Ilaria Marzetti

Female breeding behaviour in souther elephant seals (Mirounga leonina)

Degree thesis - Dip. di Biologia Animale - Universita' degli Studi di
Roma "La Sapienza" - 1997

Abstract
The target this research project was the breeding behaviour of female
elephant seals (Mirounga leonina) of the Falkland Islands. In
particular, we tried to explore the risks for the female due to male
mating tactics, and the behavioural tactics used by females while they
are on land for breeding to reduce these risks. In southern elephant
seals breeding is colonial: females have a innate tendency to group when
they come back to land to give birth and mate. The cause of this
grouping tendency is debated: we explored the hypothesis that grouping
reduce the per capita risk of harassment by males. Each group of
females, called harem, is controlled by a single male, the alpha or
harem master. The alpha has an almost exclusive and uncontested right of
access to estrus females, and he is able to effectively keep other males
away from the harem, thereby reducing the risk of harassment of females.
The field work was carried on in the Falkland Islands during the 1995
and 1996 breeding seasons. The research area was Sea Lion Island, a
small island that shelter a population of about five hundreds females.
We tagged females and marked them with hair dye to be able to recognize
them during behavioral interactions. Every day we counted the females in
every harem of the population, and we identiefied as much females as
possible in each harem, keeping a record of their breeding status.
Estrus was estimated from birth, by adding the mean number of days from
birth to first copulation calculated in a large independent sample. We
observed all the harems of the population during 1258 hours in 1995 and
1294 hours in 1996. In 1996 we observed intensively a single medium
sized harem for 258 hours, to collected precise information about timing
of reproductive activities of individual females. During behavioural
observations we recorded data about all interactions between males and
females (3163 in 1995; 2849 in 1996) using all occurrences sampling; we
videotaped many breeding interactions to study in detailed female
reaction to male approch. We also recorded ad libitum all interaction
between males and solitary females we spotted while working in the
field. Male-female interactions may happens in the harem or outside the
harem: the effect on the female of this two kinds of situation, and the
kind of males involved, are quite different. The most of approches and
copulations in and around the harem are done by alpha males. Pheripheral
males will try to get in touch with harem females at any time, but their
approaches are in mosta cases disrupted by alpha males. To evaluate the
effect on harassment of variaous demographic and social parameters we
calculated harassment indices for each harem, based on the number of
interactions, the number of females and the number of associated males.
We found a decrease of harassment per females with increase of harem
size: in large harems harassment is distributed on a larger number of
females, and hence harassment per female is lower, notwithstanding the
increase in absolute harassment rate due to concentration of pheripheral
males near larger harems. Hencen grouping fovours the reduction of
likelyhood of harassment by males. Values of per capita indices of
harassment by pheripheral males are very low: hence, while in the harem
each female has to interact mainly with a single and experieced male,
the likelyhood of heavy harassment is low, and the likelyhood of
disruption by other males is almost nil. Hence, the main risks for
females during their permanence in harems are interruption of lactation
and separation from the pup that may result form interaction with the
alpha male. In northern elephant seals (M. angustirostris) the
likelyhood of pup separation is high, and in many cases separation
results in definitive abandonment of the pup. Thus, male mating behavior
is a significant source of pup mortality in this species. In our
population of souther elephant seals the situation was quite different:
in many cases reproductive interactions resulted in disruption of
lactation and separation of of the pup from the mother, but in no case
this separation was definitive and lead to abandonment. These mild
effect of male mating behaviour depends on the density of Sea Lion
Island population, which is very low if compared with the high density
typical of the northern species and the bigger colonies of the southern.
On Sea Lion Island harems are small and not very crowded: hence the pup
has a great likelyhood to be able to come back to his mother aftera
temporary separation. Outside the harem the main risk for breeding
females is harassment from secondary males. The libido of these males is
very high, as is demonstrated by the capability to copulate at fast pace
when, due to random events, an harem remains without alpha. Chances to
be able to get actual mates is, on the contrary, very low: they are
strongly limited in their access to females by the action of alphas.
Hence, these males concentrated on consorting with females ouside the
harems (rare isolated females, pregnant females in arrival from sea,
post estrus females in departure to sea). They usually heavily harass
females, and this harassmente may potentially have a cost for the
females (disruption of energy and time budget, wounds, breeding in unfit
areas). In the northern species, harassment from pheripheral male may
result in serious wounding, and is a serious source of female mortality.
On the contrary, in our study population, serious wounding of females is
very rare, and in no case it resulted in death of the female involved or
definitive abandonment of the breeding ares. In spite of this, also in
our population, female breeding behavior revealed may different fine
tuning to reduce the risk of interception by secondary males.