Sanvito, S., F. Galimberti and E. H. Miller (2007).
Vocal signalling of male southern elephant seals is honest but imprecise.
Animal Behaviour 73: 287-299.

In the most common models of communication, it is assumed that animals provide
reliable information about phenotype, hence can settle competitive contests
without physical interactions like fights. This assumption has been tested
rarely for wild mammals. Recent studies of mammals have revealed relationships
of vocal attributes to age and body size. Here, we analyze relationships of
frequency attributes of agonistic vocalizations to phenotype (age, body size,
proboscis size, agonistic behaviour) in males of the southern elephant seal
(Mirounga leonina), a species with intense male competition for access to
females, and in which vocalizations are used frequently to settle male-male
contests. We analyze formant structure and vocal-tract size, and demonstrate
that nasal and oral components of the vocal tract contribute separately to vocal
formants; hence the male?s proboscis serves to elongate the vocal tract. Next we
demonstrate that formants in the upper part of the frequency spectrum (4th and
5th in particular) and formant dispersion convey significant information about
age, size, and resource holding potential at large, and, therefore, can be
honest signals of the phenotype of the emitter. Explained variance is
statistically significant in our and similar studies but is not high, so formant
structure cannot serve as the sole basis of acoustic assessment. Other possible
sources of information exchanged in elephant seal contests are non vocal
acoustic signals (e.g. vibrations) and optical displays.