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19th January 2009


A New Zealand study on wild horses has revealed the changing condition of the mare at conception may play a major part in determining the sex of the foal. The study found that 97 per cent of mares losing condition at conception gave birth to fillies, and 80 per cent of mares gaining condition at conception gave birth to colts.


In a six year study funded by the New Zealand Department of Conservation, researchers Dr Elissa Cameron and Dr Wayne Linklater found that the change in condition of the mare around the time of conception, as opposed to her actual condition, better predicted the sex of the foal.

The researchers, who were students in the Ecology Group of the Institute of Natural Resources at Massey University, were studying wild horses in the Kaimanawa Mountains and surrounding areas in the central North Island of New Zealand, between August 1994 and 1999.

Dr Cameron, in a previous study for her PhD thesis, set out to test the expectation that more males should be born to better-conditioned mares.

The primary motivation for the initial research on the heard was to trail remotely delivered contraceptives to control herd growth.

Dr Linklater said the subpopulation in the Moawhango River basin area was the focus of the study.

“There were few difficulties locating the horses in the wild, as there was a decent number, and they were loyal to their home ranges. Once you know which area a group is in, it’s actually pretty easy to find them,” Dr Linklater said.

“More than 400 horses were individually identifiable by either freeze brands on their rumps or natural markings.”

Research Method

Body condition scores (BCS) were estimated by visual body fat distribution on an 11-point scale (0-5 with 0.5 graduations) with the aid of strong binoculars or a telescope. Scores were recorded every time a horse was seen, provided visibility was good.

Body condition at conception was calculated by backdating from the date of foaling (accurate to + or – 5 days) by the average gestation length (336+ or – 10 days), and pre-conception (conception -20 + or – 10 days) to post conception (conception +20 + or – 10 days).

Only mares whose BCS had been recorded at least twice during each 20-day period were used in the analysis, and the sample size was 118 births. The BCS of the mares ranged from1 to 3.5, with an average of 2.5. The scores were compared before and after conception to measure the change in condition in each mare.

Research Results


When mares were categorized by whether they were losing condition, maintaining condition or gaining condition around conception, the results were striking. If gaining condition 80 per cent gave birth to a colt, but if losing condition at conception only 3 per cent gave birth to a colt.

There was no difference in foaling rates in relation to condition a conception. This makes it unlikely that the results could be explained by differential loss of the more costly sex during gestation, particularly since the sex ratio deviates significantly in both directions- towards sons when females are gaining condition and towards daughters when females are losing condition.

“Wild horses are good species for this sort of study,” Dr Linklater said.

“The mares are not supplementary fed and go through annual condition cycles, and there is only one offspring. Animals with multiple offspring make such a study very difficult.

Foals are easy to spot because horses don’t hide their newborns. Cattle and deer often take their offspring into hiding but wild horses tend to be nervous in cover.” Dr Linklater said.

“Visual body scoring also works well with horses as their fat is spread all over the body, unlike cattle and sheep which tend to concentrate fat around their internal organs.”

Previous studies into variation in the production of males and females have often produced inconsistent results.

One theory to explain these results is the Trivers-Willard hypothesis, which states that:

– Mothers in good condition with more resources to invest would be advantaged by producing sons, as highly competitive sons would out-compete highly competitive daughters, who are constrained to lower reproductive rate

– Mothers with fewer resources to invest would be advantaged by producing a daughter, as a daughter would out-reproduce an unsuccessful son.

“So our results were not surprising, as the theory predicts what we found. It is delightful when that happens,” Dr Linklater said.

Further Study

Dr Cameron has also hypothesized that glucose levels play a part in sex ratio, and a year-long study into the relationship between the sex of the offspring and glucose is about to start, using sheep. The process is simple and non-invasive, with the glucose being inserted into the uterus via a catheter in the cervix.

Rich feed and stress can cause an elevation in the glucose levels in animals. Add to this the glucose-birth sex link and there could be major repercussions for all livestock farmers, particularly horse breeders, many of whom feed rich supplements to their broodmares.

“If an early embryo gets to much glucose, it kills the females,” Dr Linklater said.

“We would love to investigate what drives the relationship in horses. We are confident that the pattern occurs, but why?”

According to Dr Linklater, with this glucose link established a shift in the birth ration by five to ten per cent may be possible, by “what you feed them, when you feed the.”

Cameron Ez et al (1999). Birth sex ratios relate to mares condition at conception in Kaimanawa horses. Behavioural Ecology Vol. 10 No. 5: 472-475
Taken from the Equine Research News, December 2008 www.rirdc.gov.au