studies in conscious rams and mice, as well as in anesthetized rams and bulls, challenged the classical paradigm regarding scrotal/testicular thermoregulation, as acute testicular hyperthermia caused increases in blood flow and in delivery and uptake of O2, with no indications of hypoxia. In contrast to the long‐standing paradigm, our data were evidence that effects of increased testicular temperature were due to testicular temperature per se and not secondary hypoxia.
Evaluation of Scrotal Surface Temperature with Infrared Thermography
Infrared thermograms of the scrotum of bulls with apparently normal scrotal thermoregulation were symmetrical left to right, with temperature at the top 4–6 °C warmer than at the bottom [24, 25]. More random temperature patterns, including a lack of horizontal symmetry and areas of increased scrotal surface temperature, were interpreted as abnormal thermoregulation of the testes or epididymides. Nearly all bulls with an abnormal thermogram had reduced semen quality [24, 25]; conversely, not every bull with poor quality semen had an abnormal thermogram. Consequently, infrared thermography is a useful tool for breeding soundness evaluation of bulls, although it does not replace collection and evaluation of semen. In one study, 30 yearling beef bulls, all deemed breeding sound on a standard breeding soundness examination, were individually exposed to approximately 18 heifers for 45 days [26]. Pregnancy rates 80 days after the end of the breeding season were similar (83 vs 85%) for bulls with a normal or questionable scrotal surface temperature pattern, respectively, but were higher (P < 0.01) than pregnancy rates for bulls with an abnormal scrotal surface temperature pattern (68%).
Effects of Increased Testicular Temperature
Increased Ambient Temperature
Effects of increased ambient temperature on semen quality have been widely reported. In one study, ambient temperatures of 40 °C at a relative humidity of 35–45% for 12 hours reduced semen quality [27]. Furthermore, B. taurus bulls are more susceptible than B. indicus bulls to high ambient temperatures [27]. In that regard, decreases in semen quality were less severe, occurred later, and recovered more rapidly in crossbred (B. indicus × B. taurus) bulls than in purebred B. taurus bulls exposed to high ambient temperatures [28].
Scrotal Insulation as a Model of Increased Testicular Temperature
Scrotal insulation is frequently used to increase testicular temperature. In one study [29], scrota of B. indicus × B. taurus bulls were insulated for 48 hours. The nature and time (day 0, start of insulation) of the morphologically abnormal sperm that resulted were as follows: decapitated, days 6–14; abnormal acrosomes, days 12–23; abnormal tails, days 12–23; and protoplasmic droplets, days 17–23. Therefore scrotal heating affected sperm in the caput epididymis as well as spermatids. Although daily sperm production was not affected, epididymal sperm reserves were reduced by nearly 50% (9.2 vs 17.4 billion), particularly in the caput (3.8 vs 6.6 billion) and cauda (3.7 vs 9.5 billion), perhaps due to selective resorption of abnormal sperm in the rete testis and excurrent ducts. In another study [30, 31], scrota of six Holstein bulls were insulated for 48 hours (day 0, initiation of insulation). The number of sperm collected was not significantly different, but the proportion of progressively motile sperm decreased from 69% (prior to insulation) to 42% on day 15. The proportion of normal sperm was not significantly different from day −6 to day 9 (80%), decreased abruptly on day 12 (53%), and reached a nadir on day 18 (14%). Although there was considerable variation among bulls in both type and proportion of abnormal sperm, specific abnormalities appeared in a consistent chronological sequence: tailless, days 12–15; diadem, day 18; pyriform and nuclear vacuoles, day 21; knobbed acrosome, day 27; and Dag defect, day 30. When sperm were collected 3–9 days after insulation and examined immediately, their motility and morphology were similar to pre‐insulation values [30]. Compared to semen collected prior to insulation, following freezing, thawing, and incubation at 37 °C for three hours [30], there were significant reductions in the proportion of progressively motile sperm (46 vs 31%, respectively) and the proportion of sperm with intact acrosomes (73 vs 63%). Freezing plus post‐thaw incubation manifested changes that had occurred in sperm that were in the epididymis at the time of scrotal insulation.
In another study [32], scrotal insulation (four days) and dexamethasone treatment (20 mg/day for seven days) were used as models of testicular heating and stress, respectively. Some bulls seemed predisposed to produce sperm with a specific abnormality. Pyriform heads, nuclear vacuoles, microcephalic sperm, and abnormal DNA condensation were more common in insulated than dexamethasone‐treated bulls. Conversely, dexamethasone treatment resulted in an earlier and more severe effect on epididymal sperm, an earlier and greater increase in distal midpiece reflexes, and an earlier increase in proximal and distal droplets. Overall, types of defective sperm and the time of their detection were similar between treatments.
Insulation of the Scrotal Neck
The scrotal neck of five bulls was insulated for seven days (days 1–8) as a model of bulls with excessive body condition (which typically have considerable fat in the scrotal neck). Sperm within the epididymis or at the acrosome phase during insulation appeared to be most affected [33]. Insulated bulls had twice as many sperm with midpiece defects and four times as many with droplets on day 5, fewer normal sperm and three times as many with midpiece defects and droplets on day 8, fewer normal sperm on days 15 and 18, and more sperm with head defects on days 18 and 21. Semen quality in insulated bulls had nearly returned to pre‐insulation values by day 35. In a second experiment [33], scrotal subcutaneous temperature increased 2.0, 1.5, and 0.5 °C at the top, middle, and bottom of the testis, respectively, and intratesticular temperature was 0.9 °C higher at the corresponding three locations 48 hours after scrotal neck insulation compared to before insulation. Clearly, the scrotal neck is an important site of heat loss.
Increased Epididymal Temperature
In most mammals, the cauda epididymis is cooler than the testes [34], facilitating its sperm storage function. Increasing cauda temperature disrupts absorptive and secretory functions, changes the composition (ions and proteins) of the cauda fluid, and increases (approximately threefold) the rate of sperm passage through the cauda [34]. Consequently, the number of sperm in the first ejaculate declines, with an even more dramatic decline in sperm number in successive ejaculates. In addition, increased temperature seems to hasten sperm maturation [34].
Effects of Increased Temperature on Testicular Cells
Although heating seems to affect Sertoli and Leydig cell function, germ cells are the most sensitive [35]. All stages of spermatogenesis are susceptible, with degree of damage related to the extent and duration of the increased temperature [35]. Spermatocytes in meiotic prophase are killed by heat, whereas sperm that are more mature usually have metabolic and structural abnormalities [36]. Heating testes usually decreases the proportion of progressively motile and live sperm, and increases the incidence of morphologically abnormal sperm, especially those with defective heads [37]. Increased testicular temperature caused a lack of chromatin protamination and subtle changes in head shape of bull sperm [38]. Despite considerable variation among bulls in the nature and proportion of defective sperm, order of appearance of specific defects is relatively consistent [31, 32]. Unless spermatogonia are affected, the interval from cessation of heating to restoration of normal sperm in the ejaculate corresponds to the interval from the beginning of differentiation to ejaculation [37]. Following scrotal insulation in bulls, blastocyst rate (in vitro system) was more sensitive than cleavage rate [39]. Even though sperm morphology has returned to normal, their utilization may result in decreased fertilization rates and an increased incidence of embryonic death [40].
Summary
