ors. 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 thawing, with males showing the response pattern 2 having the lowest quality. The lack of differences of quality in fresh semen would not allow us to characterize the behavior of sperm of a male during cryopreservation based on these parameters , –. In contrast, changes in morphometry of sperm heads observed throughout the cryopreservation have proved useful to this purpose.
A number of studies have attempted to address the role of sperm head morphometry on the prediction of freezability , , –, . Although these studies have addressed this goal in different ways, most of them have used related measures of sperm head dimensions on fresh semen with measures of sperm quality at thawing. In general, these studies have reported that males with smaller sperm heads in fresh semen showed better cryoresistance than those with larger heads. Furthermore, some studies have observed that the sperm heads that had a lower rate of change on their dimension, from fresh to thawed semen, had better cryoresistance . The present study provides no information on which sperm parameters are most valuable as predictors of cryoresistance, since we perform a freezing-thawing process to gather information on sperm cryosurvival. Instead, we use this information to characterize the sperm cryoresistance of a male. Our work was based on the hypothesis that the response of spermatozoa to the freezing-thawing process could be an inherent feature of males and therefore remain constant. This idea is based on the fact that sperm head morphometry has been showed to be a quite constant feature of males . Here, we propose to characterize the sperm cryoability of a male rather than to predict this ability. Furthermore, we believe that this should be based on the study of patterns of response, rather than on the evaluation of a particular feature at a particular time. Had we focused only in the sperm head dimensions in fresh semen, this study would not have allowed us to identify the role of this feature in predicting cryoresistance. Males that suffered change