We identified the contents of stomachs from 16 salamanders collected in
1956 and 1957; the items found in them are listed in Table 1. It is not
likely that this list is complete for prey species because _A. hardii_
eats a variety of food and probably takes prey almost indiscriminately
if it is of appropriate size. The kind of food most frequently eaten
was ants; they comprised almost 40 per cent of the total items.
Nevertheless, less than half the stomachs contained ants; this may mean
that salamanders do not make an effort to take ants over any other prey.
Such foraging behavior would result in random capture of ants, and it is
noteworthy that the frequency distribution of ants in stomachs suggests
a Poisson distribution, a mathematical description of one kind of random
distribution.
Table 1.--Numbers of Food Items Found in Stomachs of 16 Specimens
of Aneides hardii
=================================+==========+===========+=============
| | Percentage| Number of
|Individual| of total | stomachs in
Items |animals | (154) | which found
| |individuals|
---------------------------------|----------|-----------|-------------
Mollusca | | |
_Pupilla muscorum_ } | | |
_Gastrocapta_ sp. } | 3 | 1.9% } | 4
_Vallonia pulchella?_ | 4 | 2.5 } |
| | |
Arthropoda | | |
Arachnoida | | |
Arachnida | 15 | 9.7 | 9
Acarina | 13 | 8.4 | 3
| | |
Insecta | | |
Orthoptera (_Ceuthophilus_)| 2 | 1.3 | 2
Hemiptera | 1 | 0.6 | 1
Coleoptera | | |
adults (carabids and | | |
buprestids) | 8 } | 30.9 | 7
larvae | 38 } | |
| | |
Hymenoptera | | |
ants | 62 | 40.2 | 7
wasps | 2 | 1.3 | 2
| | |
Unidentified | 5 | 3.2 | 5
+----------+-----------+-------------
Total | 154 | 100.0 |
---------------------------------+----------+-----------+-------------
Adult and larval beetles comprised about 28 per cent of the total items,
but were found in only seven of the stomachs. Beetles eaten were small
representatives of beetle groups likely to occur in or under logs. A
relatively large species of spider was found in nine stomachs; it
represented only ten per cent of the items taken but was one of the most
important foods when mass is considered.
Two adult salamanders not included in Table 1 were found, in the course
of examination for parasites, to have empty stomachs. One was a male,
and the other was a female taken from a chamber that held an egg
cluster. It would not be surprising regularly to find stomachs empty in
"incubating" females, but the fact is that the one other such female
collected by us had a small amount of food in the gut; probably these
individuals take anything that enters the egg chamber, but do not leave
for active pursuit of food.
Foraging behavior of captive salamanders was observed by one of us. The
salamanders were maintained in a seven-gallon aquarium, the floor of
which was covered with soil, mosses, liverworts, certain flowering
plants, and pieces of rotten fir log. The salamanders were placed in
the terrarium in September, 1956, July, 1957, and October, 1958; one
individual lived 13 months, another 14 months.
A variety of natural foods was present in the soil and plant matter
placed in the terrarium, and these were presumably eaten as found by the
salamanders. However, the great bulk of the food used by the salamanders
was introduced for them, in the form of colonies of _Drosophila
melanogaster_ in half-pint milk bottles. We tried to keep thriving
colonies of flies, primarily of the mutant vestigial-winged type,
present in the terrarium; in 1957 this was successful to the extent that
there appeared to be a surplus of food available at all times. We did
not attempt to feed the salamanders any wholly artificial food, such as
ground beef.
Initially, the salamanders, although seemingly healthy and well-fed,
were not fat. Those that we maintained on a presumably minimal diet
remained slender and did not grow in length. Two individuals captured
in 1957, however, were maintained on food in excess, and these grew in
length and in girth; from an initial size of about 37 mm. snout-vent
length (a subadult size) they attained about 45 mm. snout-vent length
(an adult size) in a period of five months. The observations on foraging
behavior were made primarily on these latter individuals.
The salamanders captured prey by pursuit. A salamander would pursue a
fly until it was caught, or until it moved out of the field of action.
The salamanders were attracted by movements of flies, and ignored those
that were completely quiet; predation was oriented almost wholly on a
visual basis. Once they were within 2 to 4 mm. of a fly they would snap
out the tongue to secure the fly; they were successful in capturing
vestigial-winged flies in about 75 per cent of all tries. The relative
success of capture was greater when the animals were fresh from the
field and less after they had become fattened. The vigor of their
pursuit also decreased noticeably once they became fat. About two days
after any new fly colony was placed in the terrarium, a salamander would
take up a position just inside the lip of the milk bottle, which was
placed on its side. From this vantage point the salamanders took heavy
toll of the fly populations, eating both adults and larvae.
Initially the salamanders foraged indiscriminately in daylight or in
darkness. Later, as they became fat, they avoided high light intensity
and were active only at night or under artificial light of low
intensity. The latter pattern of activity is probably typical of the
pattern they maintain under natural conditions. Certainly we never saw
individuals abroad in daylight at Cloudcroft, yet under favorable
environmental conditions they were to be found in sites that required
considerable movement over open areas of the ground surface.
For several months two individuals of _Eurycea longicauda_ were kept in
with _A. hardii_. Foraging of these two plethodontids is nearly
identical, but the tongue of an adult _Eurycea_ can be extended somewhat
more than one-half inch in capturing flies; for _A. hardii_ this
distance is usually less than one-quarter inch. The relatively short
tongue of _A. hardii_ can be correlated with its life in restricted,
subsurface chambers, where prey most frequently is close to salamanders;
_E. longicauda_ inhabits significantly more open sites.