Elephant seals of the Falklands

Elephant seals of Sea Lion Island Sea Lion Island population of elephant seals is a small, localized one with a bit more than five hundred breeding females. After an increase in number during the 1970s and early 1980s the population was almost stable from 1989 onward; we observed an increase of about 7% in total number of breeding females and pup production in 1997, while in 1998 the population remained steady. Fecundity is very high with more than 97% of the females giving birth to a pup and mortality up to weaning is low (less than 2%): hence the population, notwithstanding its very small size, seems to be an healthy one. During the breeding season we gathered data on demography of the population by daily censuses of the whole breeding area, which comprises the sandy beaches on the eastern tip of the island.

Map of Sea Lion Island, with the main study area, on the right side of the arrows (click in map to see aerial pictures of island and study area)

Demography of the population

Socionomy of the population

Breeding biology of the population

Summary tables of demography and socionomy

Demography of the population Population size Pinniped population sizes are not easily estimated, as a portion of the population is at sea at any time. The easiest way to cope with this problem is to estimate the total production of pups and then to use this figure to calculate the actual size of the population using information on the population age structure from life tables. We applied a correction factor of 3.5 to calculate the number of individuals of age one year and older from the number of pups (see below). Therefore, the entire population of Sea Lion Island is estimated to be 1800 seals one year old or older. Number of breeding females It's quite difficult to obtain directly the total number of breeding females in populations of elephant seals because of the pattern of arrival and departure of females during the three-months breeding season. An exact estimate may be obtained from the marking records of females and their pups: we had 516 in 1995, 518 in 1996, 554 in 1997 and 562 in 1998. These estimates correlate well with direct counts of females (females on land at peak haul out = 90% of the total number of breeding females).

A mathematical model of the percentage of females on the breeding beaches in each day of the season was worked out from our daily counts. It is a simple Gaussian model that fits the data much better than other models (e.g., quadratic mode). The fit was very good for all breeding seasons. We used the model to standardize the irregular counts of females carried out prior to our study. These estimates were difficult to compare directly because they had been made in different periods of the breeding season. Since the timing of breeding in our population is almost constant in different years, we used our model to correct the old counts. The population was steady in the 1989-1996 period.

However, current short term stability is not a definitive evidence of stability in the long term, as the small population of Marion Island demonstrates. The other small, localized breeding population of elephant seals for which data is available, at Gough Island, is almost stable. The density of females on Sea Lion Island was quite low (about 110 females per km of the coastline suitable for breeding) compared to densities recorded in other sub-Antarctic populations: this low level of crowding of females probably depends on the small population size and the abundance of sandy beaches with a gentle slope, which are the preferred breeding habitat for elephant seal.

Production and pup mortality The gross production was almost equal in 1995 and 1996 (517 and 518 pups), while in 1997 we had an increase of 7%. Mortality rate was homogeneous between years, and hence overall estimates can be derived: pre-weaning mortality was 1.6 % and total mortality 2.1%. The mortality was mostly due to still birth. Mortality increased to 3.4% in 1998. This excess mortality was due to exceptional rough sea during a few days of the season. Pups were frequently washed up and separated by mother, some of them died during high tide, and some other were abandoned. In particular, a small harem of five females, placed in an unusual and marginal site, was completely disbanded, and all females except one lost their pups. The mortality rates found in our study are lower than those reported for other southern elephant seal populations. In the northern elephant seal the high pup mortality (10-40%) is mainly due to trampling and crushing by the males during their agonistic activities and to starvation through prolonged separation of the pup from the mother because of female aggression or male interference. The same events have been reported for high-density southern elephant seal populations. Visual inspection of dead pups did not offer any indications of crushing by adult individuals; separation of the pup from the mother was always short and in no case definitive. We may conclude that the main sources of mortality observed in crowded populations of elephant seals are of minor importance on Sea Lion Island. The Sea Lion Island population is a low-density population characterized by medium-sized harems. Low mortality could hence be a result of the low level of agonistic activity between males, the low crowding of females and the low level of female aggression within the harems, as in the Valdes Peninsula. Sex ratio On Sea Lion Island, the sex ratio between mature individuals (male of class SAM2 or older) showed a daily variation of Gaussian shape, symmetric for the variation in the number of females on land, and reached a maximum (10-12 females per male) just before the peak haul out of females. The number of females per breeding male (the males which stay on the breeding beaches for prolonged periods actively pursuing access to females) had also a Gaussian daily variation and also reached a maximum (13-15) just before the peak haul out. A stricter criterion for calculating the ratio between breeding individuals is to consider only the alpha males, the males that has control of a harem and, therefore, almost unrestricted access to breeding females (at least in our study population). This may be the best measurement of the actual sex ratio of breeding individuals in a polygynous species with a harem defense mating system. On Sea Lion Island, the overall ratio of breeding females to alpha males reached the maximum (46-47 females per male) just after the peak in the number of hauled out females. The sex ratio between breeding individuals recorded on Sea Lion Island is intermediate between that reported by for the Valdes Peninsula (females/alpha males = 11) and those reported for most sub-Antarctic populations, for example for South Georgia (females/alpha males = 74), for the Kerguelen Islands (102.3), and for Macquarie Island (277). Male age structure There was a great variation in the timing of arrival of different age classes on the breeding beaches. Consequently, the age structure of the population on land changed throughout the breeding season. The pattern of males' arrival depended on age; adult males (median day of arrival = day 9 of the breeding season) and older subadults arrived earlier than the younger animals. A peculiar pattern of this variation in age structure was the decrease in the relative number of adult males in the population, a common feature of most elephant seal populations: at peak haulout they represented less than 40% of the breeding males on land. The number of breeding males in each age class increased gradually from subadult class 1 males to adult males (SAM1 = 3.6%, SAM2 = 17.9%, SAM3 = 14.3%, SAM4 = 28.6%, AD = 35.7%). This age structure is typical of a highly competitive mating system in which older males tend to have a higher resource holding potential and tend to keep younger individuals away from breeding areas and females. Survival More than half of the breeding females tagged during one season were observed again on Sea Lion Island the next season (67-78%). As females of the genus Mirounga are known to show fidelity to a breeding site, this is likely to be a good estimate of the actual survival rate of females between breeding seasons. We estimated the rate of tags lost by using double-tagged individuals and applying a binomial model of tag loss: tag loss rate are very low. Therefore, the lack of identification of marked females would not significantly change the estimate of female survival between breeding seasons. Nevertheless, the estimate of survival was more accurate for breeding males than for females, as we marked most breeding males with more tags than females, recorded every scar or natural sign on male bodies, and checked each male for signs of lost tags on flippers. We assumed that males not present during the breeding season had died because of the strongly phylopatric nature of the species, because of the lack of evidence that males may skip a breeding season, because of the high quality of our daily resight records and because of the limited chances of breeding outside Sea Lion Islands. About fifty percent of the breeding males survived until the next breeding season.

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Socionomy of the population Demography and social system The social system of elephant seals populations is shaped by two factor: land breeding during a short breeding season and strong tendency of females to aggregate when on land. These two factors are common in land breeding pinnipeds and result in an high level of polygyny. Two additional factors should be considered when dealing with elephant seals: the low mobility of females on land and enormous sexual dimorphism. All these factors result in a mating system based on harem formation: females aggregate in large groups, each one defended by a single male (the alpha male or harem master) with exclusive (or almost exclusive) access to breeding females of the harem. This mating system permits the achievement of the highest level of polygyny recorded in mammals.

A large (more than one hundred females) elephant seals harem (the alpha is in the middle of the harem; one beta is between females on the right side)

The stability of harems in both space and time is the result of the strong tendency of females to aggregate and of the reduction of movements after parturition. The grouping of females is an autocatalytic process: the increase in size of an harem makes it much more visible and more attractive for females, and that favors the increase in size. The percentage of females in harems is higher than 90% for the most of the breeding season and isolated pupping is very unusual (a single case in 1996, with death of the pup; four cases, with three alive weanlings in 1997). Hence, breeding in an harem is an almost obliged component of female breeding strategies in this species. The structure of the mating systems depends on local demography, which defines the basal level of polygyny, and on the intensity and result of intermale competition, which defines the realized level of polygyny. We call the sum of demographic and social factors socionomy of the population. Stability of the social system There is a low change of the breeding males system across breeding seasons. 65-70 %of the breeding males of the 1996 and 1997 breeding season have been marked as resident males during the previous season (34.0 % were breeding males during the previous season). Sea Lion Island is a small island with a small local population of elephant seals and preliminary information show that there is very few breeding of elephant seals in the rest of the Falklands: so breeding males should have few opportunities to roam around, and this should in turn determines a greater stability of the social system. Spatial distribution of harems On Sea Lion Island the settlement of females is uneven and the distribution of harem is irregular: all the breeding activities happen on the sandy beaches at the eastern tip of the island. The restriction of breeding to sandy beaches could depend upon the easy of access, because their modest slope facilitate the haul-out of pregnant females. The distribution of harems shows a little variation between breeding seasons on large scale, apart from two aspects: creation of small new harems in previously unused areas and hundred meters scale variation of the initial site of the harem. The fidelity of individual females to pupping sites is very high on large scale but may be quite low on a finer one: in elephant seals there is a strong relationship between prepartum movements and male interference and so the settlement of females depends not only on preferences at arrival but also on interaction with males in arrival phase. Movements of both males and females (before parturition) are limited between breeding zones but quite common between harems. Fidelity is high at zone level (about 70% of females tagged came back to the same zone for breeding) but is lower at harem level (about 39% of females breeding in the same harem, about 55% of females breeding in another harem already present in the previous season and about 6% of females breeding in new harems). Fidelity to breeding site of individual females across seasons is high on kilometer scale but quite low at hundred meter scale: any case it's quite remarkable that more than one third of females came back to exactly the same harem for breeding. Timing of harems formation Different harems have different timing of formation and disappearance. The median day of formation was day 20 to 23 (third week of September) in different seasons; the median lifetime of harems was 53 to 57 days. There is a general relationship between the date of formation of an harem and its size, with an early beginning for larger harems, but this relationship is not an absolute one, and also late forming harem may achieve a mid range size. The variation in the timing of different harems has a strong effect on male mating strategies: males excluded from the control of harems at the beginning of the season have additional opportunity to get control of late harems. These means that the most powerful males will get control of the early harems, which are usually but not always the larger ones, but later in the season new opportunities will be available for less powerful males. Sometimes late harems will grow up to a large size giving a good breeding opportunity to secondary males. This process reduces the strength of the link between male phenotypic qualities and access to females, reducing the opportunity for sexual selection to operate. Harem size The single most important component of socionomy is harem size, which measures the tendency of females to breed in an aggregated social system. We define harem size as the maximum number of females counted in an harem during the daily census; it represents about 90% of the number of females that actually breed in the harem. Median harem size varied between 31 and 35 females in different seasons. The mean harem size recorded at Sea Lion is smaller than the ones recorded at Punta Delgada (Valdes Peninsula), and also much smaller than the mean harem size reported for South Georgia and the majority of other population of southern elephant seals, but is larger than the median harem size of 11 females calculated for the whole Valdes Peninsula population. The intraseasonal variation of the harem size is large (5-125 females): this is the first indicator of the huge variation in breeding performance between males who are able to maintain control of a harem. This variation in size of harems is one of the main source of variance in mating success, and hence of potential for sexual selection. Social and behavioural effects of harems structure The first correlate of harem structure should be the level of competition: the mating system of elephant seals is an overcompetitive one, with the most of the breeding males excluded from direct control of females. These peripheral males usually assemble around the larger harems, and so we expect a positive relationship between competition pressure and harem size. In fact an index of potential level of competition calculated from the daily number of secondary males associated to each harems has a strong correlation with harem size. This relationship is almost linear (82% of variance explained by the linear component): the linearity of this relationship is important because it implies a gradual increase in costs of defense with the increase of benefits due to access to a larger number of breeding females. The competitive interaction rate (interactions per observation period) has a wide variation among harems, with median values ranging from 1 to 18. There is a strong positive relationship between median interaction rate and harem size, but the relationship is not linear: for harems up to about 60 females of size the interaction rate is low and grows slowly, while for larger harem the increase in interaction rate with size of the harem is sharp. This two speed process implies an increase of cost of defense more than proportional for very large harems: only large, adult, experienced males are able to manage this level of competition pressure.

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Breeding biology of the population Timing of the breeding season Reproduction in elephant seals occurs during a fixed three-month period when females come on land to give birth, suckle their pups and mate. The timing of the breeding of pinnipeds is regulated photoperiodically . On Sea Lion Island, the peak presence of females on land was recorded on October 20 in 1995 and 1996, and October 19 in 1997 and 1998. There is a rough clinal variation of the day of maximum number of females hauled out in various populations of southern elephant seals, and the timing of maximum haul out on Sea Lion Island is in accordance with this cline. Females begin to come on land during the second week of September. The first birth was recorded on September 11 in 1995 and on September 17 in 1996. The last departure of females was on December 3 in 1995 and on November 27 in 1996: the late departure in 1995 was due to an isolated female with an unusual breeding pattern. On Sea Lion Island, births were recorded over a 60-days period in 1995 and a 58-days period in 1996. These periods are longer than those reported for the Isles Kerguelen (43 days) and Isles Crozet (36 to 51 days), but similar to the times estimated for South Georgia (60 days). On the whole, the length of the birth season seems to be regular both across populations and across seasons in the same population, confirming the strict control of timing of breeding in this species. Female breeding The timing of female reproduction in land-breeding pinnipeds is usually very regular, and elephant seals are no exception. The typical female comes on land a few days before parturition, gives birth, suckles the pup for about three weeks, mates once or more during a couple of days, and finally returns to sea. Median arrival date was October 5 in each season. Parturition was also very regular, and median date varied from October 10 to October 12. The departure date was more irregular, showing a median between November 1 and November 6. As a consequence of the regular timing of parturition and the predictable interval between parturition and oestrus, the distribution of days of estrus was very similar in the two years, the median date being October 28 or October 29. This regularity probably has a significant effect on male breeding, as the distribution of females in oestrus is very predictable both in space and time. The total time spent on land (arrival to departure) was very similar among females, showing a median of 27 days. The interval between parturition and the beginning of oestrus (first non-protested copulation) was also regular with a median of 20 days. This regularity is expected to promote synchronization in female mating and the predictability of the distribution of fertile periods over the breeding season. The timing of reproduction in our study area is almost the same as the one reported for the Valdes Peninsula and South Georgia. The main difference between the populations is the day the season starts, while internal timing is almost equal. Female phenotype and timing of reproduction Preliminary evidence suggests that the timing of reproduction partly depends on some aspects of the female phenotype, particularly size. Small females tended to arrive earlier than large females (median arrival day = 29 vs 34), with medium females closer to large ones. Small females also tend to give birth earlier than large females (median parturition day = 34 vs 40). This variation in the timing of breeding of females of different age classes suggests a variation in breeding strategies based on phenotypic differences. Large females are expected to have greater fat reserves and hence to be able to stay on land and suckle their pup for a longer period. Small and medium females spent a lower median number of days on land than large females (27 days vs 29 days). Small and medium females spent a lower median number of days suckling their pups than large females (21 days vs 23 days). Large females seem to be able to stay on land longer and to suckle their pups for more days than smaller females. The absolute value of the difference in timing is small, but suckling is a heavy cost for the mother, as the energy transfer rate between mother and pup is very high. Therefore, even one additional day of suckling may have a significant impact on the female's energy consumption and the weaning weight of the pup. Sex ratio at weaning In our study area, the sex ratio was slightly biased toward males (0.87 to 0.94 females per male), although the difference from an equal sex ratio was not significant. A slight bias toward males at birth is common in the majority of elephant seal populations, ranging from 0.82 in South Georgia to 0.89 in the Kerguelen and 0.90 on Heard Island, although an almost balanced sex ratio may sometimes be found. Classical theory of evolution of breeding effort predicts an adaptive variation of offspring sex ratio of different mothers with different phenotype. We compared sex ratio at birth of Sea Lion females classified by size and found small, non significant differences between size classes. This is rough, preliminary evidence against adaptive variation of sex ratio at birth according to capability of maternal investment. Previously published data are contradictory; no adaptive shift in parental investment was reported in M. angustirostris, while for M. leonina there is variation between studies.

Weight at weaning The distribution of weights was almost normal, with a mean weight of 133.6 kg. There was a modest sexual dimorphism at weaning in favor of males (males: 135.4 kg ± 21.6; females: 132.0 kg ± 23.3), but the difference was not statistically significant. Weight at weaning was partly related to the duration of suckling: the number of days between birth and weaning was positively correlated to weanling weight. The weights recorded on Sea Lion Island are similar to those reported for the Valdes Peninsula and higher than those reported for South Georgia.

In our population there was a clear relationship between mother's size and weight of pups at weaning; mean corrected weanling weight rose from 109.2 kg for small mothers to 125.1 kg for medium sized mothers, to 149.1 kg for large mothers. These weights by size class are very similar to those found at Valdes. Our results confirm that the total breeding effort of females depends on their own size.

Male breeding As shown above, there was a large variation among age classes in the median day of arrival on land of males. Large variation was also found in total time spent on land. The first males come on land at the very beginning of the breeding season, during the first week of September, before the haul out of the first female. Some of the breeding males stayed on land for the entire breeding season, fasting for as much as three months, while others were on land for just a small part of the season. We considered total presence (number of days in which a male was on land) and active presence or tenure (number of days in which a male was on land and involved in competition for access to females). The median number of days spent on land by breeding males was 55, yet the number of days in which the males were active in breeding was only 15, with a large variability between individuals (CV = 0.893): only a few males were able to sustain a large breeding effort throughout most of the breeding season. According to the predictions of an age-specific tuning of the breeding effort based on life history theories, a large variation in presence on land is expected among male age categories. The total presence on land was greater for adult males than for subadult males, but the difference was not large and only modest variation among subadult classes was revealed. There was much larger variation in active presence among age categories: there was a gradual increase in the number of days spent on land from subadult class 2 to subadult class 4 and a steep increase between SAM4 and adult males. The effect of aging on presence on land is revealed by a comparison of the number of days spent on land by males breeding during different seasons; 87 of the males that returned stayed on land longer in the following season, and that percentage becomes higher (93-96%) if we consider only males who were subadults in the previous season. For subadult males, the mean increase in presence on land in the second season was 18 days. This result confirms an increase in the breeding effort as males get older. Another correlate of length of presence on land was the seasonal breeding status of males. Principal males, which controlled females during the breeding season, spent more days on land than secondary males (med. = 73 vs 49 days). Our result is similar to that obtained at Valdes Peninsula, and is supported by anecdotal evidence from other populations. The role of time spent on land is clarified by examining its relationship with breeding performance. The number of days spent on land has a strong positive correlation with level of control of breeding females, with mating rate, and with estimated fertilization success. Length of presence on land therefore seems an important component of male breeding performance. In our study area we confirmed that older males with higher status tended to arrive on land earlier and to stay there longer, gaining large breeding benefits.

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