Janko Skok

Growth of piglets in relation to milk intake and anatomical location of mammary glands

Abstract The quantity of milk consumed by piglets was estimated with the aid of the ‘weigh–suckle–weigh’ method. During the four weeks of lactation we closely observed the changes in the quantity of milk taken in by piglets with respect to the defined area of the mammary complex (anterior (A), middle (M) and posterior (P)). Daily milk production was estimated, and comparison was made between the growth rate of the piglets and their birth weight (light, heavy). Ten sows (Landrace×Large White) with their suckled litters (9.8±1.9 piglets) were evaluated in the study. There were no significant differences between the A and M areas in the quantity of milk consumed by the piglets, yet both areas differed significantly from the P area (p<0.05). During the entire period of lactation, piglets that suckled at the cranial (A and M area) teats took in more milk and consequently achieved higher daily weight gain than the piglets positioned in the P area. The body mass growth of piglets was positively and significantly correlated with gland milk yield during lactation stages (p<0.05).

Being a weakling and surviving: Keep the fittest siblings close-by when eating

Weaklings serve as a buffer, allowing an increase in the overall prospect of offspring survival. Whereas birds are able to directly determine which offspring to invest in via selective feeding, multiparous mammals have fewer methods of excluding weaklings from milk provision. In pigs the maternal investment strategy is based on the vigorousness of the progeny, whereby weaklings can suffer hunger due to an inability to sufficiently stimulate the mammary gland. However, the vigorous massage might stimulate higher milk production in the surrounding glands and it is possible that a weaklings survivability is conditioned also by suckling at a position adjacent to heaviest littermate. Results of the present study have confirmed this prediction, showing that the surviving weaklings tended to have the heaviest littermates as nearest neighbours during suckling. It appears, therefore, that weaklings which establish a type of commensal, rather than competitive, relationship with strongest siblings might increase their survivability.

A note on precise tracking of suckling position by piglets

Piglets establish a relatively stable teat order after the first week of lactation, when each piglet chooses a preferred teat or teat pair where they tend to suckle permanently. The mechanism underlying the choice that leads each piglet to the same suckling position is still not clarified. In the present case study, piglets with different preferred suckling positions (anterior, posterior) were transferred to a foster sow after completion of the regular 4 weeks of lactation by their biological mother. For both sows, which were identical based on parity, litter size and farrowing date, we examined the suckling positions of the experimental piglets. Piglets occupied exactly the same suckling positions on both sows. Thus, piglets precisely track positions on the mammary complex of a sows udder even when previously learned discriminable stimuli have been changed, such as odours, tastes, morphologies (of sows udder or the sow itself), vocalizations, and neighbouring littermates. Our observation indicates that the spatial configuration of the mammary complex, which is determined by two parallel rows of teats arranged in pairs, can hypothetically be proposed as an important cue for piglet suckling orientation. Therefore, additional attention should be given to the exact spatial position of piglets (and vacant positions) on the mammary complex of biological and foster mothers in order to reduce stress from cross-fostering, which is standard practice in contemporary piglet production.

On the presence and absence of suckling order in polytocous mammals

Mammals have developed a variety of suckling behaviours ranging from tenacious nipple attachment in some rodents and marsupials to once-a-day suckling in rabbit. However, a common feature of suckling that was found in many mammals is the suckling order, or a partial preference to suckle a particular teat (teat fidelity) or part of the udder (suckling preference). A lack of suckling order is observed only in a few mammals. In this article, the possible background of the presence or absence of suckling order in eutherian polytocous mammals is discussed either from the maternal investment or sibling competition point of view. Characteristics related to maternal investment in species in which the suckling order has already been studied at least partially, were classified using C4.5 algorithm (J48 classifier in Weka 3.8.1), and decision tree was built. In the context of sibling competition, an extensive form game (game theory) was predicted to show the optimal suckling strategy considering the basic relations among littermates in two situations (littermates of equal strength/dominance and littermates with different strength/dominance). Although no ultimate conclusion can be drawn, it appears that the suckling order is typical for species whose reproductive system requires a lower maternal investment (up to one litter/year, monogamy, biparental care, lower litter birth weight); and, it appears that the suckling order is inherent to the weaker (inferior) siblings.