How come the males and females go in same races. In human athletics the men are much faster and so there are separate events.
Short answer: Human males are typically bigger and stronger than human females, but this is not true for all species (just the opposite, in many cases). In horses, there is apparently little difference between male and female running times, otherwise owners would surely only use the faster of the two (or three, if geldings are counted separately).
There are separate races for fillies and colts, but there are also races for both.
I’ve always been told, in general, colts are faster than fillies. However, you may have a filly that can keep up with a colt for 6 furlongs, but can’t last for a mile-and-a-half. So then you find yourself a 6 furlong race for 3 year olds. It keeps things interesting that way.
Most races are restricted by age and performance level, for example: For 3-year-old California-breds, non-winners of 2, or, For 3-year-olds and up, maidens (nonwinners). Notice there is no restriction on gender–however, such non-gender-specific races are typically filled with males. But–a female would be allowed to run; it’s just uncommon for her to.
Typically, females run in a race restricted to their gender. The example races above would then become "For 3-year-old fillies, California-breds, non-winners of 2." This pattern continues in all classes of races, all the way up to the Grade I/Group I top-class category. However, as before, the fillies and mares nearly always have the opportunity to be entered against males if her owner/trainer chooses. It is uncommon, at least in thoroughbreds.
Like their human counterparts, male horses are typically larger and more muscular than the females. They are usually faster, but it is not always the case. In sprints (under a mile), things seem to be a bit more leveled. I have seen many top class mares beat the tar out of their male counterparts in this category; most recently, Honest Lady was 2nd in the World Championship Breeder’s Cup Sprint in 2000, and Xtra Heat was 2nd in 2001 (and she was the most consistent sprinter of the year).
I actually went to a race last weekend to see my friend’s mare, Kalookan Queen, become the first female winner of the Potrero Grande stakes–a sprint.
An added note: According the The Blood Horse stakes conditions book, there is only one stakes race restricted to males-only: the World Championship Breeder’s Cup Juvenile. Run on World Championship day, there is another race–the Juvenile Fillies–for the females. (These races are for horse racing’s babies: 2-year-olds.)
Horses in general show a low level of sexual dimorphism. You can have a big muscular mare and you’d have to check to make sure she’s female when standing next to a gelding of similar musculature. “Mareishness” is more about behavior that looks.
However, stallions develop muscles a little differently (and quite a few 3 year old stallion colts are running.)
It seems like fillies in general do not run with the colts although for the most part they are allowed to. It is inaccurate to say that fillies can’t run the longer distances – 3 fillies have won the Kentucky Derby – Regret (in the 1950s) Genuine Risk (in the 1980s) and Winning Colors (in the early 1990s).
A filly named Ruffian was considered one of the best racehorses ever bred until her untimely death.
Absolutely, Hello Again. Females can and do beat males in the more classic, longer distances, but it is much less common than at the shorter distances.
(BTW, Regret won the Kentucky Derby in 1915, Genuine Risk in 1980, and Winning Colors in 1989.)
It is also notable that sometimes the male-dominating fillies are enormous, masculine-appearing creatures, such as Winning Colors, or my friend’s filly, Kalookan Queen–as well as my mentioned screenname sake. Other times, however, the filly is diminutive and decidedly feminine and still overcomes her physical stature to beat the stuffing out of males. The best example of this I can think of is Lady’s Secret, who so dominated the males in 1985 that she is one of the very few mares to be named Horse of the Year.
It’s always great to meet another horse racing fan, Hello! Is there a track you frequent?
It seems to be easier for a top class filly to beat the colts over middle distances in Europe than it does in the US.
The principle all-aged race in the UK over 12 furlongs is the Grade 1 King George VI & Queen Elizabeth Stakes run at Ascot in July. This race was won by a filly in 1966 (Aunt Edith), 1969 (Park Top), 1973 & 1974 (Dahlia), 1976 (Pawneese) and 1983 (Time Charter).
Dahlia is the only horse to win this race twice, and this filly collected at least one other middle distance Grade 1 prize against the colts when she was in her prime.
France’s premier all-aged race over 12 furlongs is the Grade 1 Prix de l’Arc de Triomphe, run at Longchamp in October.
Fillies have taken this race on many occasions, most notably 8 times in 12 years betweeen 1972 and 1983, and 5 times hand running between 1979 and 1983.
In Europe, there is a weight for sex allowance which varies between 3lb and 7lb. For example, in the English Derby a colt will carry 9st. whereas a filly would shoulder 8st 9lb.
The last filly to win an English Derby was Fifinella in 1916, although Salsabil won the Irish Derby as recently as 1990.
I think the question in the OP has been pretty well answered, but I’ll add another example of a filly beating the boys at a longer distance: Dance Smartly, in the 1991 Queen’s Plate. The Plate is one-and-a-quarter miles, and is run at Woodbine, in Toronto.
Yes, Ruffian, to anticipate your question, I’m another racing fan. The track I attend most often is Woodbine, but I’ve also been known to search out other tracks if I’m out of town and have the time to spend a day at the races.
Down under we have to be satisfied with Sunline the best mare in the world. The site is not up to date as this news indicates Sunline has now won 26 Group races including 13 Group 1 races. Many were against males.
don’t ask, spoons, Nostradamus…it seems more commonplace in virtually all other racing countries for fillies/mares to face male opposition on a more regular basis. Although there are nearly countless opportunities to do so in the States, fewer trainers have taken that option in recent years. I find this very disappointing, but racing as a whole has been taken disappointing turns in recent years.
Thinking about USA racing only, I cannot think of a single filly or mare running, and defeating, males in top competition at the classic distances in recent years. (Can anyone think of any from the past 5 years or so? Even the past 10? All the mares I’d name off the top of my head–Personal Ensign, Winning Colors, Lady’s Secret, Estrapade, Royal Heroine–are from the '80s.)
But then again, racing here has the unfortunate infatuation with speed and sprinting as it is. It’s hard enough getting a colt to go the distance. But that’s a more personal beef to this fan. 
BTW, I mis-typed–Winning Colors won in 1988, not 1989. Sunday Silence won in '89, der. (This from a person who counts not sheep, but Derby winners–chronologically–when she can’t sleep. For shame!)