All mammals are born with a sucking reflex - an instinct on which their lives depend - but human babies are unique in that they also need to suck for comfort. Or so it was thought. A new study now shows evidence suggesting that baby zebras can suckle for psychological needs, rather than just for feeding.
|Grévy's zebra foal|
The use of soothers is a sensitive topic amongst parents. Soothers (also known as pacifiers or dummies) comfort babies and help them sleeping, but many parents go through great lengths (and many sleepless nights) to avoid using a soother. These concerned parents may fear their baby will develop crooked teeth, or have problems breastfeeding, or they may simply find soothers unnatural. Regardless of their individual choices, all parents will agree that their baby has a need to suck- whether it’s a soother, a thumb or an old rag.
There are many studies showing that so-called 'non-nutritive sucking' can comfort babies, help them to settle, reduce the risk of sudden infant death syndrome (SIDS) and even increase tolerance to pain. The use of soothers is therefore often recommended in intensive care units for premature babies and sick newborns who sadly may need painful medical procedures. But is ‘comfort sucking’ widespread amongst mammals or a specific evolutionary adaptation of our species?
Scientists previously assumed that the duration and frequency of suckling reflected the energetic needs of the young – the longer an infant spent suckling, the more milk it drank. But studies in mammals directly measuring infant weight gain and time spent suckling have shown no correlation between the two. So why do babies and mums across so many species invest so much time and energy nursing? One of the reasons is bonding, but there is more to it.
In a new study published in the September issue of the Journal of Zoology, a research team from the Institute of Animal Science and Czech University of Life Sciences, in Prague, Czech Republic, compared suckling behaviour in three zebra species - mountain, plains and Grévy's zebras. These species are closely related but have very different social organisations, so the researchers could ask whether time spent suckling might reflect the social needs of the young.
Mountain and plains zebras live in stable groups, or ‘harems’, of several females, their babies (or foals) and only one male, while Grévy’s zebras prefer to graze on their own and form loose social bonds. Zebras are far from being docile creatures - to defend their position in the harem social hierarchy, mountain and plains zebra females (called mares) take their gloves off and become very aggressive. Mountain zebra mares are especially hostile, and can sometimes even harass unrelated foals. Grévy’s zebras are the least aggressive of the three species, perhaps because of their more solitary nature.
The researchers observed the suckling behaviour of 30 foals of mountain, plains and Grévy’s zebras at the Dvůr Králové Zoo throughout several years. After watching the zebra herds for an impressive total of about 1,500 hours, the results were clear: mountain zebra foals suckled for longer and more frequently, followed by plains and Grévy’s zebras. As mountain zebra herds have the highest aggression rates and Grévy’s zebra the lowest, the team concluded that baby zebras spend more time suckling in species where there is higher social tension.
Because the study was performed on zebras held in captivity - and so all zebras were exposed to the same living conditions - these differences in sucking behaviour can’t be explained by water or food availability, or by a specific adaptation of each species to its unique environment. But the authors of the study are nevertheless cautious about over-interpreting their results:
“I don’t think that the foal initiated suckling to seek comfort and stress reduction. What I think is that the suckling bout duration reflects the psychological needs of the foal rather than the milk transfer. Thus, prolonged suckling can reflect social tension”, says Jan Pluháček, an ethologist at the Institute of Animal Science and leading author in the study.
Previous studies in primates and rodents had shown a link between maternal care and social organisation, but Pluháček and colleagues provide new evidence suggesting that, at least in zebras, suckling may not only be a means for the young to feed or to bond with mum- it could also reflect their psychological needs.
“We suppose that when any tension in the herd occurs then the young try to stay for longer with the mother, and longer suckling can strengthen the bond between young and mother in all mammalian species” says Pluháček “[…] in zebras 98% of suckling is initiated by the foal. So it’s the foal who seeks soothing via suckling.”
Pluháček J. et al. (2013). Time spent suckling is affected by different social organization in three zebra species, Journal of Zoology, DOI: 10.1111/jzo.12077