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Developing the Finest Colored Huacaya Alpaca Herd in the World

Relationship Between
Testicle Size and Fertility

by P. Walter Bravo

Deciding whether to consider a male as a sire includes evaluation of testicle size and growth. Testicles are the reproductive organs that secrete the male hormone testosterone and produce spermatozoa. Testosterone gives the male his masculine characteristics, like size, muscular mass, and the sexual desire to breed females. A spermatozoon fertilizing an ovum provides one-half of the genetic material of the next generation.

A sire must have two testicles. The testicles are ovoid in shape and located beneath the anus within the scrotum, which is less pendulous than in rams or bulls.

Both testicles should be of equal size. In South America, male alpacas with testicles of unequal size are castrated when they are one and a half or two years old. Males at this age show an exacerbated sexual desire or libido that coincides with a rapid increase of testosterone concentrations in the blood. Elevated concentrations of testosterone accelerate the closure of the growth plates, at which point males stop growing. If a male is castrated at a younger age, the growth plates remain open, and it grows taller and has a somewhat finer bone structure than intact males.

As in other livestock species, there is a relationship between testicle size and age. At birth the testicles are small (2/5 inch or 1 centimeter in length) and flaccid. Sometimes they may not be in the scrotum and are difficult to palpate. Testicles should be in the scrotum by six months of age. Table 1 lists testicle sizes at different ages according to averages calculated from more than three hundred male alpacas in the Peruvian highlands. Because the left and right testicles are not different in size, only one measurement is given. To measure a testicle, hold it with one hand and with the other hand measure the long axis with a caliper.

The consistency of testicles is determined through digital palpation at the onset of puberty; which is usually around two years of age. The normal testicle should be resilient, move freely within the scrotum, and may resemble a tensely held human bicep. The epididymis, the elongated mass of tubules at the back of the testicles, should also be felt during palpation and should have three clearly differentiated anatomical parts: head, body, and tail. Normally the head of the epididymis in the alpaca is located in the upper pole of the testicle, close to the anus; the tail is at the lower end.

The detachment of the penis from the prepuce, or foreskin, is also used to determine whether a male has reached breeding age and is also determined through digital palpation. The male must be sedated during this procedure. Extrude the penis as follows: With the animal in lateral recumbency, use one hand to straighten the sigmoidal flexure of the penis and push the penis toward the prepuce; with the other hand, push back the prepuce. The initiation of penis-prepuce detachment begins from the tip and follows caudally to the glans penis. Eleven-month-old alpacas do not show any detachment of penis from prepuce; by twelve months, the tip of the penis is free, indicating initiation of detachment, in 12.2 percent. By twenty-four and thirty-four months of age, 59.5 and 94.4 percent, respectively, exhibit complete detachment from the prepuce. Once the penis is free, a male may begin breeding females. Naturally, some males are precocious .and may act ready before they are physically able.

In addition to determining breeding age, testicle size is implicated in breeding success. In a 1983-84 study conducted in Peru, two groups of male alpacas bred to two groups of thirty females each demonstrated a 30 percent difference in fertility rate depending on testicle size. The males with normal-size testicles, 11/2 inches (3.5 centimeters) in length and 11/S inches (2.9 centimeters) in width, impregnated twenty-one out of thirty females. The group with small testicles, 1 inch (2.5 centimeters) in length and 7/8 inch (2.2 centimeters) in width, impregnated only twelve of thirty females.

In conclusion, testicle size can be correlated to the male alpaca's age. The numbers given in Table 1 may be used as guidelines in the process of selecting male alpacas for breeding purposes or for correlating with age. The presence of two equal, normal-sized testicles is related to high fertility rates.

** Minimum size acceptable shown in parentheses

P. Walter Bravo is a native of Peru, where he obtained a DVM degree at the Universidad Nacional Tecnica del Altiplano, Puno, in 1973. He earned a MS degree in reproductive physiology at Utah State University, Logan, in 1986 and a PhD degree in comparative pathology in the Department of Reproduction, University of California, Davis, in 1990. He has worked as chief of animal production and health in two co-ops, Picotani and Marangani, dealing with sheep, cows, and thousands of llamas and alpacas. He also worked for San Marcos University as a researcher in animal production and was an assistant professor at the La Raya Research Center in Peru. He has both clinical and research experience concerning llamas and alpacas in Peru and the United States. He is invited to give lectures on camelid reproduction worldwide, including Scotland, France, Australia, Argentina, Peru, and the United States.