Anna Fabiani

Structural and functional aspects of male agonistic behaviour in
southern elephant seals (Mirounga leonina)

Degree thesis - Dip. di Biologia Animale - Universita' degli Studi di
Roma "La Sapienza" - 1996

The target of this research project is the agonistic behaviour of
southern elephant seals (Mirounga leonina) of two local populations of
the South Georgia stock, Punta Delgada (Valdˇs Peninsula, Patagonia -
Argentina) and Sea Lion Island (Falkland Islands). Each population was
studied for two breeding seasons with almost the same protocol for
animal marking, collection of demographic data, behavioural observations
and collection of measures of structural phenotype. Firstly, I analyse
both the fine structure of behaviour (action patterns and behavioural
sequences) and its results at individual level (indices of behavioural
performance). I analyse the effect of both structural components of
phenotype (age, size, morphology) and behavioural indices
(aggressiveness, dominance, role) on the structure of agonistic
behaviour. I then explore the functional effects of performance in
inter-male competition by evaluating the relationships between various
indices of performance (interaction rate, local rank, global dominance)
and indices of breeding success (control of females, mating success,
fertilization success). Male agonistic behaviour in southern elephant
seals comprises two components, conventional assessment and direct
aggression and are both very important in the establishment and
maintenance of dominance relationships. Assessment through visual and
acoustic threats is more frequent than direct aggression. It is used to
settle the majority of the contests, especially between males with large
differences in resource holding potential (RHP). Aggression through
direct interaction with physical contact and long all out fight,
although less frequent, is of crucial importance in the initial phase of
settlement of dominance relationships between males with almost equal
RHP. Specific tactics and strategies of competition adopted by
individual males are well correlated with both structural and
behavioural phenotype. Variance in behavioural performance between males
is very large (larger than the one measured for simple structural
traits) and its distribution is different between the two populations.
Hence, the relative importance of various components of phenotype seems
to differ between PD and SLI. In particular, the variation of structural
RHP is larger on SLI and behavioural performance has a stronger effect
on male breeding performance in the SLI population. A large variation in
the demography and socionomy of breeding areas and groups is present
both in and between populations. The specific breeding situation has a
significant effect on the relationship between performance in
competition and breeding success, but the same basic trend is present in
all places. Results of agonistic interactions set up dominance
hierarchies, and these hierarchies are long lasting and strongly linear,
both at local and population level. Excellent performance in competition
between males is a fundamental requisite to high breeding success,
however the local variation of parameters like breeding sex ratio and
density of competitors mold the strength of this link. Stochastic
factors, and factors with a deterministic nature but almost
unpredictable by individual males, changes the rewards of effective
competition tactics in different areas.