Stallions with deeper whinnies pull the mares, French research suggests.
Females in many mammal species use a range of indicators to assess male fertility. The researchers set out to investigate the possibility that stallion whinnies conveyed information to the mare about their arousal and fertility.
They used recorded whinnies from stallions and monitored the reactions of mares in relation to the pitch of the voice.
Alban Lemasson and his colleagues found clear correlations between the acoustic features of the call with arousal and reproduction success.
They found that the lower-pitched the stallion’s voice, the slower his heart beat and the higher his fertility.
“Females from three study groups preferred playbacks of low-pitched voices,” they said.
Click the start button to listen to four different whinnies:
Mares, they said, were attracted by whinny frequencies that appeared to provide them with information on large body size, calmness and high fertility.
There was evidence, too, that large females preferred the voices of larger stallions, the five researchers reported in the open-access peer-reviewed journal, PLoS ONE.
More work was needed to explore the relative importance of the biological interrelationships involved, they said.
The research team said vocal communication was primordially important in the daily social life of a large number of mammal species. “Just by hearing a given voice, some mammals, including humans, are able to assess their degree of familiarity with the caller.”
It was known that some mammals used vocal signals to attract mates.
“Horses are interesting models in this perspective for several reasons,” they said, noting the nature of male-female interactions in their social system and the long-lasting social and breeding relationships formed.
It was noted that, during their lifetime, females were not all equally receptive to males. Older and more experienced mares tended to be more receptive.
“Authors have hypothesized that females’ libido and reproductive efficiency are related to the time spent in contact with a male.”
In captivity, when presented a choice of males, females preferred some males to others, but the question of how the choice was made remained open.
Whinnies are notably exchanged when horses are separated.
The researchers explored whether stallion voices encoded their physiological characteristics, and whether mares display preferences for certain types of voice.
In February 2012, the researchers recorded the calls and heart rates of 15 breeding stallions of various breeds and ages, housed on three different French stud farms – Le Pin, Lamballe, Saint-Lô. A total of 178 whinnies were recorded.
Blood samples were also collected and tested for cortisol and testosterone levels, as the researchers were interested in respectively emotion- and reproduction-related hormones. The scientists already had access to previously measured sperm mobility data in respect of the stallions.
They analysed the data in relation to the known breeding performance of the stallions.
In the other phase of the study, they monitored 40 mares, aged 7 to 27, when the stallions’ whinnies were played to them.
In a series of tests, each mare was placed centrally in an area, and exposed to a lower-pitched whinny from a speaker on one side and a higher-pitched whinny from a speaker on the other side. The researchers noted whether the mares turned their head or moved closer to the source of one whinny or the other.
They found that the mares favored the lower-pitched whinnies by a significant margin.
The researchers noted that the taller stallions possessed lower pitched voices, and that the preference for the low-pitched whinnies was ever greater among the taller mares.
They found that the lower-pitched the stallion’s voice, the slower its heart beat and the higher its reproductive success.
No correlation was found between cortisol and testosterone profiles and sperm quality, or any other physiological and acoustic parameters.
The status of each mare’s oestrus cycle did not influence the strength of their preference for one type of voice, the researchers said. This suggested that the preference was not systematically related to mating or fertility, but may represent a step along the way.
“Indeed, directing its attention and interest towards a particular male is the first indispensable step for an active choice of partner, but not a direct proof of sexual motivation.”
They said that females’ preference for low pitched voices, as found here in mares, was a widespread characteristic, including in humans.
“Our study showed that frequency-related voice features are reliable predictors of male reproduction success,” they concluded.
“However, our study also questions the relevance of different physiological parameters for assessing male reproduction qualities. Here, male reproduction success – measured by artificial inseminations – was not related to sperm quality or hormonal profile.”
Lemasson was joined in the research by Kévin Remeuf, Marie Trabalon, Frédérique Cuir and Martine Hausberger.
Lemasson A, Remeuf K, Trabalon M, Cuir F, Hausberger M (2015) Mares Prefer the Voices of Highly Fertile Stallions. PLoS ONE 10(2): e0118468. doi:10.1371/journal.pone.0118468
The full study can be read here.