Vocalizations around the time of milk ejection in domestic piglets: A reliable indicator of their condition?

In some nursings, piglets initiate nose contacts with their mother and emit typical 'croaking' vocalizations. We examined whether the croaking vocaliz ations reflect the condition of the piglets and whether the sows increase t heir maternal investments in response to those vocalizations. The following predictions were tested: (i) Piglets with lower weight gain should Vocaliz e more than piglets with higher weight gain; (ii) piglets' milk intake is l ower in those nursings in which they vocalize after milk ejection; (iii) pi glets make more croaking vocalization in nursings which were preceded by lo nger intervals since the last nursing with milk ejection; (iv) sows permit longer post-ejection massage in nursings in which piglets vocalized immedia tely after milk ejection; (v) sows decrease the interval until the succeedi ng nursing after those nursings in which the piglets have vocalized. Hypoth eses (i) and (ii) were investigated by controlling the inter-nursing interv als in 14 sows and recording the milk intake of individual piglets' over 24 hours during days 7 or 8 post partum (Experiment 1). Hypotheses (iii) to ( v) were examined through analysing video recordings of undisturbed six h nu rsing sequences in 29 sows (Experiments 2 and 3). The majority of our predi ctions were not confirmed: piglets did not vocalize more (either before or after milk ejection) after longer intervals since last milk ejection; they did not vocalize more in nursing in which they received less milk; and it w as not the piglets or the litter with a lower milk intake or lower weightga in that emitted more vocalization. Neither of our predictions regarding the influence of croaking vocalizations on maternal investment was confirmed. The vocalizations were in no way associated with the length of the followin g inter-nursing interval or with the permission of longer udder massage. To conclude, piglet croaking vocalizations during nursings are not reliable i ndicators of piglet condition and are not used by sows to adjust their mate rnal investment.