MALE REPRODUCTIVE SUCCESS
IN SOUTHERN ELEPHANT SEALS
BEHAVIOURAL ESTIMATES AND GENETIC PATERNITY
A. Fabiani (1,2), F. Galimberti (2) and A.R. Hoelzel (1)
(1) University of Durham, Dept. Of Biological Science, Durham, UK - www.dur.ac.uk
(2) ESRG, http://web.tiscalinet.it/esrg/
INTRODUCTION
The southern elephant is the species with the highest level of sexual dimorphism and polygyny among all mammals.
They reproduce on land, are easy to approach and their mating system is thought to be among the purest forms of harem
defence polygyny. This species is therefore an ideal taxon for studying relationships between behavioural and genetic
indices of mating success in a polygynous mating system.
AIMS
The project focuses on the southern elephant seal population of Falkland Islands and its main goals are:
v The determination of genetic paternity
v The assessment of male reproductive strategy and variance in male reproductive success
v The evaluation of the relationships between observational measure of male mating success and true
reproductive success
MATERIALS AND METHODS
The field work was carried out during three breeding seasons, from 1996 to 1998 on Sea Lion Island:
v we investigated the paternity for 192 pairs mother/pup belonging to 7 harems (ranging in size from 23 to 104
females). Five harems were from the 1996 and 2 from the 1997;
v we screened 104 males, 162 females and 192 pups for 7 microsatellites. All loci were polymorphic (mean allele per
locus 7.1, range 4-10) and amplified for one locus, except BETA that amplifies for two loci (and four alleles);
v for the paternity inference we tested both exclusion and statistically based (CERVUS - http://helios.bto.ed.ac.uk/
evolgen/) methods which gave almost identical results. We used the exclusion method for all data presented here.
v we estimated 3 behavioural indices of breeding success for each male:
- index of female control (FF_DAYS): the sum of the females controlled by each male throughout the breeding season
- index of mating success (MS100): the number of copulations by the male in 100 hours of observation
- index of fertilization success (ENFI), the product between the proportion of copulations achieved by the male in one
harem by the number of females that bred in the harem, summed over harems in which the male copulated;
v observational effort was balanced among the harems, with a total of 1,030 hrs of observation and 356 copulations
recorded for the 7 harems (means 147 and 50 respectively).
v For each mother/pup pair for which we had the genetic paternity, we assigned the father
by different criteria, based on the behavioural data: HOLDER = harem holder; H_ENFI =
male with the highest ENFI in the harem; FIR_CO: male that first copulated with the female;
ASS_E: male associated with the female for the longest period during her oestrus. All the
estimates were very good predictors of the genetic paternities. Table 2 shows which
estimates were the same or different from the genetic match. While all estimates were good
predictors of genetic paternity, FIR_CO was consistent with genetic paternity significantly
more often than either HOLDER or H_ENFI (2= 4.6, p= 0.03).
Index
Same
Differ.
HOLDER
ASS_E
H_ENFI
FIR_CO
125
115
125
95
59
38
59
25
n
% same
184
153
184
120
68
75
68
79
Table 2
v The largest prportion of paternities in each harem was achieved by the harem holder
(paternities: mean 72% ± 15, range 48%-95%; ENFI: mean 80%, range 50%-100%).
There was no clear association between the size of the harem and the relative success of
the harem holder for either breeding estimates or genetic paternity (Fig.2).
1. Behaviour
v Behavioural data from the two years indicated that only 29.3% of the reproductively
mature males in the colony (n=109) achieved at least one copulation. The distributions of
the estimates for both years were all highly asymmetric, even among males that copulated
(ENFI means respectively 9.6±23.5 for 109 males and 32.7±33.9 for the 32 males that
copulated; opportunity of selection I = 6.12).
2. Genetic and Behaviour
v From the direct comparison of the genotypes of mother and offspring with that of
putative fathers, we could assign a father to 185 pups and exclud all sampled males for 7
(3.6%) cases. For each pup across all loci, the mean ± SE probability that the father
assigned was father by chance was very low, 1.595E-06 ± 4.057E-06.
100
PAT
ENFI
80
% in the harem
RESULTS
60
40
20
0
20
23
39
45
62
Harem size
v The distribution of paternities in the sampled harems was highly skewed in both years
(Fig.1, showing both years combined), with many males assigned paternity of none or a
single pup and only a few males assigned many more (2.6± 5.3 and 3.8±7.9 to a maximum
of 22 and 31/male in each year respectively; I = 4.16 for 1996 and 4.32 for 1997).
v In simple regression analysis of the breeding estimates versus paternity, the proportion of
the paternity's variability explained by the breeding estimates was always very high in both
years and for each harem (coefficients of determination R2 in Table1).
60
85
104
Fig.2
CONCLUSIONS
v The average success of harem holders (72% of paternities) was greater than
that seen for either southern elephant seals in Argentina (58%) or northern
elephant seals (38%; see Hoelzel et al. 1999, Behav. Ecol. Sciobiol. 46:298-306),
which may imply a difference in either male or female strategy at Seal Island.
v There was no correlation between harem size and the success of the harem
holder for the range of harem sizes included in this study (20-104 females).
NUMBER OF MALES
ENFI
50
40
RU96
(6)
PAT
ENFI
FF_D
MS100
30
20
10
0.89
0.90
0.90
SF96 SI196 SI296 SM96 SI297 SF97
(3)
(12)
(14)
(6)
(17)
(3)
0.99
0.99
0.99
0.96
0.98
0.96
0.92
0.79
0.92
0.96
0.92
0.97
0.98
0.91
0.98
0.96
0.98
0.96
Table1
Harems as columns, sample dimension for each harem in bracket. All p < 0.004
0
0-10 11-20 21-30 31-40 41-50 71-80
ENFI and PATERNITIES
Fig.1
v Genetic results strongly supported those from behavioural data and all the
indices used were good predictors of the relative male reproductive success.
v Simple behavioural and demographic indices predicted genetic paternity
68-79% of the time, with behaviours associated with first copulation and
association during oestrus being somewhat better than harem holding or ENFI.
Special thanks to all the people who worked in the field with us. Funding to
A. Fabiani from the University of Rome, “La Sapienza” and the CNR of Italy.