
Differences on sperm quality throughout the cryopreservation process between response patterns are presented on Table 2. In fresh and prefreezed semen, the three patterns showed a quite similar sperm quality. Only sperm from males with the response pattern 3 showed a significant higher viability and percentage of sperm motility. After thawing, sperm quality of males with the response pattern 2 decreased considerably, being the lowest. Males with the response pattern 3 showed the best sperm quality, while those males with the response pattern 1 showed an intermediate sperm quality. When we examined the ratio of change compared to the values in fresh semen (normalized percentages), the same results were observed but with some differences. In this case, the three groups showed a similar decrease in quality during the prefreezing stage, and no differences were observed in the change of the quality of sperm movement among groups. Males showing the response pattern 2 again showed a greater decline in sperm quality.
We also examined the additive relationships of each male with the other males showing the same response pattern and with those showing different pattern. The within additive relationship coefficients were twice large as that seen between coefficients among groups of response patterns (Table 3). Thus, males who share the same pattern of response were found to be more closely related to each other than with those showing different responses.
The present study has revealed the existence of three distinct patterns of response, based on changes that occur in the dimensions of the sperm head during the cryopreservation process, and how the sperm quality on thawed semen varies depending on these patterns.
The study has been designed to disentangle male effects from other environmental or management effects. Thus, our experimental design was aimed at standardizing the semen collection, as a way to assure that differences in the response to cryopreservation were due to male-specific factors rather than external factors. To achieve this, males have been collected during the breeding season. All males have been maintained in the same conditions. Each male has been collected three times and only those ejaculates showing a good quality were processed and cryopreserved.
Our results support the idea of the existence of an individual component in the response to the cryopreservation process. Many authors have already addressed this issue, and most of them have reported that different individuals exhibit significant different responses to the same freezing protocol –. These authors have considered different sperm characteristics such as sperm morphometry , , , sperm motility and velocity , , integrity of sperm membranes , – or DNA status  to evaluate the differences on male response. Here, we have studied the changes that occur in the dimensions of the sperm head during the cryopreservation process.
It is widely accepted that cryopreservation leads to a reduction of sperm head dimensions from fresh to thawed semen –, . Different hypothesis have been proposed to explain the reasons why sperm head dimensions decrease after cryopreservation: osmotic changes –, possible alterations of some cell compartments , , damage or loss of the sperm acrosome –, overcondesation of sperm nuclear chromatin –. This reduction has been observed in this study as well, but it has not been a general feature of all males. Thus, about half of males (response pattern 1) have not changed their sperm head dimensions in a significant way during the cryopreservation. In contrast, the other half of males has undergone changes on their dimensions during cryopreservation. Some of these males (response pattern 2) have suffered a progressive decrease in sperm head dimensions, while for the third group of males (response pattern 3) changes in sperm head dimensions have mainly occurred just before freezing, during the cooling period. Sperm quality of fresh semen was quite similar for all males regardless of their response pattern, but differed significantly after thawi