A few species do not fall easily into either of these groups. These include the pig, where the piglets are well developed and mobile, but have poor thermoregulatory abilities and are born into a large litter. The sow, therefore, builds a large nest to group the piglets together and keep them warm. Sows are extremely passive and non-responsive mothers (Fig. 2), which may reduce piglet deaths from maternal crushing. They also have an unusual lactation pattern, where milk ejection occurs only for a few seconds each suckle, and the piglets are called to the udder before milk ejection by a specific vocalization a suckling grunt. Other intermediate groups include primates, with relatively developed semiprecocial offspring that have limited locomotor abilities. An important maternal behavior in these species is carrying of the young, which may facilitate observational learning.
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