The snorts, whinnies and nickers exchanged by mares and their foals have been scrutinized in wild horses in the United States, with researchers finding the chances of offspring survival are better when the mothers are more communicative.
The findings of the study, published in the open-access journal PLOS ONE, highlight the importance on multiple levels of the communication between mares and foals.
And, in what will come as little surprise to human mums, foals perhaps don’t always listen to their mothers.
When mares initiated communication, the outcome was more likely to be no change in activity or distance between the mare and foal. However, when the foals had something to say, there was more likely to be suckling, or a decrease in the distance between the two.
The research by Cassandra Nuñez, of the University of Memphis in Tennessee, and Daniel Rubenstein, of Princeton University in New Jersey, also provides important insights into the meaning of mare-foal communications.
They point out that acoustic signaling plays an important role in mother-offspring recognition and subsequent bond-formation.
“It remains unclear, however, if mothers and offspring use acoustic signaling in the same ways and for the same reasons throughout the juvenile stage, particularly after mutual recognition has been adequately established.”
They note that, despite its crucial role in mother-offspring bond formation, there is a lack of research explicitly linking mother-infant communication to offspring survival.
For their study, they examined the communicative patterns of mothers and their foals during their first year of development among free-living horses on Shackleford Banks, a 15km long barrier island about 3km off the coast of North Carolina.
They observed 34 females and their 45 foals, constituting 63%, 65%, and 93% of the mare-foal pairs present in 1995, 1996, and 1997, respectively.
Observations were made on foot across the entire island, with the pair identifying 10 bands of horses in 1995, 18 bands in 1996, and 11 in 1997.
The pair recorded 956 communication events during the first year of foal development, 470 (49%) of which were mare-initiated and 486 (51%) were foal-initiated.
The 522 snorts heard during the study constituted 55% of signals, nickers (96) comprised 10% and whinnies (315) made up 33%. The remaining 2% were categorized as either whinny/nickers (a combination of the two sounds) or “other”.
Mares and foals initiated communication at similar rates. Mares had an average rate of 0.497 initiations per hour, while foals had an average rate of 0.499 initiations per hour.
Mares tended to initiate communication at shorter mare-foal distances than did their foals.
Mares were more likely to use snorts — the softest form of communication — than foals.
Foals used louder calls more often than their mothers did. “For example, foals were more likely to use whinnies and, to a lesser extent, nickers than mares were.
Mares seemed to play a more active role with their foals when aged 25 weeks or older, initiating communication more often than their foals did.
Habitat visibility, foal sex, and mare age did not appear to affect whether mares or foals initiated communication.
Nuñez and Rubenstein examined the different types of communication:
Like their mothers, foals used snorts when initiating communication from closer distances. As foals aged, there was a moderate increase in the chances that mares would initiate communication with snorts, while there was a marked increase in the probability that foals would initiate communication with snorts.
While mares were less likely to use snorts in areas of low visibility, it did not seem to affect whether foals used snorts.
There was no effect of foal sex or mare age on the probability of snort use by mares or foals.
Mares were more likely to use nickers at closer distances, but not foals.
The probability that foals used nickers did not seem affected by the initial mare-foal distance.
While the probability that mares used whinnies to initiate communication remained constant throughout foal development, the probability that foals used whinnies to initiate communication decreased with foal age.
Mares tended to use whinnies even more when visibility was limited, while the foals did not.
Overall, communication initiated by the foals was more likely to result in suckling or a decrease in mare-foal distance than mare-initiated communication.
The researchers also found a correlation with foal age, whereby suckling behavior and/or decreases in mare-foal distance were less likely to occur with increasing foal development.
The decreases in mare-foal distances averaged 12.10 meters.
Communication initiated with whinnies were more likely to result in suckling behavior and/or a decrease in mare-foal distance than events initiated with snorts, regardless of who initiated the communication.
On the other hand, communication events initiated with nickers were more likely to result in suckling or decreases to mare-foal distance when used by foals, but not mares.
When mares used nickers to initiate communication, the outcome was more likely to be no change to the animals’ distance or current activity.
“These effects were especially evident during the first 10 weeks of development.”
The researchers found that, even when making adjustments for average suckling rate, average mare-foal distance, and average mare condition, the rates at which mares initiated communication with their foals were linked with offspring survival.
“Foals with mothers that initiated communication at higher rates during the first ten weeks of development survived longer than did foals with mothers that initiated communication at lower rates.
“The fact that foals of more communicative mares were more likely to survive indicates the importance of maternal ‘style’ to offspring survival.”
Rates of mare and foal-initiated communication occurring during 10–20 weeks of age were not linked with foal survival, pointing to the importance of communication in the first 10 weeks of life.
Average suckling rate, average mare-foal distance, and mare age did not predict foal survival to nutritional independence.
The rates at which foals initiated communication did not appear to affect offspring survival at all.
Discussing their findings, Nuñez and Rubenstein said mares and foals differed in how and when they initiated communication during the first year of development.
On average, mares initiated communication at closer mare-foal distances and preferred snorts, while foals initiated communication at farther distances and preferred whinnies and, to a lesser extent, nickers.
Moreover, the outcomes of mare versus foal-initiated communication differed: when mares initiated, the outcome was more likely to be no change to the animals’ activity or distance; conversely, when foals initiated, suckling and/or decreases to the mare-foal distance were more likely to occur.
“Taken together, these results indicate an important difference in the function of mare vs. foal-initiated communication.
“Mares seemed to use communication primarily as a means of balancing maternal attentiveness and foal independence.
“Mares that maintain auditory contact with their foals, even when they are close by, consistently make their position known to the offspring which may enable foals to stray farther, enabling more exploration of the environment and socialization with fellow group members, two factors likely critical to foal survival.
“On the other hand, foals seemed to use communication largely as a way of acquiring proximity to and/or nutrients from their mothers: indicating that for foals, communication functioned to ensure safety and nutritional resources.”
Importantly, the specific signals mares and foals used to initiate communication were associated with the outcomes of communication events: whinnies were more likely than snorts to result in suckling behavior and/or decreases to mare-foal distance, regardless of whether mares or foals initiated communication.
On the other hand, nickers seemed to elicit different outcomes depending on initiator type: while they were more likely to result in no change to the animals’ distance or current activity when used by mares to initiate communication, they were more likely to result in suckling and/or decreases in the mare-foal distance when used by foals.
“The use of nickers by mares and foals in lieu of the signals they more commonly use (snorts and whinnies, respectively) may be indicative of situational ‘urgency’ or the level of mare and foal arousal.”
On the while, mares use softer vocalizations more frequently, which are unlikely to elicit suckling or changes in distance, but may serve to maintain contact.
“On the other hand, foals, and particularly young foals, in need of nutrition and protection from their mothers, are more likely to use nickers and whinnies, signals that more often elicit suckling or decreased mare-foal distance.
“As offspring become more autonomous, their need for maternal proximity and milk decreases. As such, the function of offspring communication may converge with that of their mothers, shifting from resource acquisition to contact maintenance.”
The researchers noted that the association between mare-initiated communication early in development and foal survival to nutritional independence provides evidence that mare behavior exhibited during the juvenile stage can be critical for future benefits as well.
Offspring allowed to play with other foals, interact with elders, and explore their environment are more likely to survive, previous research has shown.
“It may be that more communicative mares are facilitating these experiences for their foals, contributing to their future survival.
“Notably, the fact that mare behavior seems to be more critical than foal behavior regarding foal survival highlights the importance of studying the behavior of both mothers and offspring when examining the mammalian juvenile stage.”
Nuñez CMV, Rubenstein DI (2020) Communication is key: Mother-offspring signaling can affect behavioral responses and offspring survival in feral horses (Equus caballus). PLoS ONE 15(4): e0231343. https://doi.org/10.1371/journal.pone.0231343