and has up to four leaves, each of which may be 1 m
in length and width. It is considered a weed by Monte-
verde residents, for it grows quickly in damp sunny
areas such as pastures. In the early 1980s, the over-
grown pastures at the head of the Pefias Blancas val-
ley contained large patches of hundreds of Xantho-
soma, but trees have now grown over them and only
a few remain. The small isolated patches at higher ele-
vations within Monteverde are fairly typical of its
original distribution. The inflorescence, a character-
istic feature of the family Araceae (aroids), consists
of two parts: the spathe, a modified leaf that covers
the inflorescence as it develops, and the spadix, a
spongy stalk covered by flowers (Fig. 8.7). In Xantho-
soma, the male flowers on the top of the spadix and
the female flowers below these are separated by a ring
of sterile flowers. The lower part of the spathe en-
closes the female flowers, forming a chamber. A flow-
ering individual annually produces 5-15 inflores-
cences, spaced about 9 days apart. Flowering occurs
primarily during the rainy season.
Two species of scarab beetles (Scarabaeidae; Cole-
optera) visit Xanthosoma in Monteverde; both belong
to the genus Cyclocephela and are similar in size
(ca. 2.5 cm) and behavior. Cyclocephela nigerrima
is nearly black, and C. sexpunctata is light brown with
dark spots. In Penas Blancas, C. sexpunctata outnum-
bers C. nigerrima by 2 to 1; within Monteverde, the
Figure 8.7. An inflorescence of Xanthosoma robustum,
The upper part of the spathe is white, and the lower part
is purple or green. The entire structure is 20-30 cm tall.relative abundance is reversed and the ratio is about
1 to 20.
On the first day a Xanthosoma inflorescence opens,
its spadix begins to heat up intensely around 1800 hr
(Fig. 8.8). The heat volatilizes a fresh, slightly sweet
odor that is easily detected by humans and scarabs
from a distance. The temperature peaks at 40-42°C,
much hotter than both the air temperature (about
18°C) or human body temperature (37°C); the scarabs
then begin to arrive. Their buzz as they hover and land
can be loud in the gathering twilight. The scarabs land
on the spathe and descend to the chamber, where they
remain for 24 hr except for occasional forays up the
spadix. The pollen they carry comes off into a sticky
orange secretion on the surface of the receptive female
flowers. The scarabs eat the ring of sterile flowers.
Meanwhile, the spadix cools to the temperature of the
night air.
Over the next day, the scarabs mate and consume
nectar within the inflorescence. The number of scar-
abs in an inflorescence of Xanthosoma robustum at
Monteverde is typically around 7; the most I have seen
in one inflorescence was 38. Late during the second
afternoon, the inflorescence begins to heat again (Fig.
8.8). This time, the temperature peaks around 34°C;
the male flowers release their pollen, The scarabs mill
about, crawling up the spadix and becoming covered
with pollen, and then fly off to a newly opened inflo-
rescence where the sequence repeats.
During the next few days, the top of the inflores-
cence (the part with male flowers) rots and falls away.
The bottom of the inflorescence (with the lower part
of the spathe enclosing the female flowers) remains
intact and matures for the next two months. When the
fruit is ripe, the spathe splits open and peels back to
reveal a ripe "cob" of about 300 soft green fruits with
20—30 seeds each. Bats appear to be their primary
consumers and dispersers, biting away fruits as they
fly past. Many bats prefer Xanthosoma fruit over other
foods. Seeds take about half an hour to pass through
the bats' guts, which is long enough for dispersal
to occur throughout the forest (E. Dinerstein, pers.
comm.).
The cue for heating in aroids is light. In Sauro-
matum and Dracunculus, a flash of light at a critical
time initiates heating as much as several days later
(Chen and Meeuse 1972, Meeuse 1975). The chemi-
cal mediator is salicylic acid (Raskin et al. 1987), and
the sequence, once initiated, runs its course regard-
less of perturbations. The peak temperature in Philo-
dendron is remarkably constant over a 35°C range of
ambient temperatures, a feat that at lower tempera-
tures requires metabolic rates in excess of those of
hummingbird flight muscles (Nagy et al. 1972). Using
a refrigerator as an impromptu cooling chamber, I269 Plant-Animal Interactions