Testicular length as an indicator of the onset of sperm production in alpacas under Swedish conditions

Abstract

Background: The popularity of alpacas (Vicugna pacos) is increasing in Sweden as well as in other countries; however, knowledge about optimal management practices under Swedish conditions is still limited. The wide age range reported when the onset of puberty can occur, between 1 and 3 years of age, makes management decisions difficult and may be influenced by the conditions under which the alpacas are kept. The aim of this study was to find out when Swedish alpacas can be expected to start producing sperm, by using testicular length and body condition score as a more precise indirect indicator than age.

Results: This study suggests that animals with a testicular length ≥3.8 cm would be producing sperm; however, if it is crucial to know that there is no sperm production for management purposes, the threshold level for testicular length used to differentiate between sperm-producing and non-sperm producing animals should be ≤1.6 cm instead. If only one variable is considered, testicular length appears to better than age alone to predict sperm production. Body condition score together with testicular length explains the individual onset of puberty and better guide management recommendations.

Conclusions: Using a combination of these parameters (testicular length, body condition score and age) as a tool for decision making for alpaca husbandry under Swedish conditions is suggested.

Fig. 1

Measurement of testicles with a calliper

Fig. 2

Length increment model for alpaca testicle length. Experiment 1, “farm”, n = 72; solid line (mean), and dark grey polygon (95 % CI); Experiment 2 (from castrations (n = 22) and cadavers (n = 6); dashed line (mean) and light grey polygon (95 % CI). Note: the measurements on living animals included the scrotal layers whereas organs obtained after castration or from cadavers did not

Fig. 3

Length increment model for alpaca testicle length, incorporating age and body condition score (BCS). Testicle length measured in vivo (Experiment 1; n = 72); Circles represent individual values, lines represent means. Black line and circle = body condition score range from 2 to 3; red line and circle = body condition score range from 3 to 4, blue line and circle = body condition score range from 4 to 5. Shaded area indicates 95 % confidence interval

Fig. 4

Probability of sperm presence in alpaca testicles. Samples from Experiment 2 (n = 28) explained by a age, and b mean testicle length. Black circles represent organs from castrations, black triangles represent organs from cadavers; filled circles represent organs containing sperm, open circles represent organs without sperm. Animals with testicular length ≥3.8 cm would be producing sperm (median probability of sperm presence is ≥0.99); and animals with testicular length ≤1.6 cm will not be producing sperm (median probability of sperm presence is ≥0.01)