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Mating preferences of Saint Croix rams to related or unrelated ewes
Small Ruminant Research 83 (2009) 82–84
Contents lists available at ScienceDirect
Small Ruminant Research journal homepage: www.elsevier.com/locate/smallrumres
Short communication
Mating preferences of Saint Croix rams to related or unrelated ewes A. Orihuela ∗ , R. Vázquez Facultad de Ciencias Agropecuarias de la Universidad Autónoma del Estado de Morelos, Avenida Universidad 1001, Colonia Chamilpa, Cuernavaca, Morelos 62210, Mexico
a r t i c l e
i n f o
Article history: Received 12 September 2008 Received in revised form 10 March 2009 Accepted 18 March 2009 Available online 19 April 2009 Keywords: Mating preferences Inbreeding Sheep Sexual behaviour
a b s t r a c t Fifty mating tests were performed comparing the mating preferences of Saint Croix rams to unrelated ewes or their daughters. In each test, two ewes were induced into oestrus and restrained by the use of portable metal stanchions holding the head of the ewes, at equal distances from the point of entrance of 10 rams. The rams were individually tested with an unrelated ewe and his daughters as mating options in each test. No difference in mating preference was found in the choices that rams made regarding firstly the approach to the ewe, the number of approaches to each ewe, and the ewes mated. It was concluded that in Saint Croix rams, genetic relatedness did not influence the choice of mating partner. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The choice of a mating partner is an integral part of sexual selection in species like sheep, where individual males mate with several females. Rams select ewes, and ewes’ attractivity has been shown to be consistent. Different rams also select oestrous ewes in a similar order (Tilbrook, 1987). However, the fact whether the choice of mate occurs independently of genetic relatedness among mating pairs has received little attention, although inbreeding may have fitness consequences (Holand et al., 2007). Avoidance of mating between primary kin has been extensively reported across many animal species (Kokko and Ots, 2006), and the adaptive value of such behaviour is explained through inbreeding. The negative effects of inbreeding are well documented: the survival of inbred offspring is reported to be reduced in many species and inbred progeny are often less attractive as potential mates (Frommen and Bakker, 2006). Thus, individuals should avoid mating with close kin. However, experimental evidence regarding inbreeding avoidance in sheep is rare.
∗ Corresponding author. Tel.: +52 777 3177046. E-mail address: [email protected] (A. Orihuela). 0921-4488/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2009.03.005
The aim of this study was to determine if rams display mating preferences to unrelated or related ewes under controlled conditions. 2. Materials and methods The study was conducted at the University of Morelos, Mexico, located at 18◦ 37 N and 99◦ 19 W (899 m above sea level, average annual rainfall of 800 mm and temperature of 23 ◦ C). Ten Saint Croix rams (3 years of age) and 100 intact 18-month-old ewes were used in 50 mating tests, comparing the mate preferences of rams to unrelated or related ewes as mating partners. Five tests were conducted every other day for each ram. In each test, the males were exposed to two ewes restrained by the use of a portable stanchion at equal distances from the point of entrance of the ram. Mating tests were designed to last 5 min in duration, unless ejaculation occurred (whichever occurred first). In each test, one ewe was the daughter of the ram under evaluation, while also paired with an unrelated ewe—similar in weight, height and age. Different ewes were used in each test. Since born, ewes had been kept in a unisexual group 100 m apart of the males until the day of testing. During the experiment only oestrous induced ewes were used. Ewes were induced into oestrus in preparation for the mating tests with the aid of intravaginal sponges containing 30 mg FGA (Chronogest, Intervet, Mexico). The sponges were inserted for 12 days, followed by an intramuscular injection of 500–600 IU PMSG (Foligon, Intervet, Mexico) at sponge withdrawal. During each test the order of testing and the position of the related and unrelated ewes were reversed. The first approach to the ewe, the number of approaches toward each ewe, and the ewe mated by the ram (ejaculation) were registered.
A. Orihuela, R. Vázquez / Small Ruminant Research 83 (2009) 82–84 Data were analyzed determining whether the proportion of choices deviated significantly from a 50:50 ratio within each variable (Altman, 1997).
3. Results and discussion All rams ejaculated within the 5 min period, approaching unrelated ewes in 60% of the tests, displaying 55% of their approaches toward these same ewes and mating (ejaculating) 45% of the times. These findings were not statistically different (P > 0.05) from the 50% probability of success (or failure) expected at random. It has been recognized that rams select certain characteristics in their mating choices. Tilbrook (1987), established that ewes differ in their sexual attractiveness to rams, and that this attractiveness is constant during successive oestrous periods, depending at least in part, on wool and the breed as such. Woolly ewes are preferred to shorn sheep (Tilbrook and Cameron, 1989) and rams also exhibit a preference to their own breed of ewe and heavier ewes when offered a choice (Owens and Thompson, 1994; Preston et al., 2005). In addition, rams prefer ewes that have not been recently mounted (Houpt, 1998). Tilbrook (1987), also established that attractiveness is not related to level of oestrogen used to induce oestrus or the amount of soliciting behaviour. The results of the present study show that in Saint Croix rams, genetic relatedness did not influence the actual choice of a mating partner. This disagrees with the findings in other species, where males, when given the choice between courting a familiar or unfamiliar non-sib prefer to mate with the non-sib and thus avoid the disadvantages of inbreeding (Frommen and Bakker, 2006). Individuals in some species prefer partners carrying dissimilar genes at the major histocompatibility complex (MHC), which may help to increase the MHC or overall heterozygosity of progeny (Penn, 2002). Many studies in mice and rats indicate that the MHC genes influence an individual’s odour, playing a role in kin recognition (Olsén et al., 1998; Eggert et al., 1999; Yamazaki et al., 2000). In addition, research is examining the potential role of MHC-linked olfactory receptor genes in mating preferences (Penn, 2002). These MHC-disassortative mating preferences may help to avoid kin mating (Potts and Wakeland, 1993; Brown and Eklund, 1994; Penn and Potts, 1999). However, Paterson and Pemberton (1997) found no evidence of MHC-dependent mating preferences in feral sheep, suggesting that perhaps rams are not able to recognize their daughters, or do not use this strategy to avoid inbreeding. In other species different factors have been observed that deserve attention in sheep. So for example: inbreeding in some bird species is reduced by post-copulatory mechanisms that favour fertilization by sperm from unrelated males (Zeh and Zeh, 2006). The prevalence of this mechanism in captive ungulates such as sheep is not known. Other polygynous ungulates avoid inbreeding by dispersing from their birth site to other geographical areas occupied by genetically unrelated conspecifics (Holand et al., 2007). It could also be possible that due to husbandry practices that involve the isolation of ewe lambs from the
83
rams, until the breeding season when ewes are mature, rams are unable to recognize any familiarity in the ewes. In other words, males could avoid mating with females they have grown up with, which suggest that familiarity early in life could be more important than genetic relatedness with respect to avoiding inbreeding. So for example, non-reproductive females in families of eusocial common mole-rats (Cryptomys sp.) are not suppressed by their mother (either behaviourally or pheromonally) as is generally assumed. They do not mate with their father and kin, simply as they are not sexually attractive to them (and vice versa). Incest avoidance is based on the capability to recognize (and remember for up to 3 weeks) each family member individually (Burda, 1995). However, no conclusive data are available in sheep. Disruption of the natural herds (groups) and suppressing the female partner choice, as was the case in the present experiment, could be masking adaptive responses like those found in other species to avoid inbreeding in social mammals. Rams do select ewes, but ewes are also selective (Estep et al., 1989). In fact, in most polygynous mammalian species, the female choice (for mating) is very common if not the rule. In the present experiment, female choice was not possible. Höner et al. (2007) found that females preferred sires that were born into or immigrated into the female group after the female was born. The extent of male dispersal as a response to such female mate preferences depends on the demographic structure of breeding groups, rather than the genetic relatedness between females and males. The lack of choice between related and unrelated ewes suggests that inbreeding problems could surge in sheep herds, since inbreeding avoidance mechanisms may have been selected out during domestication—or there are other mechanisms involved. Inbreeding has been identified as a major factor in the emergence of some genetic disorders of sheep (Jolly et al., 2004). Birth weights, average daily gain from birth until 2 months of age and litter size are depressed due to inbreeding (Norberg and Sorensen, 2007). 4. Conclusion It was concluded that in Saint Croix rams, genetic relatedness did not appear to influence the actual choice of mating partner. References Altman, D.G., 1997. Practical Statistics for Medical Research. Chapman and Hall, London, pp. 232–239. Brown, J.L., Eklund, A., 1994. Kin recognition and the major histocompatibility complex: an integrative review. Am. Nat. 143, 435–461. Burda, H., 1995. Individual recognition and incest avoidance in eusocial common mole-rats rather than reproductive suppression by parents. Experientia 51, 411–413. Eggert, F., Müller-Ruchholtz, W., Ferstl, R., 1999. Olfactory cues associated with the major histocompatibility complex. Genetica 104, 191–197. Estep, D.Q., Price, E.O., Wallach, S.J.R., Dally, M.R., 1989. Social preferences of domestic ewes for rams. Appl. Anim. Behav. Sci. 24, 287–300. Frommen, J.G., Bakker, T.C., 2006. Inbreeding avoidance through nonrandom mating in sticklebacks. Biol. Lett. 2, 232–235.
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A. Orihuela, R. Vázquez / Small Ruminant Research 83 (2009) 82–84
Holand, O., Askim, K.R., Roed, K.H., Wladji, R.B., Gjostein, H., Nieminen, M., 2007. No evidence of inbreeding avoidance in a polygynous ungulate: the reindeer (Rangifer tarandus). Biol. Lett. 3, 36– 39. Houpt, K.A., 1998. Domestic Animal Behavior for Veterinarians and Animal Scientists. Iowa State University Press, Iowa, pp. 133–139. Höner, O.P., Wachter, B., East, M.L., Streich, W.J., Wilhelm, K., Burke, T., Hofer, H., 2007. Female mate-choice drives the evolution of malebiased dispersal in social mammal. Nature 16, 798–801. Jolly, R.D., Blair, H.T., Johnstone, A.C., 2004. Genetic disorders of sheep in New Zealand: a review and perspective. New Zeal. Vet. J. 52, 52–64. Kokko, H., Ots, I., 2006. When not to avoid inbreeding. Int. J. Org. Evol. 60, 467–475. Norberg, E., Sorensen, A.C., 2007. Inbreeding trend and inbreeding depression in the Danish populations of Texel, Shropshire, and Oxford Down. J. Anim. Sci. 85, 299–304. Olsén, K.H., Grahm, M., Lohn, J., Langefors, A., 1998. MHC and kin discrimination in juvenile Arctic charr Salvelinus alpinus (L.). Anim. Behav. 56, 319–327. Owens, I.P.F., Thompson, D.B.A., 1994. Sex differences, sex rations and sex roles. Proc. Roy. Soc. B 258, 93–99.
Paterson, S., Pemberton, J.M., 1997. No evidence for major histocompatibility complex-dependent mating patterns in a free-living ruminant population. Proc. Roy. Soc. B 264, 1813–1819. Penn, D.J., 2002. The scent of genetic compatibility: sexual selection and the major histocompatibility complex. Ethology 108, 1–21. Penn, D., Potts, W.K., 1999. The evolution of mating preferences and major histocompatibility genes. Am. Nat. 153, 145–164. Potts, W.K., Wakeland, E.K., 1993. Evolution of MHC genetic diversity: a tale of incest, pestilence and sexual preference. Trends Genet. 9, 408–412. Preston, B.T., Stevenson, I.R., Pemberton, J.M., Coltman, D.W., Wilson, K., 2005. Male mate choice influences female promiscuity in Soay sheep. Proc. Roy. Soc. B 272, 365–373. Tilbrook, A.J., 1987. The influence of factors associated with oestrus on the sexual “attractiveness” of ewes to rams. Appl. Anim. Behav. Sci. 1–2, 117–128. Tilbrook, A.J., Cameron, A.W.N., 1989. Ram mating preferences for woolly rather than shorn ewes. Appl. Anim. Behav. Sci. 24, 301–312. Yamazaki, K., Beauchamp, G.K., Curran, M., Bard, J., Boyse, E.A., 2000. Parent-progeny recognition as a function of MHC odortype identity. Proc. Nat. Acad. Sci. U.S.A. 97, 10500–10502. Zeh, J.A., Zeh, D.W., 2006. Outbred embryos rescue inbred half-siblings in mixed-paternity broods of live-bearing females. Nature 439, 201–203.
Contents lists available at ScienceDirect
Small Ruminant Research journal homepage: www.elsevier.com/locate/smallrumres
Short communication
Mating preferences of Saint Croix rams to related or unrelated ewes A. Orihuela ∗ , R. Vázquez Facultad de Ciencias Agropecuarias de la Universidad Autónoma del Estado de Morelos, Avenida Universidad 1001, Colonia Chamilpa, Cuernavaca, Morelos 62210, Mexico
a r t i c l e
i n f o
Article history: Received 12 September 2008 Received in revised form 10 March 2009 Accepted 18 March 2009 Available online 19 April 2009 Keywords: Mating preferences Inbreeding Sheep Sexual behaviour
a b s t r a c t Fifty mating tests were performed comparing the mating preferences of Saint Croix rams to unrelated ewes or their daughters. In each test, two ewes were induced into oestrus and restrained by the use of portable metal stanchions holding the head of the ewes, at equal distances from the point of entrance of 10 rams. The rams were individually tested with an unrelated ewe and his daughters as mating options in each test. No difference in mating preference was found in the choices that rams made regarding firstly the approach to the ewe, the number of approaches to each ewe, and the ewes mated. It was concluded that in Saint Croix rams, genetic relatedness did not influence the choice of mating partner. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The choice of a mating partner is an integral part of sexual selection in species like sheep, where individual males mate with several females. Rams select ewes, and ewes’ attractivity has been shown to be consistent. Different rams also select oestrous ewes in a similar order (Tilbrook, 1987). However, the fact whether the choice of mate occurs independently of genetic relatedness among mating pairs has received little attention, although inbreeding may have fitness consequences (Holand et al., 2007). Avoidance of mating between primary kin has been extensively reported across many animal species (Kokko and Ots, 2006), and the adaptive value of such behaviour is explained through inbreeding. The negative effects of inbreeding are well documented: the survival of inbred offspring is reported to be reduced in many species and inbred progeny are often less attractive as potential mates (Frommen and Bakker, 2006). Thus, individuals should avoid mating with close kin. However, experimental evidence regarding inbreeding avoidance in sheep is rare.
∗ Corresponding author. Tel.: +52 777 3177046. E-mail address: [email protected] (A. Orihuela). 0921-4488/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2009.03.005
The aim of this study was to determine if rams display mating preferences to unrelated or related ewes under controlled conditions. 2. Materials and methods The study was conducted at the University of Morelos, Mexico, located at 18◦ 37 N and 99◦ 19 W (899 m above sea level, average annual rainfall of 800 mm and temperature of 23 ◦ C). Ten Saint Croix rams (3 years of age) and 100 intact 18-month-old ewes were used in 50 mating tests, comparing the mate preferences of rams to unrelated or related ewes as mating partners. Five tests were conducted every other day for each ram. In each test, the males were exposed to two ewes restrained by the use of a portable stanchion at equal distances from the point of entrance of the ram. Mating tests were designed to last 5 min in duration, unless ejaculation occurred (whichever occurred first). In each test, one ewe was the daughter of the ram under evaluation, while also paired with an unrelated ewe—similar in weight, height and age. Different ewes were used in each test. Since born, ewes had been kept in a unisexual group 100 m apart of the males until the day of testing. During the experiment only oestrous induced ewes were used. Ewes were induced into oestrus in preparation for the mating tests with the aid of intravaginal sponges containing 30 mg FGA (Chronogest, Intervet, Mexico). The sponges were inserted for 12 days, followed by an intramuscular injection of 500–600 IU PMSG (Foligon, Intervet, Mexico) at sponge withdrawal. During each test the order of testing and the position of the related and unrelated ewes were reversed. The first approach to the ewe, the number of approaches toward each ewe, and the ewe mated by the ram (ejaculation) were registered.
A. Orihuela, R. Vázquez / Small Ruminant Research 83 (2009) 82–84 Data were analyzed determining whether the proportion of choices deviated significantly from a 50:50 ratio within each variable (Altman, 1997).
3. Results and discussion All rams ejaculated within the 5 min period, approaching unrelated ewes in 60% of the tests, displaying 55% of their approaches toward these same ewes and mating (ejaculating) 45% of the times. These findings were not statistically different (P > 0.05) from the 50% probability of success (or failure) expected at random. It has been recognized that rams select certain characteristics in their mating choices. Tilbrook (1987), established that ewes differ in their sexual attractiveness to rams, and that this attractiveness is constant during successive oestrous periods, depending at least in part, on wool and the breed as such. Woolly ewes are preferred to shorn sheep (Tilbrook and Cameron, 1989) and rams also exhibit a preference to their own breed of ewe and heavier ewes when offered a choice (Owens and Thompson, 1994; Preston et al., 2005). In addition, rams prefer ewes that have not been recently mounted (Houpt, 1998). Tilbrook (1987), also established that attractiveness is not related to level of oestrogen used to induce oestrus or the amount of soliciting behaviour. The results of the present study show that in Saint Croix rams, genetic relatedness did not influence the actual choice of a mating partner. This disagrees with the findings in other species, where males, when given the choice between courting a familiar or unfamiliar non-sib prefer to mate with the non-sib and thus avoid the disadvantages of inbreeding (Frommen and Bakker, 2006). Individuals in some species prefer partners carrying dissimilar genes at the major histocompatibility complex (MHC), which may help to increase the MHC or overall heterozygosity of progeny (Penn, 2002). Many studies in mice and rats indicate that the MHC genes influence an individual’s odour, playing a role in kin recognition (Olsén et al., 1998; Eggert et al., 1999; Yamazaki et al., 2000). In addition, research is examining the potential role of MHC-linked olfactory receptor genes in mating preferences (Penn, 2002). These MHC-disassortative mating preferences may help to avoid kin mating (Potts and Wakeland, 1993; Brown and Eklund, 1994; Penn and Potts, 1999). However, Paterson and Pemberton (1997) found no evidence of MHC-dependent mating preferences in feral sheep, suggesting that perhaps rams are not able to recognize their daughters, or do not use this strategy to avoid inbreeding. In other species different factors have been observed that deserve attention in sheep. So for example: inbreeding in some bird species is reduced by post-copulatory mechanisms that favour fertilization by sperm from unrelated males (Zeh and Zeh, 2006). The prevalence of this mechanism in captive ungulates such as sheep is not known. Other polygynous ungulates avoid inbreeding by dispersing from their birth site to other geographical areas occupied by genetically unrelated conspecifics (Holand et al., 2007). It could also be possible that due to husbandry practices that involve the isolation of ewe lambs from the
83
rams, until the breeding season when ewes are mature, rams are unable to recognize any familiarity in the ewes. In other words, males could avoid mating with females they have grown up with, which suggest that familiarity early in life could be more important than genetic relatedness with respect to avoiding inbreeding. So for example, non-reproductive females in families of eusocial common mole-rats (Cryptomys sp.) are not suppressed by their mother (either behaviourally or pheromonally) as is generally assumed. They do not mate with their father and kin, simply as they are not sexually attractive to them (and vice versa). Incest avoidance is based on the capability to recognize (and remember for up to 3 weeks) each family member individually (Burda, 1995). However, no conclusive data are available in sheep. Disruption of the natural herds (groups) and suppressing the female partner choice, as was the case in the present experiment, could be masking adaptive responses like those found in other species to avoid inbreeding in social mammals. Rams do select ewes, but ewes are also selective (Estep et al., 1989). In fact, in most polygynous mammalian species, the female choice (for mating) is very common if not the rule. In the present experiment, female choice was not possible. Höner et al. (2007) found that females preferred sires that were born into or immigrated into the female group after the female was born. The extent of male dispersal as a response to such female mate preferences depends on the demographic structure of breeding groups, rather than the genetic relatedness between females and males. The lack of choice between related and unrelated ewes suggests that inbreeding problems could surge in sheep herds, since inbreeding avoidance mechanisms may have been selected out during domestication—or there are other mechanisms involved. Inbreeding has been identified as a major factor in the emergence of some genetic disorders of sheep (Jolly et al., 2004). Birth weights, average daily gain from birth until 2 months of age and litter size are depressed due to inbreeding (Norberg and Sorensen, 2007). 4. Conclusion It was concluded that in Saint Croix rams, genetic relatedness did not appear to influence the actual choice of mating partner. References Altman, D.G., 1997. Practical Statistics for Medical Research. Chapman and Hall, London, pp. 232–239. Brown, J.L., Eklund, A., 1994. Kin recognition and the major histocompatibility complex: an integrative review. Am. Nat. 143, 435–461. Burda, H., 1995. Individual recognition and incest avoidance in eusocial common mole-rats rather than reproductive suppression by parents. Experientia 51, 411–413. Eggert, F., Müller-Ruchholtz, W., Ferstl, R., 1999. Olfactory cues associated with the major histocompatibility complex. Genetica 104, 191–197. Estep, D.Q., Price, E.O., Wallach, S.J.R., Dally, M.R., 1989. Social preferences of domestic ewes for rams. Appl. Anim. Behav. Sci. 24, 287–300. Frommen, J.G., Bakker, T.C., 2006. Inbreeding avoidance through nonrandom mating in sticklebacks. Biol. Lett. 2, 232–235.
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A. Orihuela, R. Vázquez / Small Ruminant Research 83 (2009) 82–84
Holand, O., Askim, K.R., Roed, K.H., Wladji, R.B., Gjostein, H., Nieminen, M., 2007. No evidence of inbreeding avoidance in a polygynous ungulate: the reindeer (Rangifer tarandus). Biol. Lett. 3, 36– 39. Houpt, K.A., 1998. Domestic Animal Behavior for Veterinarians and Animal Scientists. Iowa State University Press, Iowa, pp. 133–139. Höner, O.P., Wachter, B., East, M.L., Streich, W.J., Wilhelm, K., Burke, T., Hofer, H., 2007. Female mate-choice drives the evolution of malebiased dispersal in social mammal. Nature 16, 798–801. Jolly, R.D., Blair, H.T., Johnstone, A.C., 2004. Genetic disorders of sheep in New Zealand: a review and perspective. New Zeal. Vet. J. 52, 52–64. Kokko, H., Ots, I., 2006. When not to avoid inbreeding. Int. J. Org. Evol. 60, 467–475. Norberg, E., Sorensen, A.C., 2007. Inbreeding trend and inbreeding depression in the Danish populations of Texel, Shropshire, and Oxford Down. J. Anim. Sci. 85, 299–304. Olsén, K.H., Grahm, M., Lohn, J., Langefors, A., 1998. MHC and kin discrimination in juvenile Arctic charr Salvelinus alpinus (L.). Anim. Behav. 56, 319–327. Owens, I.P.F., Thompson, D.B.A., 1994. Sex differences, sex rations and sex roles. Proc. Roy. Soc. B 258, 93–99.
Paterson, S., Pemberton, J.M., 1997. No evidence for major histocompatibility complex-dependent mating patterns in a free-living ruminant population. Proc. Roy. Soc. B 264, 1813–1819. Penn, D.J., 2002. The scent of genetic compatibility: sexual selection and the major histocompatibility complex. Ethology 108, 1–21. Penn, D., Potts, W.K., 1999. The evolution of mating preferences and major histocompatibility genes. Am. Nat. 153, 145–164. Potts, W.K., Wakeland, E.K., 1993. Evolution of MHC genetic diversity: a tale of incest, pestilence and sexual preference. Trends Genet. 9, 408–412. Preston, B.T., Stevenson, I.R., Pemberton, J.M., Coltman, D.W., Wilson, K., 2005. Male mate choice influences female promiscuity in Soay sheep. Proc. Roy. Soc. B 272, 365–373. Tilbrook, A.J., 1987. The influence of factors associated with oestrus on the sexual “attractiveness” of ewes to rams. Appl. Anim. Behav. Sci. 1–2, 117–128. Tilbrook, A.J., Cameron, A.W.N., 1989. Ram mating preferences for woolly rather than shorn ewes. Appl. Anim. Behav. Sci. 24, 301–312. Yamazaki, K., Beauchamp, G.K., Curran, M., Bard, J., Boyse, E.A., 2000. Parent-progeny recognition as a function of MHC odortype identity. Proc. Nat. Acad. Sci. U.S.A. 97, 10500–10502. Zeh, J.A., Zeh, D.W., 2006. Outbred embryos rescue inbred half-siblings in mixed-paternity broods of live-bearing females. Nature 439, 201–203.