SEOANE´S VIPER - Vipera seoanei   Lataste, 1879

This species was initially described in 1879 as a subspecies of the adder Vipera berus, and consequently classified as Vipera berus seoanei. The subspecific status was maintained for almost a century until the year 1976, when SAINT GIRONS & DUGUY elevated it to specific level and classified it as Vipera seoanei.

Distribution: The Seoane´s viper occupies the north and northwest of the Iberian Peninsula from Galicia and the extreme north of Portugal in the west to Navarra in the east. Even though its distribution range enters a few kilometres into the extreme southwest of France, this species is considered endemic to the Iberian Peninsula.

With an average length of 45-50 cm (18-20 inch) the Seoane´s viper is slightly smaller than the other Iberian vipers.

The head is quite narrow and only slightly separate from the rest of the body.

Unlike the Lataste´s and asp vipers, the tip of the snout is not upturned in the Seoane´s viper, and the labials are heavily marked.

Vipera seoanei
female Vipera seoanei, head detail - photo: Daniel Gómez

As far as colouration is concerned, Vipera seoanei is the most variable of the Iberian vipers. There are five different colour morphs (see illustration).

The activity period begins in March and continues until the end of October. This is a very active species which wanders in the open almost on a daily basis. It may also be active in cloudy days and even with drizzle and fog.

Vipera seoanei
From left to right:: classic form, bilineata form
unicolor form, melanic form and cantabrica form

Seoanei comiendo
Vipera seoanei, feeding - film by Juan Timms

Like other vipers, Vipera seoanei usually lies in wait for prey, killing the prey with its venomous apparatus.

However, in certain occasions it will search actively for prey. Adult specimens feed mainly on small rodents (80%) completing their diet with lizards and small birds.

Juvenile specimens feed primarily on lizards and to a lesser extent on amphibians.

In normal or slightly favourable climatic conditions adult females will reproduce every year. When the climatic conditions during the year are colder than usual and the activity period becomes shorter, the females will reproduce every second year.

From 3 to 10 baby vipers are born by the end of August or the beginning of September. Newborn vipers shed their skin shortly after birth, and only a few days later they will start feeding.
Because of the high number of gravid females present every year and that the life expectancy of the newborn is also very high, Vipera seoanei is without doubt the most prolific of the Iberian vipers.

A bite from Vipera seoanei is normally not life threatening for a human being, except in the cases of elderly people, children or unhealthy persons. However, a bite should always be monitored by a doctor and treated in a hospital. The Seoane´s viper´s venom toxicity varies considerably depending on the regions where the specimens are found (Detrait & Saint-Girons, 1986; Detrait et al., 1990).

The Vipera seoanei cantabrica specimens have the highest toxicity, with a LD50* of 6,9 to 9,9 mg. Specimens from the Basque Country and the north of Cantabria have the lowest toxicity, with a LD50 of 23,1 to 23,6 mg.

Finally, the specimens from the eastern parts of Asturias and the west of Cantabria have an intermediate toxicity, with a LD50 of 13,2 to 16,2 mg.
The outcome of these analysis reveals that the venom of the Vipera seoanei cantabrica subspecies is at least twice as toxic as that of the nominate form Vipera seoanei seoanei, and that it is comparable in dangerousness to the venom of Vipera aspis.

* LD50: (Lethal Dose 50% or median lethal dose) is the dose required to kill half the members of a tested population (laboratory mice) after a specified test duration. Expressed in milligrams, it indicates the degree of toxicity of a venom; lower values of LD50 indicate a higher toxicity.

There are currently two subspecies recognized; the nominate form Vipera seoanei seoanei and the subspecies Vipera seoanei cantabrica.

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