Abstract
For the most part solutions to farm animal welfare issues, such as piglet mortality, are likely to lie within the scientific disciplines of environmental design and genetic selection, however understanding the ecological basis of some of the complex dynamics observed between parent and offspring could make a valuable contribution. One interesting, and often discussed, aspect of mortality is the propensity for it to be sex-biased. This study investigated whether known physiological and behavioural indicators of piglet survival differed between the sexes and whether life history strategies (often reported in wild or feral populations) relating to parental investment were being displayed in a domestic population of pigs. Sex ratio (proportion of males (males/males+females)) at birth was 0.54 and sex allocation (maternal investment measured as piglet birth weight/litter weight) was statistically significantly male-biased at 0.55 (t35 = 2.51 P = 0.017), suggesting that sows invested more in sons than daughters during gestation. Despite this investment in birth weight, a known survival indicator, total pre-weaning male mortality was statistically significantly higher than female mortality (12% vs. 7% respectively z = 2.06 P = 0.040). Males tended to suffer from crushing by the sow more than females and statistically significantly more males died from disease-related causes. Although males were born on average heavier, with higher body mass index and ponderal index, these differences were not sustained. In addition male piglets showed impaired thermoregulation compared to females. These results suggest male-biased mortality exists despite greater initial maternal investment, and therefore reflects the greater susceptibility of this sex to causal mortality factors. Life history strategies are being displayed by a domestic population of pigs with sows in this study displaying a form of parental optimism by allocating greater resources at birth to males and providing an over-supply of this more vulnerable sex in expectation of sex-biased mortality.
Highlights
Applying ecological theories to situations concerning domestic livestock is a method of addressing applied biological issues in farm animal welfare [1,2]
Piglet mortality is one such issue still to be effectively addressed and, solutions are likely to lie within the scientific disciplines of environmental design and genetic selection, understanding the ecological basis of some of the complex dynamics observed between parent and offspring could make a valuable contribution
It seems unlikely that discrimination would exist at this postnatal stage and general litter size adjustment may be more important than sex ratio variation for ensuring reproductive fitness
Summary
Applying ecological theories to situations concerning domestic livestock is a method of addressing applied biological issues in farm animal welfare [1,2]. The two main and applicable theories are: (i) an adaptive manipulation of the sex ratio (% of males) by mothers unable to rear successful sons [3] and; (ii) differential energetic requirements between the sexes in a sexually dimorphic species, where the larger sex are more susceptible to food shortages associated with their faster growth rates and increased nutritional requirements [4] The first of these theories, the Trivers-Willard Model (TWM) is frequently cited, and is based on the premise that reproductive success is realised in the sex with the greater reproductive returns. Often discussed in terms of birth sex ratios and sex allocation (the resources invested in offspring), may support the TWM, as there are greater opportunities for sexspecific maternal investment. This is a topic of considerable discussion [12,13,14,15,16,17,18]
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