Why do mares get moody?

By Gayle Hallowell

23 February 2010 11:06

Horses reach puberty (the age they can conceive) between 12 and 24 months of age – fillies born late in the season may reach puberty at an earlier age than those born early.

Most horses generally have reproductive cycles throughout the spring, summer and early autumn (called long-day seasonal breeders) but, for some, the cycles can be all year round.

During this time, the two female hormones, oestrogen and progesterone, change and this can make the mare behave out of character (moody!).

For around five to six days of each cycle (which lasts 21 days) the mare is said to be in oestrus or in season. During this phase, eggs are maturing and
are released from the ovaries so they can be fertilised, and the mare is receptive to the stallion.

Mares or fillies in season may regularly whinny and/or squeal in addition to winking or squirting urine at other horses or people. Some mares turn their rear end towards approaching horses (and people) when they are in season, which is a sign to stallions that they’d like him to approach.

Some mares may even jump out of fields to be with other horses during this part of their cycle.

At the time of ovulation, there can be some mild discomfort or a change in behaviour (quieter, grumpier) as the release of the egg may result in ovarian tenderness. Whether this is the cause of mares being difficult to ride or saddle is unknown.

Oestrogen concentrations start to increase about seven days before mares come into season, peak during their season and are responsible for much of the above behaviour.

This hormone, in excess and on its own, is thought to be associated with depression and anxiety.

Another hormone, oxytocin, is released in response to approach of the stallion or his call. This is produced by a particular area of the brain and helps prepare the mare to accept the stallion.

In addition, it also causes the uterus (womb) to contract slightly, pushing the sperm towards the egg. This hormone may also be released in some mares inappropriately by the approach of other animals and people.

Occasionally, this contraction can cause the mare mild discomfort. So the behaviour your mare is showing when she is in season is completely normal, although it can be annoying, especially as other horses may react to it and lash out.

However, some mares, particularly young, nervous or excitable animals, may be reluctant to show signs of oestrus and, by suppressing this behaviour, can appear withdrawn.

They may segregate themselves from other horses in the field and become unwilling to work when they are normally keen to do so. These mares may lash out with teeth or hooves at approaching animals – including people!

Another time that oestrus behaviour may be suppressed is in mares with foals at foot or if they are separated from their foals – this may be due to stress responses associated with separation anxiety or protectiveness towards the foal.

After mares ovulate, oestrogen concentrations fall and another hormone, progesterone, takes over. At this stage, the mare’s body is preparing for the potential impending pregnancy and she will reject the stallion.

Behaviour for the next two- to two-and-a-half weeks tends to be calmer and more predictable.

Progesterone can have varying effects on behaviour – at normal concentrations, it’s thought to have antidepressant and anti-anxiety effects and is a natural mild sedative. However, if produced in excess, it can cause depression and irritability.