Olfactory imprinting in sheep

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Imprinting is most often used to describe an early-life bond that can later affect an animal's mate choice. More broadly, the term refers to a rapid and selective learning process that only can occur during certain times in an animal's life. In sheep, ewes having just given birth imprint onto their lambs on the basis of olfactory cues, allowing mothers to distinguish their own offspring from other lambs in the flock. This olfactory-based imprinting is dependent on a ewe's behavior after giving birth, on the presence of amniotic fluid, and on a specialized odor-influenced learning process which allows the ewe to quickly memorize the smell of her offspring, to whom she then forms an exclusive maternal bond.

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Bonding and selective attachment

Some species of mammals show selective maternal bond between mother and infant. This type of bonding is characterized by an exclusive relationship; selective mothers will not provide maternal care to young with which they have not formed a bond. [1]

Domesticated sheep, Ovis aries, are one such mammal, forming a strong and exclusive bond with their young. In sheep, olfaction, the sense of smell, plays a vital role in the establishment of the exclusive bond, though other senses, particularly sight and to an extent hearing, are involved as well. [2] This bonding process appears to operate in the same fashion in wild and feral sheep populations (Genus ‘’Ovis’’, various species) as in domesticated breeds, though wild and feral populations have received less study. [3]

Sheep are ideal for studying attachment because there are ethical and logistic difficulties that limit laboratory and research use of most other species known to develop selective mother-young social relationships, namely seals, primates, and other ungulates. Additionally, domestic sheep are common worldwide, easily bred and handled, and well understood in terms of behavior and natural history, providing a solid base for more intricate study. [1] Olfaction plays a role in responsiveness, as most non-maternal animals find the smell of infants and amniotic fluid repulsive, while in maternal animals, those with the appropriate hormone profiles, these same smells increase maternal responsiveness. [4]

Maternal care is rarely selective, particularly outside of ruminant species. Most animals displaying maternal instincts will care for any infant introduced to the nest. In contrast to this, selective mothers form bonds to specific individual young and will show maternal responsiveness only to these young. In sheep, young to whom the ewe is not maternally bonded will be received with aggression, generally head-butting and turning in circles away from the lamb. [5] Another feature of this maternally selective bond is that it is difficult to break, and very difficult to establish new bonds outside of the period immediately surrounding parturition. [6]

Behavioral ecology

In natural populations, selective attachment in parenting appears to emerge as a result of the combination of the mobility of the offspring and the social structure of the species. Natural selection is assumed to favor the evolution of mechanisms ensuring that the correct offspring receive the (often costly) parental investment involved in rearing. Sheep are fully mobile shortly after birth and young are likely to mix with unrelated offspring at an early age due to the flock social structure, so the ability to identify and preferentially interact with one's own offspring is crucial for the correct allocation of maternal resources, principally milk. This is commonly seen in other ungulates and also in seals and primates, all species where the chance of misdirecting parental care is substantial. [1]

Maternal care: Responsiveness and selectivity

Maternal care in sheep can be seen as composed of two components – selectivity and responsiveness. Maternal responsiveness can be observed broadly across species, mammalian and otherwise. This is the drive for a mother to provide care for her young. Maternal responsiveness in sheep is characterized by allowing lambs to nurse, by an absence of aggressive behavior, anxiety upon removal of their young and by low-pitched, close-mouthed vocalizations or “rumbles”. These rumbles calm and quiet the lamb, and serve as an auditory signature as the lamb ages, a cue by which lambs can identify their own mothers. [4] Olfaction plays a role in responsiveness, as most non-maternal animals find the smell of infants and amniotic fluid repellant, while in maternal animals, those with the hormone profiles which occur in late pregnancy, these same smells increase maternal responsiveness. [5]

Maternal care is rarely selective, particularly outside of ruminant species. Most animals displaying maternal instincts will care for any infant introduced to the nest. In contrast to this, selective mothers form bonds to specific individual young and will show maternal responsiveness only to these young. In sheep, young to whom the ewe is not maternally bonded will be received with aggression, generally head-butting and turning in circles away from the lamb. Another feature of this maternally selective bond is that it is difficult to break, and very difficult to establish new bonds outside of the period immediately surrounding parturition. [6]

Behaviors important for olfactory imprinting

The expression of maternal behavior in sheep follows a fairly rigid pattern in the period leading up to and following birth, a large part of which involves licking and sniffing. These licking and sniffing behaviors expose the ewe to the smell of the lamb and the amniotic fluid in which it is covered. During the period leading up to parturition, ewes will display maternal responsiveness and an attraction to amniotic fluid. After giving birth, the ewe will lick her lambs clean of fluid and membranes. Only a few minutes of this licking behavior is required for some ewes to butt away a foreign lamb, and most will show selectivity and aggression toward foreign lambs within 2 to 4 hours, but complete selectivity can take around 24 hours to develop. [7] [8]

The role of amniotic fluid

The presence of amniotic fluid is important for proper acceptance and selective bonding. It is not only the natural smell of the lamb that elicits maternal response and moves forward the bonding process, but the smell of the amniotic fluid as well. In this way, amniotic fluid plays a dual function, with a role in both the development of responsiveness and selectivity. The presence of amniotic fluid is particularly important for ewes with no maternal experience. If a lamb that has been washed with water is presented to an inexperienced ewe, the ewe will not accept the lamb at her udder and will display aggressive behaviors toward the lamb. This can also occur with washed lambs and experienced dams, but the occurrence is not as reliable. [8]

Ewes will generally display stronger attraction to their own amniotic fluid than to that of another ewe, [7] though they will display some attraction to the amniotic fluid of any animal of their own species. If a dry alien lamb is smeared with amniotic fluid, an ewe presented with that lamb is more likely to display aggressive behavior if the lamb is smeared with the amniotic fluid of an alien ewe, compared to if it is smeared with that ewe's own fluid. It has been speculated that this is due to some olfactory signature of the ewe's amniotic fluid, though whether this is a response innate to the ewe or a result of her exposure to and ingestion of amniotic fluid from the ground after rupture of the water bag. [8]

The attraction to amniotic fluid is important during the period of time before the ewe has developed selectivity; the presence of amniotic fluid appears to play a role in the development of this selectivity as well. It was found that depriving newly lambed ewes of access to amniotic fluid during the first 4 hours after giving birth had significant consequences on maternal selectivity as well as responsiveness. The effect on responsiveness appear more severe, with nearly 50% of ewes in one experiment rejecting their own lambs when completely deprived of exposure to amniotic fluid. In those that did accept their own lamb, if their lamb was washed, they were more likely to also accept a washed alien lamb; an ewe's discrimination ability without the presence of amniotic fluid appears to be reduced. The effect of amniotic fluid deprivation appears to come from a combination of the decreased maternal attention paid to an unwashed lamb and the lessened olfactory cues to which the mother can have access through licking. [8]

Involvement of the primary and/or secondary olfactory systems

When lambs and their mothers are physically separated by distance, but the lambs remain unwashed and coated in amniotic fluid, neither the ewe's acceptance of her lamb nor her selectivity against alien lambs was affected. [8] This result supports the fairly well established idea that the compounds in amniotic fluid are volatile and perceived by olfaction. A series of studies in which ewes have been rendered anosmic, or incapable of smell, provide evidence for this. However, whether the primary olfactory system, the accessory olfactory system or both are responsible for the establishment of these responsive and selective maternal behaviors remains somewhat unclear.

In one 2006 study, ewes whose primary olfactory systems were incapacitated by nasal irrigation with zinc-sulfate procaine solution displayed rejection behaviors toward foreign lambs to the same degree as did control ewes. Ewes whose vomeronasal organs were rendered non-functional by electro-cauterization failed to reject alien lambs at the udder, and were noted to perform more maternal flehmens, a behavior related to vomeronasal activity. The lack of vomeronasal function was confirmed based on the formation of complete mucosal scarring over both oral and nasal openings of the nasoincisive duct. This suggests that the vomeronasal organ plays an active role in maternal identification of the lamb at the udder [9]

In contrast, a 1995 study [10] examined ewes under a very similar setup, with different results. Ewes were made anosmic by irrigation of the nostrils with a zinc sulfate and procaine solution. For the group in which vomeronasal perception was rendered inactive, the vomeronasal nerves only were severed; this was later confirmed by use of both an anterograde tracer and postmortem examination of the accessory olfactory bulb. This experiment found that vomeronasal-lesioned animals showed little difference from controls in both responsiveness and selectivity. The ewes whose primary olfactory system was disabled were impaired in responsiveness if they were inexperienced, showing a delay in maternal responsiveness, which was not seen in experienced ewes. In both inexperienced and experienced ewes, selective behavior was disrupted. [10]

One study sought to examine compounds in lamb wool that could contribute to this individual olfactory signature. While a number of compounds were isolated, a synthetic reproduction of these substances was not sufficient to “trick” a ewe into accepting an alien lamb. [11]

Neurobiological underpinnings

The mechanical stimulation of the ewe's vagina and cervix by expulsion of the fetus induces a neural release of oxytocin in the paraventricular nucleus of the hypothalamus, as well as in the bed nucleus of the stria terminalis, medial preoptic area and the olfactory bulb. It appears that this oxytocin release indirectly primes the main olfactory bulb such that this region may optimally respond to cues from the newborn lamb. [6]

The ewe's learning of her lamb's odor involves synaptic changes within the olfactory bulb. Electrophysiological recordings from olfactory bulb mitral cells of a recently lambed ewe show these cells respond preferentially to general lamb odors, and a subset respond preferentially to the odor of a ewe's own lamb. [6] These mitral cells become increasingly responsive to the learned odor, and this increased response stimulates increased release of glutamate and GABA between these excitatory mitral cells and inhibitory granule cells. [12]

Pharmacological studies using the centrally acting muscarinic antagonist scopolamine inhibited lamb recognition, which may be due to a blockade of muscarinic receptors in projection areas of basal forebrain cholinergic neurons. It has been suggested that olfactory recognition of the lamb could be dependent on cholinergic neurons of the horizontal limb of the diagonal band of Broca projecting to olfactory targets. This does appear to be the case, as lesions of the nucleus basalis did severely impair olfactory recognition of the lamb – these ewes were not selective against foreign lambs, nor were they as apt at identifying their own lamb. [13]

Additional olfactory structures for which there is evidence for action on the establishment and display of maternal selectivity include the secondary and tertiary olfactory processing regions (piriform cortex, medial and cortical nuclei of the amygdala; orbitofrontal and frontal medial cortex, and entorhinal cortex). [6]

Related Research Articles

Vomeronasal organ Smell sense organ above the roof of the mouth

The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth in various tetrapods. The name is derived from the fact that it lies adjacent to the unpaired vomer bone in the nasal septum. It is present and functional in all snakes and lizards, and in many mammals, including cats, dogs, cattle, pigs, and some primates. Some humans may have physical remnants of a VNO, but it is vestigial and non-functional.

Flehmen response Behavior in which an animal curls back its upper lip exposing its front teeth

The flehmen response, also called the flehmen position, flehmen reaction, flehmen grimace, flehming, or flehmening, is a behavior in which an animal curls back its upper lip exposing its front teeth, inhales with the nostrils usually closed, and then often holds this position for several seconds. It may be performed over a sight or substance of particular interest to the animal, or may be performed with the neck stretched and the head held high in the air.

Olfactory bulb Neural structure

The olfactory bulb is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus where it plays a role in emotion, memory and learning. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. The accessory olfactory bulb resides on the dorsal-posterior region of the main olfactory bulb and forms a parallel pathway. Destruction of the olfactory bulb results in ipsilateral anosmia, while irritative lesions of the uncus can result in olfactory and gustatory hallucinations.

Olfactory system Sensory system used for smelling

The olfactory system, or sense of smell, is the sensory system used for smelling (olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main olfactory system and an accessory olfactory system. The main olfactory system detects airborne substances, while the accessory system senses fluid-phase stimuli.

Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants which give rise to the sense of smell. Activated olfactory receptors trigger nerve impulses which transmit information about odor to the brain. These receptors are members of the class A rhodopsin-like family of G protein-coupled receptors (GPCRs). The olfactory receptors form a multigene family consisting of around 800 genes in humans and 1400 genes in mice.

Body odor is present in all animals and its intensity can be influenced by many factors. Body odor has a strong genetic basis, but can also be strongly influenced by various diseases and physiological conditions. Though body odor has played an important role in early humankind, it is generally considered to be an unpleasant odor amongst many human cultures.

Licking Action of passing the tongue over a surface

Licking is the action of passing the tongue over a surface, typically either to deposit saliva onto the surface, or to collect liquid, food or minerals onto the tongue for ingestion, or to communicate with other animals. Many animals both groom themselves and eat or drink by licking.

Androstenone Chemical compound

Androstenone (5α-androst-16-en-3-one) is a 16-androstene class steroidal pheromone. It is found in boar's saliva, celery cytoplasm, and truffle fungus. Androstenone was the first mammalian pheromone to be identified. It is found in high concentrations in the saliva of male pigs, and, when inhaled by a female pig that is in heat, results in the female assuming the mating stance. Androstenone is the active ingredient in 'Boarmate', a commercial product made by DuPont sold to pig farmers to test sows for timing of artificial insemination.

Rhinarium

The rhinarium is the furless skin surface surrounding the external openings of the nostrils in many mammals. Commonly it is referred to as the tip of the snout, and breeders of cats and dogs sometimes use the term nose leather. Informally, it may be called a "truffle", "wet snout" or "wet nose," because its surface is moist in some species: for example, healthy dogs and cats.

Olfactory tubercle Area at the bottom of the forebrain

The olfactory tubercle (OT), also known as the tuberculum olfactorium, is a multi-sensory processing center that is contained within the olfactory cortex and ventral striatum and plays a role in reward cognition. The OT has also been shown to play a role in locomotor and attentional behaviors, particularly in relation to social and sensory responsiveness, and it may be necessary for behavioral flexibility. The OT is interconnected with numerous brain regions, especially the sensory, arousal, and reward centers, thus making it a potentially critical interface between processing of sensory information and the subsequent behavioral responses.

Domestic sheep reproduction Reproduction of sheep

Domestic sheep reproduce sexually much like other mammals, and their reproductive strategy is furthermore very similar to other domestic herd animals. A flock of sheep is generally mated by a single ram, which has either been chosen by a farmer or has established dominance through physical contest with other rams. Most sheep have a breeding season (tupping) in the autumn, though some are able to breed year-round.

Odor Volatile chemical compounds perceived by the sense of smell

An odor or odour is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive by their sense of smell. An odor is also called a "smell" or a "scent", which can refer to either a pleasant or an unpleasant odor.

Sense of smell Sense that detects odors

The sense of smell, or olfaction, is the special sense through which smells are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.

Olfactory memory refers to the recollection of odors. Studies have found various characteristics of common memories of odor memory including persistence and high resistance to interference. Explicit memory is typically the form focused on in the studies of olfactory memory, though implicit forms of memory certainly supply distinct contributions to the understanding of odors and memories of them. Research has demonstrated that the changes to the olfactory bulb and main olfactory system following birth are extremely important and influential for maternal behavior. Mammalian olfactory cues play an important role in the coordination of the mother infant bond, and the following normal development of the offspring. Maternal breast odors are individually distinctive, and provide a basis for recognition of the mother by her offspring.

Odour is sensory stimulation of the olfactory membrane of the nose by a group of molecules. Certain body odours are connected to human sexual attraction. Humans can make use of body odour subconsciously to identify whether a potential mate will pass on favourable traits to their offspring. Body odour may provide significant cues about the genetic quality, health and reproductive success of a potential mate. Body odour affects sexual attraction in a number of ways including through human biology, the menstrual cycle and fluctuating asymmetry. The olfactory membrane plays a role in smelling and subconsciously assessing another human's pheromones. It also affects the sexual attraction of insects and mammals. The major histocompatibility complex genes are important for the immune system, and appear to play a role in sexual attraction via body odour. Studies have shown that body odor is strongly connected with attraction in heterosexual females. The women in one study ranked body odor as more important for attraction than “looks”. Humans may not simply depend on visual and verbal senses to be attracted to a possible partner/mate.

Parental brain

Parental experience, as well as changing hormone levels during pregnancy and postpartum, cause changes in the parental brain. Displaying maternal sensitivity towards infant cues, processing those cues and being motivated to engage socially with her infant and attend to the infant's needs in any context could be described as mothering behavior and is regulated by many systems in the maternal brain. Research has shown that hormones such as oxytocin, prolactin, estradiol and progesterone are essential for the onset and the maintenance of maternal behavior in rats, and other mammals as well. Mothering behavior has also been classified within the basic drives. Less is known about the paternal brain, but changes in the father's brain occur alongside the mother once the offspring is born.

Sniffing is a perceptually-relevant behavior, defined as the active sampling of odors through the nasal cavity for the purpose of information acquisition. This behavior, displayed by all terrestrial vertebrates, is typically identified based upon changes in respiratory frequency and/or amplitude, and is often studied in the context of odor guided behaviors and olfactory perceptual tasks. Sniffing is quantified by measuring intra-nasal pressure or flow or air or, while less accurate, through a strain gauge on the chest to measure total respiratory volume. Strategies for sniffing behavior vary depending upon the animal, with small animals displaying sniffing frequencies ranging from 4 to 12 Hz but larger animals (humans) sniffing at much lower frequencies, usually less than 2 Hz. Subserving sniffing behaviors, evidence for an "olfactomotor" circuit in the brain exists, wherein perception or expectation of an odor can trigger brain respiratory center to allow for the modulation of sniffing frequency and amplitude and thus acquisition of odor information. Sniffing is analogous to other stimulus sampling behaviors, including visual saccades, active touch, and whisker movements in small animals. Atypical sniffing has been reported in cases of neurological disorders, especially those disorders characterized by impaired motor function and olfactory perception.

Endocrinology of parenting has been the subject of considerable study with focus both on human females and males and on females and males of other mammalian species. Parenting as an adaptive problem in mammals involves specific endocrine signals that were naturally selected to respond to infant cues and environmental inputs. Infants across species produce a number of cues to inform caregivers of their needs. These include visual cues, like facial characteristics, or in some species smiling, auditory cues, such as vocalizations, olfactory cues, and tactile stimulation. A commonly mentioned hormone in parenting is oxytocin, however many other hormones relay key information that results in variations in behavior. These include estrogen, progesterone, prolactin, cortisol, and testosterone. While hormones are not necessary for the expression of maternal behavior, they may influence it.

Odor molecules are detected by the olfactory receptors in the olfactory epithelium of the nasal cavity. Each receptor type is expressed within a subset of neurons, from which they directly connect to the olfactory bulb in the brain. Olfaction is essential for survival in most vertebrates; however, the degree to which an animal depends on smell is highly varied. Great variation exists in the number of OR genes among vertebrate species, as shown through bioinformatic analyses. This diversity exists by virtue of the wide-ranging environments that they inhabit. For instance, dolphins that are secondarily adapted to an aquatic niche possess a considerably smaller subset of genes than most mammals. OR gene repertoires have also evolved in relation to other senses, as higher primates with well-developed vision systems tend to have a smaller number of OR genes. As such, investigating the evolutionary changes of OR genes can provide useful information on how genomes respond to environmental changes. Differences in smell sensitivity are also dependent on the anatomy of the olfactory apparatus, such as the size of the olfactory bulb and epithelium.

Insect olfaction

Insect olfaction refers to the function of chemical receptors that enable insects to detect and identify volatile compounds for foraging, predator avoidance, finding mating partners and locating oviposition habitats. Thus, it is the most important sensation for insects. Most important insect behaviors must be timed perfectly which is dependent on what they smell and when they smell it. For example, olfaction is essential for locating host plants and hunting prey in many species of insects, such as the moth Deilephila elpenor and the wasp Polybia sericea, respectively.

References

  1. 1 2 3 Nowak, R., Keller, M & Lévy, F., 2011. Mother-young relationships in sheep: a model for a multidisciplinary approach of the study of attachment in mammals. Journal of neuroendocrinology, (1), p.1042-1053.
  2. ↑ Dwyer, C.M., 2008. Genetic and physiological determinants of maternal behavior and lamb survival: implications for low-input sheep management. Journal of Animal Science, 86(14 Suppl), p.E246-58.
  3. ↑ Shackleton, D. & Shank, C., 1984. A review of the social behavior of feral and wild sheep and goats. Journal of Animal Science, 58(2), p.500.
  4. 1 2 Sèbe, F. et al., 2010. Early vocal recognition of mother by lambs: contribution of low- and high-frequency vocalizations. Animal Behaviour, 79(5), p.1055-1066.
  5. 1 2 Lévy, F., Keller, M & Poindron, P, 2004. Olfactory regulation of maternal behavior in mammals. Hormones and behavior, 46(3), p.284-302.
  6. 1 2 3 4 5 Poindron, Pascal, Lévy, F. & Keller, Matthieu, 2007. Maternal responsiveness and maternal selectivity in domestic sheep and goats: the two facets of maternal attachment. Developmental Psychobiology, 49(1), p.54–70.
  7. 1 2 Vince, M.A., 1993. Newborn lambs and their dams: the interaction that leads to sucking. Advances in the Study of Behavior, 22, p.239–268.
  8. 1 2 3 4 5 Poindron, P. et al., 2010. Amniotic fluid is important for the maintenance of maternal responsiveness and the establishment of maternal selectivity in sheep. Animal, 4(12), p.2057-2064.
  9. ↑ Booth, K., 2006. The significance of the vomeronasal organ for offspring recognition in sheep☆. Small Ruminant Research, 62(1-2), p.39-41.
  10. 1 2 Lévy, F. et al., 1995. Involvement of the main but not the accessory olfactory system in maternal behavior of primiparous and multiparous ewes. Physiology & behavior, 57(1), p.97-104.
  11. ↑ Burger, B.V. et al., 2011. Olfactory Cue Mediated Neonatal Recognition in Sheep, Ovis aries. Journal of Chemical Ecology, 37(10), p.1150-63.
  12. ↑ Kendrick, K.M. et al., 1997. Formation of olfactory memories mediated by nitric oxide. Nature, 388(6643), p.670-4.
  13. ↑ Ferreira, G. et al., 2001. Extensive immunolesions of basal forebrain cholinergic system impair offspring recognition in sheep. Neuroscience, 106(1), p.103-16.
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