BREASTFED babies cry more, laugh less and generally have "more challenging temperaments" than formula-fed infants, a study has found.
But such behaviour is normal, and mothers should learn to cope with it rather than reach for the bottle, according to researchers.
Infant irritability was said to be a natural part of the "dynamic communication" between mothers and babies.
Bottle-feeding was akin to "comfort eating" - producing quieter and apparently more content babies who may be over-nourished and putting on weight too rapidly.
The study rated the temperament of 316 babies at age three months using a 191-item behaviour questionnaire completed by their mothers.
Scores differed between babies that were exclusively breast-fed, bottle-fed or mixed-fed.
In three broad areas, breast and mixed-fed babies were rated as being more difficult to deal with than bottle-fed babies.
The study authors wrote in the online journal Public Library of Science ONE: "In particular, compared to formula-fed infants, breastfed infants were reported to show greater distress, less smiling, laughing and vocalisation, to be slower to calm down following distress or excitement, and more difficult to soothe by care givers."
The findings may help explain why so many mothers give up on breastfeeding after a short time, despite the strong health message that "breast is best" for growing infants.
Department of Health guidelines say mothers should exclusively breastfeed their babies for the first six months after birth.
According to a 2005 survey, three-quarters of new UK mothers start out breastfeeding their babies, but only a third are still doing so after four months.
Study leader Dr Ken Ong, from the Medical Research Council Epidemiology Unit in Cambridge, urged mothers to persevere despite the difficulties - and seek help if they need it.
"There is an overwhelming body of evidence supporting breastfeeding as the normal and most healthy form of infant nutrition, and our findings do not contradict this," he said.
"Bottle-fed babies may appear more content, but research suggests that these infants may be over-nourished and gain weight too quickly. Our findings are essentially similar to other stages of life; people often find that eating is comforting."
He added: "Rather than being put off breastfeeding, parents should have more realistic expectations of normal infant behaviour and should receive better understanding and support to cope with difficult infant behaviours if needed. These approaches could potentially promote successful breastfeeding, because currently many mothers attempt to breastfeed but give up after the first few weeks."
Babies involved in the research were among 1,526 infants enrolled into the Cambridge Baby Growth Study.
Mothers assessed their babies' behaviour at three months using the Revised Infant Behaviour Questionnaire, a recognised method of rating various aspects of infant temperament.
Of the infants, 137 were exclusively breastfed, 88 were exclusively bottle-fed, and 91 were fed with a mixture of formula and breast milk. No differences in behaviour were seen between boys and girls.
The researchers concluded: "Infants who were breastfed or mixed-fed at three months of age were rated by their mothers as having overall more challenging temperaments."
They pointed out that similar behaviour had been widely studied in a variety of bird and mammal species.
"Humans often perceive infant crying as stress, but for infant animals irritability is a normal component of signalling to parents," the authors wrote.
Crying and irritability were likely to act as an "honest signal" of nutritional need during early infancy, when a lot of energy was needed for growth, said the scientists.
Rosie Dodds, senior policy adviser at the National Childbirth Trust, Britain's biggest parents' charity, said: "Mothers and babies may experience starting to breastfeed as demanding or stressful in this society where bottle feeding is seen as the norm and breastfeeding is unfamiliar to many new parents.
"It would be interesting to compare this with countries where almost all babies are breastfed. We often hear from mothers who say that once both they and their baby got the hang of it, breastfeeding was a breeze."
Dr Janine Stockdale, from the Royal College of Midwives, said: "It cannot be presumed that physical signalling by the baby automatically means it is hungry; other evidence suggests that babies engage in sucking behaviour called non-nutritive sucking.
"As breastfeeding mothers are often advised not to use a soother while their baby is learning to breastfeed, this means there is a need to look at babies' behaviours, taking into consideration whether or not a soother was introduced.
"Factoring this type of information into the research would enable us to understand more about the suggested differences between the behaviours of breastfed and bottle-fed babies.
"The evidence needs to be seen in a greater context before we start to draw conclusions on this research and we should continue to do all we can to promote and increase the rates of breastfeeding."