Die Geburtshelferkröte (Alytes obstetricans) - Karch

MÁRQUEZ RAFAEL:
From zoological oddity to model organism: communication in the midwife toads
(Alytes obstetricans and Alytes cisternasii
Midwife toads are a primitive group of anurans with a peculiar reproductive behavior.
Mating is terrestrial and males exhibit obligate parental care, carrying the egg strings
twined around their hindlegs for several weeks on land, until tadpoles are fully functional
and the eggs are released in the water. In addition to its parental behavior, Alytes have
a highly simplified vocal communication. Mating calls are short tonal calls - virtually pure
tones - with no harmonic structure, and lacking elaborate amplitude modulation. Several
populations of Alytes have been studied in continental Spain since 1987. Descriptive
studies established that call duration and call dominant frequency were static
characteristics and call interval was much more variable or (dynamic characteristic).
Significant correlations were found between the static call characteristics and other
variables: call duration was highly correlated with temperature, and call frequency was
correlated with male size. The distributions of advertisement call duration and frequency
overlapped between different populations of Alytes, even if they belonged to different
species. However, the covariance between temperature and call duration varied
significantly between populations of different taxonomic groups. Large male
reproductive advantage was found in several populations of both species and in several
seasons. This trend resulted primarily from the fact that larger males obtained more
matings and hence carried larger egg masses from more females than smaller males.
Two types of two-speaker phonotaxis tests showed that calls with lower frequencies
(emitted by larger males) were more attractive to females than calls with high
frequencies. In syntopic populations of the two species there was no sign of character
displacement in the male calls, but female preference was different than in allopatric
situations: female preference for low frequency calls (that could be emitted by males of
the other species) disappeared in A. cisternasii, and female A. obstetricans, did not
exhibit preference for calls with longer duration (that could be emitted by heterospecific
males). Studies of acoustic interactions between males show that males typically
increase their calling rate when exposed to acoustic stimuli representing a nearby
competitor. The response is different if the competitor's call is high or low in frequency,
and also if the competitor's calling rate is fast or slow. Call alternation between males is
non-random, both species having similar phase angles. Although call overlap between
two males affects negatively female preference, in natural events, call alternation does
not decrease call overlap and therefore can't be explained as a method to maximize
signal transmission. Female preference for (duetto) call leaders or followers is different
between species, although it appears that preferences can be reversed at different
temperatures.
Many of the differences observed in acoustical competition between males can be
related with the differences in mating season length between the two populations (A.
cisternasii is generally explosive and A. obstetricans has a more prolonged breeding
season), and with the differences in selective pressures that these two scenarios
incorporate. The resulting picture of the mating systems is one in which male
competition is strictly acoustical, and female choice is based in both dynamic and static
call characteristics.
14