SEX RATIO IN DOMESTIC PIGS
111
BIOL. LETT. 2003, 40(2): 111–118
Available online at http://www.biollett.amu.edu.pl
Sex ratio in litters of domestic pigs
(Sus scrofa f. domestica Linnaeus, 1758)
MARCIN TADEUSZ GÓRECKI
Department of Sheep and Goat Breeding, August Cieszkowski Agricultural University of Poznañ,
S³oneczna 1, Z³otniki, 62–002 Suchy Las, Poland
e-mail: [email protected]
(Received on 4th July 2003; Accepted on 15th October 2003)
Abstract: The offspring sex ratio is a subject of considerable interest from both the theoretical and the
practical point of view. In domestic animals, including pigs, the offspring sex ratio is also a feature of
economic value. The gilts from litters with a higher proportion of females can deliver and feed more
piglets as they have more teats, a higher fertility rate, and a better reproductive performance. The aim of
this paper was to assess whether litter size, maternal age and parity, paternal breed, maternal birth year
and month, and litter birth year and month influenced the offspring sex ratio in domestic pigs. A total of
436 litters on 21st day of life were considered. It was found that paternal breed and litter size significantly
affected the offspring sex ratio (fewer males in larger litters). Also maternal month of birth had a significant
influence on offspring sex ratio (sows born in September-February delivered litters with a higher male
proportion than those born in March-August). There was also a correlation at a marginally significant
level between the offspring sex ratio on 21st day of life and proportion of stillborn piglets (more stillborn
ones in the litters with the future higher male proportion).
Key words: pig, offspring sex ratio, month of birth, litter size, paternal breed
INTRODUCTION
The offspring sex ratio is a subject of considerable interest from both the theoretical (for reviews, see GODFRAY & WERREN 1996, HARDY 1997) and the practical point of view. In case of domestic animals, including pigs, the offspring sex ratio
is a feature of economic value. The gilts from litters with a higher proportion of
females can deliver and feed more piglets, as they have more teats (DRICKAMMER et
al. 1999), a higher fertility rate (DRICKAMMER et al. 1997) and a better reproductive
performance (HUEHN et al. 2002). Some hypotheses have been proposed to explain
the variation in offspring sex ratio in domestic pigs. BROOKS et al. (1991) suggested
that the time between insemination and ovulation should influence the offspring sex
ratio. JAMES (2001) predicted that also the sex of foetuses among which the gilt was
gestated during her mother’s pregnancy could influence her offspring sex ratio.
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Marcin Tadeusz Górecki
MEIKLE et al. (1993) reported that the offspring sex ratio was connected with maternal social status (proportion of males increased with maternal social rank). By contrast, MENDL et al. (1995) found that maternal social status, adrenal activity and
nutrition status did not influence the offspring sex ratio. M EIKLE et al. (1993) and
MENDL et al. (1995) referred their results to two main adaptive explanations of offspring sex ratio variation: the TRIVERS & WILLARD hypothesis (1973) and the local
resource competition model (CLARK 1978, modified by SILK 1983).
According to TRIVERS & WILLARD (1973), if one sex receives more parental
investment, parents in good condition will bias their investment towards the sex with
greater rates of reproduction returns. The theory deals especially with polygynous
species. So mothers being in good condition (which can depend on high social rank)
should invest more resources in sons and mothers in poor condition ought to bias
their investment towards daughters.
The local resource competition model (CLARK 1978, modified by SILK 1983)
postulates that high-quality females should produce more offspring of philopatric sex,
because they will ‘inherit’ maternal social rank and home range. In mammals, males
are usually the sex that is more dispersing (GREENWOOD 1980), so mothers in good
condition should bias their investment towards daughters, while poor-quality mothers towards sons.
Therefore, the aim of this paper was to assess whether litter size, maternal age
and parity, paternal breed, maternal birth year and month, and litter birth year and
month influence the offspring sex ratio in domestic pigs.
MATERIAL AND METHODS
A total of 3,852 piglets from 436 litters were examined on 21st day of life (tattooing day). No data were available about stillborn piglets’ sex. Data about all liveborn piglets’ sex was available only for 246 litters (including 217 litters in which no
piglet died to 21st day of life). The examined litters were born in 1999–2002 at the
Experimental Farm in Z³otniki. The gilts were mated first at the age of about 9 months
(if their body weight was ca 100 kg) and then every 5.5 month in Z³otniki. The piglets were fostered if necessary. They were weaned at the age of 4-6 weeks. The studied piglets were the offspring of 152 mothers (whose age ranged from 9 to 54 months)
and 19 fathers. All sows were of the Z³otnicka White breed. They were sired by males
of two breeds: Z³otnicka White and Polish Landrace.
Linear regression was used to assess whether litter size (number of all born
piglets, number of live-born ones and number of piglets on 21st day of life), paternal breed, proportion of piglets dead to 21st day of life, maternal age, and parity
affected the offspring sex ratio, i.e. the ratio of live female piglets to all live piglets
(Table 1). Pearson correlation between the offspring sex ratio and seven factors was
calculated: number of all born piglets, number of live-born ones and number of piglets on 21st day of life, proportion of stillborn piglets, proportion of piglets dead to
21st day of life, maternal age, and parity (Table 2). Moreover, the influence of paternal breed on offspring sex ratio was checked with Student’s t-test. Effects of maternal and litter year of birth on offspring sex ratio were tested with one-way ANOVA.
The cosine functions were used in order to check if maternal and litter month of birth
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SEX RATIO IN DOMESTIC PIGS
Table 1. Determinants of offspring sex ratio (proportion of females) – results of linear regression
(N = 378)
Factor
Constant
Estimated values
Standard error
P level
0.484
0.059
0
Paternal breed
-0.042
0.021
0.042
Number of born piglets
-0.007
0.009
0.453
0.048
0.019
0.012
Number of piglets on 21st day of life
-0.034
0.021
0.102
Proportion of piglets dead to 21st day
-0.332
0.223
0.136
0.001
0.003
0.699
-0.014
0.016
0.386
Number of live-born piglets
Maternal age
Parity
Table 2. Pearson correlation between offspring sex ratio and tested factors
Factor
r
P
Number of born piglets
0.131
0.006
436
Number of live-born piglets
0.153
0.001
436
Number of piglets on 21st day of life
0.115
0.017
436
-0.091
0.062
420
0.010
0.830
420
Maternal age
-0.005
0.925
422
Parity
-0.021
0.659
420
Proportion of stillborn piglets
Proportion of piglets dead to 21st day
N
influenced the offspring sex ratio (after HENNEBERG & LOUW 1993, who tested the
influence of month of birth on human body size in this way). The cosine equation
can be expressed as follows:
π((litter month of birth – 0.5) :12 + b)) + 0.482,
offspring sex ratio = a cos(2π
where a is an amplitude and b is a phase.
A cosine equation was also calculated to check if maternal month of birth
affected litter size on 21st day of life. The cosine equation was:
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Marcin Tadeusz Górecki
litter size = a cos (2π
π((maternal month of birth – 0.5) : 12 + b)) + 8.83
All calculations were conducted with StatSoft Statistica 2001.
RESULTS
The number of piglets per litter on 21st day of life ranged from 1 to 16 (mean:
8.83, standard deviation: 2.48). The number of all born piglets ranged from 3 to 19,
and the number of live-born ones, from 1 to 18. There were 1,880 females and 1,972
males among the piglets on 21st day of life, so the offspring sex ratio was 0.488 (not
significantly different from 0.5, P > 0.05). The maternal and litter year of birth, litter
month of birth, proportion of piglets dead to 21st day of life, maternal age, and parity, appeared to have no significant effect on offspring sex ratio. Only the paternal
breed and number of live-born piglets significantly affected the offspring sex ratio
(P < 0.05, results of linear regression analysis, Table 1). Pearson correlation coefficients between the offspring sex ratio and all three measures of litter size were positive and significant. The strongest correlation was found between the number of liveborn piglets and the offspring sex ratio (r = 0.153, P = 0.0013, Table 2).
The paternal breed’s significant influence on offspring sex ratio was confirmed
with Student’s t-test: litters sired by Polish Landrace boars had a significantly lower
female proportion (mean = 0.448, N = 122), than those sired by Z³otnicka White boars
(mean = 0.493, N = 272, P = 0.025). Maternal age positively correlated with litter
size at birth (r = 0.301; P < 0.001), number of live-born piglets (r = 0.26; P < 0.001)
and live piglets on 21st day of life (r = 0.119; P = 0.014). The correlation between
parity and litter size was almost the same as mentioned above, because parity was
strongly connected with maternal age (r = 0.944; P < 0.001). The number of
stillborn piglets per litter was negatively correlated with the offspring sex ratio
(r = -0.091; P = 0.063).
The maternal month of birth had a significant influence on offspring sex ratio.
The resultant cosine equation was:
π((month of birth – 0.5) :12 + 0.066 )) + 0.482,
offspring sex ratio = -0.033 cos(2π
with P = 0.025 for amplitude, which means that maternal month of birth significantly influenced the offspring sex ratio. The litters with higher offspring sex ratios were
delivered by females born in March-August, whereas the litters with lower offspring
sex ratios were typical for sows born in September-February.
The maternal month of birth also had a significant influence on litter size. The
cosine equation was:
π((month of birth – 0.5) :12 + 0.12 )) + 8.83
litter size = -0.894 cos(2π
with P = 0.000004 for amplitude. The sows born in March-August generally produced larger litters than others.
DISCUSSION
The offspring sex ratio in the studied litters was 0.488. Similar results were
obtained by MEIKLE et al.(1993, 0.5) and HUEHN et al. (2002, 0.498) in domestic
pigs at birth as well as by FERNANDEZ-LLARIO et al. (1999, 0.477) in wild boar foetuses.
SEX RATIO IN DOMESTIC PIGS
115
The paternal breed influenced the offspring sex ratio. This result may be caused
by differences in levels of hormones (especially testosterone) in sires. JAMES (1996)
suggested that paternal testosterone level and coital frequency affected the offspring
sex ratio. It is known that domestic pig breeds differ in aggressiveness (NOWICKI &
ZWOLIÑSKA-BARTCZAK 1983, KAPELAÑSKI et al. 1992) and also that aggression is
influenced by testosterone level (e.g. FRYE et al. 2002, ORENGO et al. 2002). Thus
the differences in sex ratio between litters descending from fathers belonging to different breeds could be explained by different levels of testosterone in different breeds.
MEIKLE et al. (1993) did not observe any significant effect of parental breed on offspring sex ratio. However, those authors examined only 46 litters sired by boars of
four breeds. Thus it is possible that the number of litters in every category was too
small to detect the differences. No influence of paternal breed on sex ratio was also
observed in domestic goat litters sired by bucks of two breeds (GÓRECKI & KOŒCIÑSKI,
2003).
In the presented study, maternal birth month influenced the offspring sex ratio
in such a way that litters with the higher proportion of males were delivered by sows
born in September-February, and litters with a lower proportion of males, by sows
from March-August. This pattern was observed also for litter size. The sows born in
March-August had larger litters on 21st day of life. It is possible that the factors
affecting organisms in early stages of life influenced their features also in further
life. HENNEBERG & LOUW (1993) reported that dogs from South Africa born in
August-January were heavier than those born in February-July. HENNEBERG & LOUW
(1990) observed also this dependency in people born in South Africa. In the Northern Hemisphere, the results for humans were either similar (SHEPHARD et al. 1979,
KOŒCIÑSKI et al. in print) or contrary (WEBER et al. 1998, BANEGAS et al. 2001).
HENNEBERG & LOUW (1990, 1993) suggested that this effect occurring in both hemispheres is not dependent on climatic factors, but is due to different distances between
the Sun and Earth and related phenomena, e.g. changes in total amount of energy
reaching the Earth and changes in electromagnetic field.
In this study, the offspring sex ratio was positively correlated with litter size.
JACOBSEN et al. (1999) found that the proportion of males decreased with increasing
number of human children per plural birth. CASINELLO & GOMENDIO (1996) observed
that Barbary sheep (Ammotragus lervia) females produced the following sequence
with increasing social rank (parallel to increasing available resources): female
singleton or twins, male singleton, mixed-sex twins, male twins. Those researchers
argued that the female fitted her investment in relation to available resources, and
males are a more costly sex.
Interestingly, in the present paper, both the offspring sex ratio and litter size
were found to be affected by maternal month of birth. One can speculate that sows
born in September-February were ‘better’, like dogs and humans born in similar
seasons (SHEPHARD et al. 1979, HENNEBERG & LOUW 1990, 1993; KOŒCIÑSKI et al.
in print), so they delivered smaller litters with higher proportion of males. However,
the results obtained in the present paper suggested also that older sows with higher
parity (probably with more available resources) delivered larger litters. Some authors
(e.g. ORZECHOWSKA & MUCHA 1999) observed a similar dependency, whereas
others (e.g. QUINIOU et al. 2002) reported that litters from the second pregnancy were
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Marcin Tadeusz Górecki
smaller than those from the first and the third ones, and that prolificacy decreased
after the fourth pregnancy. The dependence between maternal age and litter size
reported here testifies against the above hypothesis that ‘better’ sows are born in
September-February. Thus, the results presented here do not support the TriversWillard hypothesis. This can be explained by the local resource competition model:
sows born in March-August were ‘better’, like humans born in similar seasons (WEBER et al. 1998, BANEGAS et al. 2001), so they delivered larger litters with higher
female proportion. However, Sus scrofa fits rather the pre-assumptions of the TriversWillard hypothesis than those of the local resource competition hypothesis. MENDL
et al. (1995) found that their results provided weak support to the local resource
competition model, whereas MEIKLE et al.’s (1993) results supported the TriversWillard hypothesis. The relation between litter size and offspring sex ratio has not
been observed so far in domestic pigs (MEIKLE et al. 1993, SOEDE et al. 2000) and
wild boars (FERNANDEZ-LLARIO et al. 1999), whereas it has been established that
these features correlated with maternal available resources: proportion of males with
maternal social rank (MEIKLE et al. 1993) and litter size with maternal body size
(FERNANDEZ-LLARIO et al. 1999).
No effects of litter birth month on offspring sex ratio were observed in this
study, contrary to NONAKA et al.’s (1999) results in humans. It was also found in this
paper that neither maternal nor litter year of birth influenced the offspring sex ratio.
GÓRECKI & KOŒCIÑSKI (2003) found that maternal year of birth influenced the offspring sex ratio in domestic goats, but this could be due to the fact that dams came
from different farms and also conditions provided to dams differed between years on
the studied farm. The maternal age and parity had no influence on piglets’ sex ratio.
MEIKLE et al. (1993) also did not observe such an effect.
The offspring sex ratio in Sus scrofa has been so far investigated in foetuses
(FERNANDEZ-LLARIO et al. 1999) and in newborn piglets (MEIKLE et al. 1993, MENDL
et al. 1995, SOEDE et al. 2000). In the presented study, the sex ratio was examined
in piglets on 21st day of life. The question is whether the offspring sex ratio investigated at different stages of life is affected by the same factors. MEIKLE et al. (1993)
found that excluding stillborn and perinatal deaths did not change results of analysis
of all litters. However, HUEHN et al. (2002) observed more males (56%) among stillborn piglets. Contrary to the above statements, a negative relationship between proportion of stillborn piglets and offspring sex ratio was recorded in this study, suggesting that perinatal mortality is female biased. Thus, it is possible that there is no
obvious trend in perinatal mortality. Also the mortality in the first weeks of life should
not be sex biased. The main factor influencing piglet mortality is body weight at birth
(GRUDNIEWSKA 1987), which is connected with their weight on 40th day of pregnancy (CHEN & DZIUK 1993). FERNANDEZ-LLARIO et al. (1999) observed in wild
boars that male foetuses were heavier, but this relationship is not so obvious in domestic pigs, where litters are much larger as a result of human selection for prolificacy. However, there are reports (e.g. QUINIOU et al. 2002) indicating that light piglets
are usually females. It was found in domestic pigs that piglet body weight was influenced by its uterine position (WISE et al. 1997) and available space in the uterus (CHEN
& DZIUK 1993). The latter factor affected the sexes in different ways: in crowded
sections of the uterus, males were lighter than females, whereas in sections with more
available space per specimen, females were lighter than males. No influence of the
SEX RATIO IN DOMESTIC PIGS
117
proportion of live piglets on 21st day on sex ratio on that day was observed in the
presented study. Thus it seems that the factors affecting the sex ratio of piglets on
21st day of life reported in this paper (paternal breed, litter size, and maternal month
of birth) also influenced the sex ratio in all born piglets and in live-born ones.
Acknowledgements: I am grateful to Mr Stanis³aw Szmania, the Head of the Z³otniki Experimental Farm,
for making the data available to me. I am also indebted to Dr Krzysztof Koœciñski for conducting the
statistical analysis and access to unpublished human data.
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