While the idea of these subgroups and races is an interesting one, it should be noted that not all studies support it – in 2005 nuclear studies (MHC) published in the journal Heredity by Berggren and colleagues only found support for the two E. concolor subgroups (north and south of the Bosphorous strait that separates the European and Asian parts of Istanbul). Nonetheless, genetic profiling of Western hedgehogs from Oxford has demonstrated significant genetic differentiation between neighbouring populations, suggesting that the populations don’t mix very often – genetic isolation is one factor involved in the formation of new species and subspecies. So, despite a few arguments, the general consensus at the moment is that the Eastern and Western hedgehogs represent two separate species. Whatever the true number of species and subspecies may eventually turn out to be, it should be mentioned that these are merely constructs of our own convenience – they have little relevance outside of satiating our desire to classify things into groups!
Early in development, the embryos are sexually undifferentiated, unpigmented, and possess filamentous external gills; the external yolk sac in this stage weighs 120–130 g (– oz). Recognizable sex organs develop by an embryonic length of 92 mm ( in), and tissue differentiation is complete by a length of 150 mm ( in). Body pigmentation appears when the embryo is 100–150 mm (– in) long; the external gills regress at around the same time. An internal yolk sac develops when the embryo is 140 mm ( in) long, which begins to take in yolk as the external yolk sac shrinks; by the time the embryo is 233–300 mm (– in) long the external yolk sac has been completely resorbed.  Off Portugal, the young seem to be born in May and December following a gestation period of over a year. As they near giving birth, the females undergo ovarian atresia (regression of the follicles), suggesting that they enter a resting period afterwards.  The litter size ranges from 1 to 29 (typically 12), and is not correlated with female size.   Parturition may occur in a yet-unknown nursery area, as newborns are rarely ever caught.  The length at birth has been reported as 23–30 cm (– in) in the Atlantic,   and 30–35 cm (12–14 in) in the Pacific.  
Common threshers are active, strong swimmers; there are infrequent reports of them leaping completely out of the water.  Like the fast-swimming sharks of the family Lamnidae , the common thresher has a strip of aerobic red muscle along its flank that is able to contract powerfully and efficiently for long periods of time.  In addition, they have slow- oxidative muscles centrally located within their bodies and a blood vessel countercurrent exchange system called the rete mirabile ("wonderful net"), allowing them to generate and retain body heat . The temperature inside the red muscles of a common thresher averages 2 °C ( °F) above that of the ambient seawater , though there is significant individual variation.  Unlike the pelagic and bigeye threshers, the common thresher lacks an orbital rete mirabile to protect its eyes and brain from temperature changes.