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discourage allo-suckling by targeted aggressive behavior (Olsen
et al.
, 1998). This process
needs to be taken into account when cross-fostering of piglets is performed. Similarly,
piglets also learn the olfactory identity of their mother by early exposure to her odor
(Horrell and Hodgson, 1992).
Moreover, mutual recognition is possible through the acoustic channel. Individual sows
differ both in the absolute rates of grunting and in frequency composition of individual
grunts (Schön
et al.
, 1999; Špinka
et al.
, 2002). These features can serve as acoustic
signatures of individual sows. Piglets are thus able to find their mother quickly even
when several sows are grunting at the same time due to nursing synchronization. By
timely arrival to the udder, own piglets are able to fight off allo-sucklers and to consume
the milk. Piglets within the same litter share similarities in the acoustic qualities of their
calls (Illmann
et al.
, 2002) and sows use these cues to attend preferentially to their own
piglets, e.g. when they get isolated from her.
Two other features in nursing behavior may also have the role of reducing the incidence
of allo-suckling; namely, nursing synchronization and non-nutritive nursings. There
exists an interesting interaction between nursing synchronization, occurrence of non-
nutritive nursings and allo-suckling. Most nursings are synchronized and in these nursing
allo-suckling is hindered by the concurrent occurrence of milk ejections. Interestingly,
when nursings are not synchronized, the proportion of non-nutritive nursings becomes
greater (Illmann
et al.
, 2005), indicating that nursing synchronization and the fact of not
releasing milk during non-synchronized nursings are two complementary strategies that
lactating sows use to reduce allo-suckling.
13.4.5
Gradual weaning during lactation?
It was proposed that the process of weaning, in the broad meaning of gradual reduction
of the maternal milk investment, already starts within days after birth and continues
throughout lactation (Drake
et al.
, 2008; Puppe and Tuchscherer, 2000). This process
was interpreted as a case of parent-offspring conflict in which the progeny attempt
to withdraw more resources than the parents are prepared to give because excessive
investment into the current young may endanger future reproduction by the parent.
There are indeed prominent changes in nursing behavior over lactation. In early lactation,
almost all nursings are initiated by the sow and almost none are terminated by her. Thus,
the sow actively promotes milk transfer and offers unlimited opportunity for the litter to
massage the teats during their first days of postnatal life. However, already by the second
week of lactation, only half of the nursings are sow-initiated and by week 4 the proportion
drops to near zero. In parallel, the proportion of sow-terminated nursings increases to
50-100% by week 4. There is therefore an almost absolute reversal of roles between very
early and later in lactation. However, developments of other aspects of nursing behavior
do not indicate a clear offspring-dam conflict or weaning process as lactation advances.
Nursing frequency declines by only about 10% between the 1
st
and 4
th
week of lactation
(Puppe and Tuchscherer, 2000; Valros
et al.
, 2002). Non-nutritive nursings, which are
a sign of conflict between the mother and the litter over milk output, seem to peak at
week 2-3 and then decline, instead of increasing continuously as would be expected if
