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Induced Ovulation

Alpacas are reproductively very different than cows, sheep, goats or horses. Unlike all those other species, female alpacas do not have a regular estrous cycle where they come into heat at regular intervals. Camelids are induced ovulators. This means that the female does not ovulate any eggs (oocytes) until she has been bred by the male. The act of breeding stimulates hormonal events inside the female which result in ovulation and hopefully conception.

Estrous cycles include a period of receptivity to the male and a period of definite non-receptivity to the male. Estrogen is produced by follicles and is the predominant hormone in a receptive female. After ovulation the follicle stops producing estrogen and the remaining cells develop into a corpus luteum. Progesterone is produced by the corpus luteum and is the predominant hormone in a non-receptive female. After a period of time, the corpus luteum regresses in response to prostaglandins (another group of hormones) and a new follicle begins to develop. When there is a pregnancy the fetus tells the corpus luteum to remain for part or all of the pregnancy (depending on the species). If the female does not become pregnant, she will go through the entire cycle again. This is the scenario for cows, sheep, goats and horses who ovulate spontaneously.

Even though the alpaca does not have an estrous cycle with definite periods of receptivity and non-receptivity, there is still cyclic activity on the ovaries. Several follicles are stimulated to develop and mature together in "waves" on the ovary. The wave of stimulated follicles develop from primordial follicles to primary follicles and then to secondary follicles which contain some fluid and are capable of producing estrogen. Only one single follicle dominates and continues developing to become a tertiary or Graffian follicle which is larger and able to ovulate. This follicle also produces estrogen and is the follicle which will ovulate if the alpaca is bred at the appropriate time. The follicles which do not ovulate will regress and shrink. Each oocyte (egg) can only be stimulated to develop once and then is not used again. Females connot produce more oocytes than what was already present in the ovaries at birth. Despite this and despite the fact that several oocytes are stimulated to develop and then never used with each follicular wave, there is still plenty of oocytes for the lifetime of the female.

A follicular wave is composed of three stages of growth, maturity, and regression. Each stage takes an average of 4 days, while the entire follicular wave takes about 11 to 12 days. This means that while the follicles are at the mature stage the alpaca is more receptive to the male and breeding should result in ovulation. Breeding is much less likely to result in ovulation during the follicular growth stage or regression stage. The female is usually less receptive during this time. The mature stage occurs 3 to 4 out of every 12 days of a follicular wave. Follicular waves will occur on both ovaries and often the waves do not coincide. For example, as one ovary enters the regression stage, the other ovary will begin the growth stage and have a mature follicle ready within a few days. This maintains follicles which are ready to ovulate more often than without the overlap or lack of syncronization between the ovaries.

This overlap of follicular waves keeps estrogen circulating in the female. Estrogen contributes to making her receptive to the male at almost any point during the follicular wave. An open (non-pregnant) female may appear to be receptive all the time. Just because the female will accept the male does not mean that there is a follicle ready to ovulate. There may only be smaller follicles in the growth or regression stage which cannot ovulate and result in a pregnancy. A breeding which does not result in ovulation could easily occur.

Once ovulation occurs and the corpus luteum is formed, it remains for only about 14 days if a pregnancy did not result from the breeding. The corpus luteum remains throughout the entire pregnancy if a pregnancy has resulted. If the female becomes receptive to the male again in 5 to 7 days, then no corpus luteum formed to produce progesterone because the female did not ovulate during breeding. In a female who becomes receptive again 14 to 16 days after breeding, ovulation occurred but did not result in a pregnancy.

 
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