A SURVEY OF ADDERS IN AN AREA OF EPPING FOREST 185
months it appears that females are more plentiful. The latter
observation is more easily explained than the former. After mat-
ing, the females take every opportunity to absorb heat, for the
development of the embryos is dependent upon sufficient warmth.
As a consequence, they lie in more visually exposed spots to be in
the sun's warmth and are thus more easily seen. Our results
suggest that generally the males awake from hibernation earlier
than the females, and move away from the hibernation area into
the open grassland before them. It is difficult to explain this other
than in terms of the availability of food in the respective zones.
Without doubt there are more small mammals and birds in the
large area of grassland than in the relatively small oak wood, and
this may be the reason for the early migration of the males. This
does not, however, explain why there is an excess of males in the
woodland in autumn; indeed, the contrary would have been ex-
pected.
It can be seen from Table 1 that the highest number of adders
seen on one day was 31 (May 27), but on differing occasions 17
males and 12 females were recorded, which, with 4 immature
snakes, makes a total minimum population of 33. To these may
be added the three young of the year, seen on October 15. Earlier,
we have given reasons for estimating that we only see and identify
one in three of the adders present in the area. If this estimate is
valid, it would suggest that the total adder population of the area
is between 90 and 100. The area in which adders have been
observed measures approximately 300 by 200 yds (275 by 182 m)
or 60,000 square yards (54,860 sq. m). The observed adder
density is thus 1 per 1,780 square yards (1,630 sq. m), while the
estimated density may be as high as 1 per 600 square yards (550
sq. m).
Densities of this order pose a number of interesting problems,
the answers to which we have not yet produced. One such is the
availability of food. According to Smith (1951), the Viviparous
Lizard is the main food of the adder. In the area under considera-
tion lizards, although present along with grass snakes and slow
worms, are not common. It seems unlikely that here this lizard,
with a lower reproductive rate than the snake, and being less
common locally, can be a major item in the diet. On the other
hand, one of us (J.P.) considers that young adders may take a
large toll of lizards where they occur commonly. In another Forest
area one year a large brood of adders fed principally on lizards
(confirmed by regurgitation), which were then abundant. The
summer following the adders' birth the lizard population had
dropped sharply. In captivity too young adders feed very readily
on lizards, even adult ones. They have an extraordinary capacity
to swallow large lizards but after a while in captivity they are less
keen to take them, and this may be related to a graduation in the
wild to a more adult diet of mammals. Doubtless, in the area
under consideration lizards are eaten, and their comparative scar-
city may be due to the high density of the adder population.