department of Psychology, and *Department of Anthropology, University of British Columbia, British Columbia, Canada V6T 1 Y7 Female gerbils (Meriones unguiculatus) of three distinct coat colors (agouti, black, and sandy or pink-eyed dilution) were tested in a Y-maze whose arms led to compartments containing unfamiliar male gerbils of varying coat colors. The stimulus animals were separated from the females by a Plexiglas door. The trials lasted for 2 min and each female was exposed to the following four combinations: two males of the same coloring as the female; one male of the same color and another of a different color from the female; both males of different color from the female. The number of crossings to the left and right arms was relayed by photocells to an IBM PC computer. The results indicate that agouti females preferred visiting the arm occupied by agouti males while those of the other coat colors showed no preference for the "wild-type" males. Instead, sandy and black female gerbils preferred to be in proximity with those of nonwild types. © 1990 Academic Press, Inc.
When considering the bases of mate choice among females in a variety of species, there is substantial evidence for the influence of factors such as the quality of the territory held by males, the vigor and persistence of male courtship behavior, and phenotypic characteristics of the males (Bateson, 1983). An excellent example of the latter can be found in snow geese which occur in two color morphs, white and dark grey ("blue"). Because more blue than white males were observed among mixed pairs, Cooke and Davies (1982) argued that females appear to be using plumage color as a criterion of mate choice. In rodents the influence of phenotype on mate choice has been illustrated by research on the effects of olfactory i Reprints should be addressed to R. Wong. This research was supported by a NSERC of Canada grant to R.W. We thank Nancy Burley for her suggestion that artificial selection might affect female preference. This paper is dedicated to the memory o f Rob Gray who collected the data> 184
0163-1047/90 $3.00 Copyright © 1990 by Academic Press, Inc. All fights of reproduction in any form reserved.
GERBIL SOCIAL PREFERENCE
185
cues on mate preference. Recently, Beauchamp, Yamazaki, and Boyse (1985) reported that mice (Mus musculus) differing only in the MHC complex would preferentially mate with others of a dissimilar H-2 type. Mice can distinguish between H-2 types and this results in a mating bias. Although the influence of olfactory cues on mate choice in rodents has been established, it has not been established whether other phenotypic characteristics such as coat color play a role. For this reason we studied the effects of the latter variable on social preference of gerbils. The present experiment involved Mongolian gerbils (Meriones unguiculatus) that exhibit three distinct coat colors. The most common color among wild and domesticated gerbils is agouti (wild-type). Its dorsal hairs are dark with a yellow band, and the belly is light in color. The second coat color is black or non-agouti (first reported in gerbils by Waring and Poole (1980)). These animals' hairs are all black with small patches of white hairs on the wrists and chin, and the color is manifested when the agouti gene is homozygous recessive. The third coat color is sandy or pink-eyed dilution (Henley & Robinson, 1981). These gerbils' hairs have a gray base (lighter than in agouti), a yellow center band, and no coloring in the tips. The belly color is similar to that of the agoutis and their eyes are pink rather than black as seen in agoutis and blacks. The sandy color is the result of recessive homozygosity at the pink-eyed dilution locus (Silvers, 1979). Using these phenotypes we studied the possible effects of coat color on the social preferences of female gerbils. The females were placed in a Y-maze whose arms led to compartments containing unfamiliar males with different coat colors. The stimulus animals were separated from the females by a Plexiglas door. Because there have not been any reported observations of the black and sandy phenotypes in the field, we hypothesized that coat color may be related to success in social competition and that there may be a strong selection for the agouti wild-type coat color or selection against any variants. On that basis we predicted that there would be a tendency for females to be more responsive to agouti males and thus "visit" them more often than males of another coat color. There are five possible outcomes when females are tested under the conditions of the present experiment. (1) They could show a preference for the side of the maze occupied by a male of the same coat color (familiar) or (2) they could show a preference for a male of a different phenotype (novel). (3) They may prefer to visit the side housing the agouti male as hypothesized or (4) not show any preference and thus respond at a chance level in the choice test situation. (5) It is also possible that they may show a position preference in the Y-maze independent of the type of males in the end-arms. A total of 30 female and 60 male gerbils (M. unguiculatus) were used with 10 females and 20 males of each of the three coat colors (agouti,
186
WONG ET AL.
sandy, or black). The sandy and black animals were bred for recessive coloring over a 2-year period. All females came from litters where all members were of the same color, and none of the males were littermates of the females to whom they were presented. All gerbils were weaned at 25 days and were sexually naive when tested at age 70 days. All were housed individually in 12 x 17 x 27-cm plastic cages (Carworth) and maintained on a 12-h light-dark cycle. The animals were tested in a clear Plexiglas Y-maze which had alleyways 15 cm wide and 15 cm deep. The stem of the maze was 30 cm long and each arm was 60 cm long. In order to confine the males to the ends of their respective arms, a clear Plexiglas partition was inserted into slots located 15 cm from the ends of each arm. Olfactory cues were sealed off in the end arms by a plastic sealant that was sprayed along the edges of the partition and the top of the maze was enclosed. Photocells were used to record each female's m o v e m e n t in the maze. Three pairs of photocells were placed 2, 19, and 36 cm from the partition of each arm. Crossings were relayed to an IBM PC computer which calculated the number of visits and time spent in each arm. During any given trial the female gerbil was placed in the start box of the maze and the test did not begin until after she had adapted to the conditions; i.e., at the beginning, many females experienced seizures just after placement in the start arm of the maze. By Day 3, 20% (6 of 30) of the females showed seizures and after that period only 1 out of 30 continued to do so. The trials lasted for 2 min and each female was exposed once to each of the following four combinations: two males of the same coloring as the female (one trial); one male of the same color and another of a different color from that of the female (two trials); both males of different color from the female (one trial). The left-right positions of the stimulus animals are indicated in Table 1. The estrous of the female was not assessed because vaginal smears of gerbils disturb their cycle and thus could affect the results. After each trial the floor of the maze was wiped with a mixture of vinegar and water to remove odor cues.
Comparisons were made of the number of times that females crossed the photocells positioned 2 cm in front of the ends of right and left arms of the maze in which the males were confined. The number of crossings made by females to the end arms housing the males varying in coat color was analyzed b y computing the ratio of the number of times the female chose the left arm to the number of times she chose the right arm. This ratio was then transformed to its natural logarithm in order to linearize the expression. The statistical significance of the transformed value was determined by a t test (two-tailed) for departure from zero, this value indicating no preference (Alfred, 1987). A negative t value that reached significance indicated a preference for the male in the right arm. Similar
* p < .05.
Black (Blk)
Sandy (San)
Agouti (Agt)
Female
Blk Agt Agt San
San Agt Agt Blk
Agt San Agt Agt
(18.2 (11.6 (11.5 (11.5
(29.7 (14.7 (15.0 (12.6
(18.2 (17.7 (20.9 (16.4
2.2) 2.3) 2.2) 1.5)
± ± ± ±
2.6) 1.9) 2.2) 3.3)
± 3.0) ± 1.5) ±_ 3.5) ± 2.0)
± ± ± ±
Male (L)
(2.80 (2.20 (2.32 (2.07
(3.33 (2.65 (2.48 (2.40
(2.83 (2.80 (2.97 (2.75
± ± ± ±
± ± --_ ±
± ± ± ±
.15) .30) .15) .31)
.13) .09) .23) .18)
.13) .13) .13) .10)
Log value
Blk San Blk Blk
San Blk San San
Agt Blk San Blk
(14.8 (18.2 (15.9 (17.8
(26.7 (21.1 (16.0 (19.7
(14.9 (22.7 (14.0 (11.1
± ± ± ±
± ± ± ±
± _+ ± ±
Male (R)
1.7) 2.4) 2.0) 2.4)
3.1) 1.8) 1.1) 1.6)
1.7) 2.2) 1.6) 1.3)
(2.63 (2.81 (2.70 (2.78
(3.21 (3.02 (2.75 (2.95
(2.63 (3.05 (2.59 (2.34
± ± ± ±
± --_ ± ±
± ± ± ±
.11) .15) .11) .16)
.13) .08) .08) .09)
.14) .13) .11) .13)
Log value
TABLE 1 Mean Number and Standard Error of Crossings by Female Gerbils to Region of Y-Maze Occupied by Male Gerbils
+1.29 -2.57* -1.81 -2.40*
+0.96 -2.83* -1.51 -2.75*
1.65 -1.90 3.26* 2.24
t value
Z
©
F
7z
188
WONG ET AL.
analyses of the data on time spent in the sectors adjacent to the end arms did not reveal any significant effects (p > .05). The results on number of crossings are summarized in Table 1. The above results indicate that agouti females show a tendency to visit the side occupied by an agouti male. There was a significant difference (p < .01) when they were given a choice between agouti and sandy males and the result approached significance (p < .053) when they were tested with agouti and black males. There were no left-right position arm preferences when the females were tested with agouti males in both arms. The analysis of the data on sandy females indicated their tendency to prefer a black male when they were given a choice between it and an agouti male (p < .05) but did not show a preference when given a choice between an agouti or a sandy male (p > .05). However, when given a choice between a black or a sandy male, they made more visits to the side occupied by a sandy male (p < .05). No position preferences were evident when the sandy females were tested with sandy males in both arms (p > .05). The black females showed a preference for the side housing the sandy male over that occupied by the agouti male (p < .05) but no preference when given a choice between either a black or an agouti male (p > .05). When given a choice between a black or a sandy male, the black females preferred the side occupied by the black male (p < .05). No position biases were revealed when these females were tested with black males in both arms (p > .05). Different patterns of preference seem to be reflected in the Y-maze behavior of the female gerbils as a function of their coat color. Agouti females definitely preferred agouti over sandy males and tended to prefer agouti males over black males. This finding is consistent with the prediction that females should show a preference for wild-type males. The natural environment of Mongolian gerbils is sandy, semi-arid terrain (Thiessen & Yahr, 1977) for which the agouti coloring provides camouflage against predation. Consequently, if coat color is a factor for survival, there is a basis for the preference of wild-type females for agouti males. The fact that black and sandy gerbils did not show such a preference may explain why such animals are not seen in the wild, since their offspring would be less likely to evade predation. The agouti females' choice might also be interpreted as evidence of phenotype-matching, an effect where females direct preferential behavior toward those conspecifics that most closely resemble their littermates (Holmes & Sherman, 1982), i.e., ones that are familiar. Males with sandy and black coat colors were novel to the agouti females, who had only agouti littermates. Black and sandy females, when given a choice between two novel males, chose the male exhibiting the other recessive coat
GERBIL SOCIAL PREFERENCE
189
color rather than males with the dominant agouti pelage. When given a choice between males carrying the familiar recessive coat and a novel recessive one, they chose the familiar, but showed no preference when given a choice between males of familiar and agouti color. It is interesting that both sandy and black females showed the same patterns of social preference and neither seemed to prefer males with the dominant agouti coloring. However, an explanation for this pattern is not clear. It could be the result of artificial selection produced when breeding for the recessive coat colors. Sandy and black females who were reluctant to mate when "forced pair" with males of the same coat color may have been selected against. However, we found that all females would eventually mate with any male if he was the only one available when they were in estrus. The only criterion for choosing females for mating pairs was coat color, and willingness to mate was not a factor. Bateson (1978) noted that male Japanese quails (Coturnix colturnbc japonica) preferred mates that were slightly different in color from their littermates. He did, however, find limits to their preference for novelty and stated that these quails preferred familiar and slightly novel colored females to those with grossly unfamiliar plumage. While our agouti female gerbils always showed preference for familiar males, the sandy and black females did not. Perhaps like Bateson's quails, they preferred slightly novel coat colors. However, this hypothesis of optimal discrepancy implies that black and sandy gerbils look more like each other than they do agouti. This implication is based on the premise that in terms of color, agouti is intermediate to sandy (light) and black (dark). However, from the phenomenology of the animals, the novel recessive coat color may be viewed as being less novel than the agouti. Alternatively, these females may not be choosing males on the basis of coat color, but on some other factor which we have not identified. REFERENCES Alfred, B. (1987). Elements of statistics for the life and social sciences. New York: Springer Verlag. Bateson, P. P. G. (1978). Sexual imprinting and optimal outbreeding. Nature (London), 273, 659-660. Bateson, P. P. G. (1983). Mate Choice. Cambridge, England: Cambridge Univ. Press. Beanchamp, G. K., Yamazaki, K., & Boyse, E. A. (1985). The chemosensory recognition of genetic individuality. Scientific American, 253(1), 86-92. Cooke, F., & Davies, J. C. (1982). Assortative mating, mate choice and reproductive fitness in Snow Geese. In P. Bateson (Ed.), Mate choice. Cambridge, England: Cambridge Univ, Press. Henley, M., & Robinson, R. (1981). Non-agouti and pink-eyed dilution in the Mongolian gerbil. Journal of Heredity, 72, 60--61. Holmes, W. G., & Sherman, P. W. (1982). The ontogeny of kin recognition in two species of ground squirrels. American Zoologist, 22, 491-517.
190
WONG ET AL.
Silvers, W. K. (197%/. The coat colors o f mice: A model for mammalian gene action and interaction. New York: Springer-Verlag. Thiessen, D. D., & Yahr, P. (1977). The gerbil in behavioral investigation. Austin: Univ. of Texas Press. Waring, A. D., & Poole, T. W. (1980). Genetic analysis of the black pigment mutation in the Mongolian gerbil. Journal o f Heredity, 71, 428-429.