www.elsevier.comrlocaterapplanim
Sexual performance of twin ram lambs and the
effect of number and sex of contemporary siblings
E.O. Price
), C.J. Bench
1, R.E. Borgwardt, M.R. Dally
2Department of Animal Science UniÕersity of CA, DaÕis, CA 95616-8521 USA
Accepted 26 January 2000
Abstract
The objectives of this investigation were to determine the degree of correspondence in the sexual performance of twin male sheep and to compare the sexual performance of ram lambs born
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co-twin to another male M–M , co-twin to a female M–F and as single S offspring. Individual
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ram lambs Ns117 , including 13 M–M twin sets, were exposed to four estrous ewes for 30 min
at 7-day intervals over a 4-week period at 8 to 9 months of age.
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Frequencies of ejaculation serving capacity and mounts without ejaculation were recorded. M–M males averaged 2.9 ejaculations and 13.1 mounts per test. Individuals within 9 of the 13
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M–M twin sets 69% differed by an average of 0.5 or fewer ejaculations per test. Although variability within M–M twin sets was very low, variability between the 13 M–M twin sets was
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also low coefficient of variations10.6% . Consequently, frequencies of both ejaculation and
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mounting without ejaculation within the 13 twin sets were not correlated Ps0.50 and
.
Ps0.10, respectively . When the progeny of individual sires were compared, mounting
frequen-Ž . Ž .
cies were correlated P-0.02 but not ejaculation rates Ps0.72 . M–M, M–F and S ram lambs
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did not differ for either ejaculation frequency Ps0.26 or mounts without ejaculation Ps0.98 .
It was concluded that the sexual performance of related individuals is likely to be very similar and that the number and sex of contemporary siblings does not necessarily influence rates of mounting
and ejaculation.q2000 Elsevier Science B.V. All rights reserved.
Keywords: Sheep; Reproduction; Sexual behavior; Sexual performance; Libido
)Corresponding author. Tel.:q1-530-752-6789.
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E-mail address: eoprice@ucdavis.edu E.O. Price .
1
Present address: Prairie Swine Centre, P.O. Box 21057, Saskatoon, SK, Canada, S7H 5N9. 2
Present address: UC Hopland Research and Extension Center, 4070 University Road, Hopland, CA 95449, USA.
0168-1591r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved. Ž .
1. Introduction
The primary objectives of this investigation were to determine the degree of similarity in the sexual performance of full-sib domestic ram lambs and to compare sexual performance based on the number and sex of contemporary siblings. A number of studies have demonstrated an important genetic basis for the traits that enable male farm
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animals to attain successful matings. Boyd and Corah 1986 reported that the sons of
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four A.I. bulls exhibited significantly different ejaculation rates serving capacity when
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exposed to estrous females. Dunnington and Siegel 1983 demonstrated successful artificial selection for sexual performance in male chickens. More recently, Bench et al.
Ž1999 demonstrated that artificial selection for sexual performance in rams can be. Ž
achieved in a single generation. Rams selected for high serving capacity rate of
.
ejaculation produced a higher-than-expected proportion of sons who exhibited high
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serving capacity. Blockey et al. 1978 and Kilgour 1985 have reported relatively high heritability estimates of 0.59"0.16 and 0.33"0.62 for serving capacity in bulls and rams, respectively. In light of these findings, one might expect the sexual performance of male siblings to be more similar than the sexual performance of non-sibs. This
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hypothesis is supported by Bane 1954 and Blockey and Galloway 1978 who reported a relatively strong correlation for the serving capacity of identical twin bulls.
In the production of full-sib male twin sets for this study, a number of male–female twin sets and single-born offspring were obtained. This outcome provided the opportu-nity to examine differences in the sexual performance of rams based on the sex and
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number of offspring born to individual ewes. Fitzgerald et al. 1993 reported that rams born co-twin to another male exhibited higher serving capacity scores than rams born co-twin to a female. They suggested two possible explanations for this result, namely either prenatal and postnatal effects of androgens on masculinization of sexual behaviors
ŽFord and D’Occhio, 1989 andror the animal’s rearing environment Zenchak and. Ž .
Anderson, 1980; Orgeur and Signoret, 1984; Price et al., 1994 . Thus, a second objective of the present study was to compare the sexual performance of rams born and reared with a male co-twin, born and reared with a female co-twin and born and reared as single offspring.
2. Animals, materials and methods
2.1. Ram lambs
Subjects were 117 Targhee-type ram lambs born during a 22-day period in January 1998 and maintained at the University of CA Hopland Research and Extension Center,
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Hopland, CA. Thirty-six ram lambs were born and reared co-twin to a male M–M , 14
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were born and reared co-twin to a female M–F and 67 were born and reared as a
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single S . All subjects were conceived by single-sire matings using rams previously
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classified as exhibiting either high HP or low LP sexual performance Ns6 and 4
.
rams, respectively . The proportion of ram lambs sired by HP and LP males did not
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likely all dizygotic fraternal since monozygotic identical twins in sheep are extremely
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rare Henning, 1939; Morley, 1948; Rae, 1956 . After weaning at 3 to 4 months of age, animals were maintained as a single group on pasture until testing at which time they were transferred to a smaller paddock near the test facilities and supplemented with alfalfa pellets.
2.2. Stimulus females
Thirty-four laparoscopically ovariectomized adult ewes were used as stimulus fe-males in ram lamb sexual performance tests. They were induced to exhibit behavioral estrus by IM injections of 25 mg of progesterone 5 and 3 days prior to testing and 200
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mg of estradiol benzoate 24 h before testing Kilgour and Whale, 1980 .
2.3. Ram lamb sexual performance tests
Ram lambs were evaluated for sexual performance every 7 days for 4 weeks during
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September and early October fall breeding season when they were approximately 8–9
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months of age. Sexual performance serving capacity tests consisted of individually exposing the ram lambs to four unrestrained hormone-induced estrous ewes in 5=5 m2
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enclosures for 30 min each test between 0600 and 1400 h. Test order of the subjects was systematically balanced over the four tests to minimize the possible effects of time of day, temperature, test pen, and observer bias, and to insure that no ram was tested with the same estrous ewes more than once. Each of two experimenters observed two
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ram lambs and accompanying ewes in two adjacent test pens separated by a visual barrier. The observers recorded all occurrences of ejaculations and mounts without ejaculation.
After testing had been completed on each of the first three test days, all ram lambs that were sexually inactive or failed to attain an ejaculation during testing were given an additional 60 min of exposure, as a group, to eight estrous ewes in a 10=10 m2 corral. These additional exposures to estrous ewes were administered to facilitate the onset of copulatory behavior as early as possible in the testing period. Additional sexual performance tests were administered during weeks 5 and 6 for those ram lambs that attained their first ejaculation in test 2 or test 3 so that each animal was tested four times starting with the first test in which he attained an ejaculation.
At the end of the fourth week of testing, three M–M, two M–F and eight S ram lambs were still sexually inactive and were eliminated from the study. One M–M and one S male were eliminated for inability to achieve intromissions in their last two to three tests in spite of numerous mounts. This left 32 M–M, 12 M–F and 58 S subjects for the statistical analyses. Of the 32 M–M ram lambs remaining in the study, there were 13 sets of M–M twins in which both individuals of the pair provided useable data. In addition to the four M–M animals removed from the study, the co-twin of two males had died prior to testing.
The number of ejaculations and mounts without ejaculation were averaged for each individual for the third and fourth tests in which ejaculations were attained. Preliminary
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tests did not differ and were higher than ejaculation rates in the first and second tests. As
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in previous studies Price et al., 1991 , it was concluded that sexual performance in the third and fourth tests provided reliable measures of sexual performance.
Similarities in the sexual performance of co-twins in the M–M group were deter-mined by calculating Pearson product–moment correlation coefficients. Animals within twin sets were assigned to samples 1 and 2 for these analyses by consecutive ear tag numbers. Correlation coefficients were computed for the sexual performance scores of all 13 M–M twin sets and for the progeny of the eight sires providing M–M ram lambs. In the latter case, scores were averaged for the progeny of each sire listed in each of the two samples used in the computation. The sexual performance scores of all M–M, M–F and S subjects were compared by one-way ANOVAs. An additional ANOVA was conducted using the mean ejaculation frequency scores of ram lambs born to each sire in
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each of the three ram-lamb groups. A sign test Siegel and Castellan, 1988 was used to compare the mean ejaculation rate of M–M and S ram lambs born to each of eight sires.
3. Results and discussion
Mean frequencies of ejaculation and mounts without ejaculation during the third and fourth sexual performance tests are presented in Table 1. Frequencies of both ejaculation
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and mounting were not correlated for the 13 M–M twin sets rs y0.20, dfs11,
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Ps0.50 and rs0.48, dfs11, Ps0.10, respectively . When correlation coefficients were recomputed using the mean scores of the progeny of individual sires, ejaculation
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frequencies were again not correlated rs y0.15, dfs6, Ps0.72 but a significant
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correlation was obtained for mounting frequency rs0.78, dfs6, P-0.02 . With very young males, mounting frequency may be a better measure of sexual motivation than frequency of ejaculation since it is less affected by mating competency.
When all subjects were considered individually, M–M, M–F and S ram lambs did
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not differ for either ejaculation frequency Fs1.35, dfs2r99, Ps0.26 or the
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frequency of mounts without ejaculation Fs0.02, dfs2r99, Ps0.98 . This same result was obtained using the mean ejaculation frequencies of ram lambs born to each
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sire represented in each of the three ram-lamb groups Fs1.30, dfs2r21, Ps0.29 . In addition, there was no difference in the mean ejaculation frequencies of M–M and S
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ram lambs born to each of eight sires sign test: Ps0.29 .
Table 1
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Mean "SE frequencies of ejaculations and mounts without ejaculation for ram lambs born co-twin to
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another male M–M , co-twin to a female M–F or as a single offspring S during the third and fourth sexual performance tests
M–M M–F S
No. of rams 32 12 58
No. of ejaculations per test 2.91"0.09 2.75"0.40 3.19"0.15
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Previous studies demonstrating significant correlations in the sexual performance of
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twin bulls Bane, 1954; Blockey and Galloway, 1978 employed genetically identical
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males. Kilgour 1985 reported a greater similarity in the serving capacity of related rams than among unrelated rams of five different genetic lines. Based on previous
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reports Henning, 1939; Morley, 1948; Rae, 1956 that monozygotic twins are extremely rare in sheep, it was assumed that all M–M twin sets in the present study were dizygotic. Still there was relatively little variation in sexual performance within M–M
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twin sets in this study. Individuals in 9 of the 13 twin sets 69% differed by 0.5 or fewer services per test, one of the twin sets differed by 1 serve and the remaining three differed by 1.5 ejaculations per test. It should also be noted that there was less-than-nor-mal variability between twin sets. When the mean serving capacity scores for the 13
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twin sets were computed, all but three 77% averaged 2.75, 3.00 or 3.25 with the remaining twin sets averaging either 2.5 or 3.5. These scores are remarkably homoge-neous both within and between twin sets.
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The coefficient of variation CV for the ejaculation frequencies of the 26 M–M ram lambs was 18.2% and the CV of the means of the 13 pairs was a very close 10.6%. In contrast, the ejaculation frequencies of the 12 experimental rams born co-twin to a female ranged from 0.5 to 5.0 and the CV was 50.5% while the scores of the single-born rams ranged from 1.0 to 7.5 services with a CV of 36.1%. These CVs suggest that there was considerably less variation in the sexual performance of the M–M subjects, taken as a whole, than in their M–F or S counterparts.
The fact that several of the M–M twin sets were sired by the same ram may have contributed to this group’s relatively low between twin-set variability and CV score. However, the mean ejaculation rate and corresponding CV of M–M ram lambs was not appreciably changed by using average performance scores of all M–M ram lambs born
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to common sires 2.89 ejaculations per test and 11.3% vs. 2.92 ejaculations per test and
.
10.6% when all 13 twin sets were used in the computations . In contrast, the CV of the S ram lambs was reduced from 36.1% to 21.8% when ram lambs born to common sires were averaged for the computation. Even so, the CV of the M–M subjects was only
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about half that of the S ram lambs 11.3% vs. 21.8% .
In a previous year, 15 M–M ram lamb twin sets produced in a multiple-sire mating
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program twins could be half- or full-sibs were given the same series of sexual performance tests as in the present study. The mean ejaculation rates of the 15 twin sets yielded a CV of 18.7%, somewhat higher than the CV of the mean scores of the 13 twin sets used in the present study. This result is reasonable considering that some of the twin sets in this earlier population were undoubtedly half sibs.
The relatively close correspondence in sexual performance scores within M–M twin sets could not be statistically verified due to the unexplained unusually low variability between M–M twin sets.
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The present study failed to replicate the finding of Fitzgerald et al. 1993 that male rams born co-twin to another male have higher ejaculation rates than males born co-twin to a female. No ready explanation can be offered for this discrepancy. It was noted that
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Fitzgerald et al. had very large sample sizes Ns183 and 198 for M–M and M–F
.
counterparts. Mean differences of that magnitude in the present study would likely have been significant in spite of small sample sizes. The Fitzgerald et al. study also employed stock from five sheep breeds that differ in prolificacy and libido. The authors did not state whether the various breeds were equally represented in the two twin groups and in the rams born as singles.
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In addition, Fitzgerald et al. 1993 reported that yearling rams from multiple births were more likely to exhibit high levels of sexual performance than rams born as singles. This finding was not replicated in the present study, which found no difference in the sexual performance of twin- and single-born ram lambs even when averages of ram lambs born to common sires were used in the computations. Although the subjects were
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of different ages in the two studies Fitzgerald and colleagues used yearling rams , it is
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unlikely that age could account for the relative differences observed. Kilgour 1985 found no difference in the serving capacity of twin- and single-born yearling rams.
In conclusion, the present study demonstrated very close correspondence in the sexual performance of full-sib males born as twins. However, the present study did not statistically verify that twin ram lambs are necessarily more similar in their sexual performance than non-twins. It was also concluded that the sexual performance of ram lambs does not differ based on the number and sex of contemporary siblings. From the standpoint of sexual performance there is no advantage in selecting twins or non-twins for use in sheep breeding programs
Acknowledgements
The authors acknowledge the assistance of J. Hays and R. Rittenhouse in caring for the sheep used in this study.
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