The Annual Cycle on Land

The annual cycle of the two species is similar except that the time scale is shifted (fig. 1.3). M. leonina breeds in the austral spring (early September to mid-November), while M. angustirostris breeds in the northern winter (December to February). Temporal intervals between reproductive events are similar (Carrick et al. 1962; Le Boeuf and Petrinovich 1974). There are slight shifts in peak events, such as the maximum number of females during the breeding season, among rookeries distributed widely across latitudes. This is especially well documented in the southern elephant seal (Laws 1956a ; Condy 1979; McCann 1985; Hindell and Burton 1988). As one proceeds from northern rookeries such as Peninsula Valdez, Argentina, to southern ones like the islands of King George and South Georgia (across 22 degrees of latitude or about 1, 736 km), the peak number of females is delayed by 22 days, from October 3 to October 25 (McCann 1985; Campagna, Lewis, and Baldi 1993). Variation in the onset of the season in the northern species, from one rookery to the next, has not been reported, but differences, if they exist, are thought to be slight.

The annual cycle of northern elephant seals at Año Nuevo, California, seems to be typical of that of other colonies of seals in the Northern Hemisphere (with the possible exception of numbers during the female and juvenile molt; see fig. 1.3a) but differs in some respects from southern colonies, not only in the shift in seasons but in the timing of haul-out in different age and sex groups. The annual cycle is described below for Año Nuevo and for Macquarie Island.

Northern Species

It is useful to divide the annual cycle into four terrestrial phases: breeding season, female and juvenile molt, male molt, and juvenile haul-out. The relative numbers present on the rookery during each phase is shown in figure 1.3a.

Breeding Season:
December to Mid-March

The breeding season at Año Nuevo begins in early December with the arrival of the adult males. Usually, the older bulls arrive first, and all serious competitors are on the rookery by the end of December. Concurrent with the arrival of adult males, there is a rapid decline in the number of juveniles, 1 to 4 years old, that previously predominated on the rookery. Pregnant females begin arriving in mid-December, reach a peak during the period from January 26 to February 2, and then their numbers decline until all of them have returned to sea by the end of the first week in March (fig. 1.4). Younger males begin leaving the rookery in late February, but the larger bulls remain on the rookery until the end of March, long after the last female has departed.

The pupping period is from about the third week in December to the end of the first week in February. Copulations occur from the first week in January through the first week in March, with February 14 being the peak day of copulation frequency.

Female and Juvenile Molt:
Mid-March through May

As the last females wean their pups and return to sea, the first females that gave birth early in December begin returning from sea in mid-March to molt, a process that takes about one month to complete. This influx of adult females continues for about two months. The adult females are joined by juveniles, 1 to 4 years old, of both sexes. The highest number of animals present on the rookery are seen in late April. Present in the spring but declining in number are adult males, all of whom return to sea by the end of March, and the newly weaned pups, 80% of whom leave the rookery by the end of April. Rather suddenly, in early to late May, there is a rapid decline in total numbers on the rookery.

Male Molt:
June through August

The lowest number of animals are observed on the rookery in June, July, and August (fig. 1.3a), when breeding-age males molt. There is a tendency for the younger pubertal males, 5 to 6 years old, to arrive in early summer, and they are followed by the older males in late summer (fig. 9.1, this volume). A few young of the year and 1 ½-year-olds are observed early in this period, but they make up less than 5% of the total number of seals in residence. Juvenile numbers begin to increase in August, and in some years, they outnumber molting males by the end of the month.

Fig. 1.3
A schematic representation of the annual cycle of northern and southern
elephant seals showing the relative number of animals present throughout the
year. The times of breeding and other events during the annual cycle are
indicated by horizontal bars. (A)  The relative number of northern elephant
seals on a rookery such as Año Nuevo Island during the late 1970s (solid line).
The dashed line indicates that for some rookeries (e.g., San Nicolas and San
Miguel), the number of animals during the molt of cows and juveniles is
nearly the same as the number observed during the breeding season
(Le Boeuf and Bonnell 1980). (B)  The relative number of southern elephant
seals present on a typical rookery during the annual cycle (based on
data from Laws 1956a and Hindell and Burton 1988).

Fig. 1.4
A schematic diagram showing the increase and decrease in categories of elephant
seals present during the breeding season on Año Nuevo Island (solid line;
adapted from Le Boeuf 1972) and a typical rookery of the southern elephant seal
(dashed line; adapted from Laws 1956a  and Hindell and Burton 1988).

Juvenile Haul-Out:
September through November

By the first week in September, all but a few males have gone to sea to feed and fatten up for the approaching breeding season. Young of the year and juveniles up to four years of age increase steadily in number to a peak in October. At Año Nuevo, many of these juveniles are immigrants from southern rookeries. Some yearlings exhibit a pathological skin and pelage called "scabby molt," but as a rule, normal molting does not occur during this period. Pubertal, subadult males begin to arrive on the rookery during the middle of November, a time when juveniles are decreasing in number.

The schematic representation of the total number of elephant seals present on Año Nuevo throughout the year (fig. 1.3a) shows the relative number of animals present in each phase of the annual cycle. The temporal patterning of peaks and troughs in total animals, as well as the relative size of the peaks to each other, has varied little at this rookery during the last two decades. Elephant seals are in residence throughout the year, but their

numbers fluctuate greatly because animals of both sexes and different age groups are moving in and out predictably.

The pattern in figure 1.3a resembles that observed at other northern elephant seal colonies, such as San Nicolas, San Miguel, and Santa Barbara islands (Odell 1974; Bonnell et al. 1979), during the same period that Año Nuevo was monitored (Le Boeuf 1981). The difference is that the female-juvenile molt numbers are equal to or only slightly higher than breeding season numbers. This is because animals from southern California rookeries may haul out to molt in a different place from where they breed, showing up to be counted on northern California rookeries like Año Nuevo. (Similarly, some South Georgia elephant seals haul out to molt at Signy Island, Elephant Island, and South Shetland Islands; Macquarie Island animals haul out on the Antarctic continent. Also, bulls haul out to molt in limited numbers at Año Nuevo compared with colonies of the southern species.)

Southern Species

Information for the southern species is drawn from R. M. Laws (1956a ), R. Carrick et al. (1962), and M. A. Hindell and H. R. Burton (1988) (fig. 1.3b).

Breeding Season:
August to Mid-December

At Macquarie Island, the adult males arrive in early August, and all serious competitors are present by mid-September (fig. 1.4). Small numbers of juveniles (females of 1 to 4 years; males of 1 to 6 years) haul out in winter but decline to near zero by the end of August. Pregnant females begin to arrive in mid-September, reaching a peak about October 16, subsequently declining so that all have returned to sea by the end of November. Adult bulls remain until mid-December. The pupping period is from late September to the end of October. Copulations occur from mid-October to mid-November, with peak frequency in early November.

Female and Juvenile Molt:
January and February

Mature females begin returning from sea about the beginning of January, to molt for about one month, reaching peak numbers about the end of January; all have returned to sea by the end of February. The juvenile haul-out begins much earlier, in mid-November, reaches a peak about mid-December, and ends in late January.

Male Molt:
November to April

Subadult males (6 to 8 years old) molt from early November to early March, with a peak at the end of December. Adult males haul out for molt from early February until May, with a peak in mid-March. The lowest numbers are observed on the rookeries in winter, July to August, representing a haul-out of adult males and juveniles.

Juvenile Haul-Out:
Mid-March to the End of August

By the end of August, males have completed their molt and gone to sea to feed and lay down blubber for the breeding season. At Macquarie Island, there is a winter haul-out of juveniles from April to August, with peak numbers in early June less than half of the peak molting haul-out. At Macquarie, juveniles are present ashore for a greater proportion of the year than any other population component. At Signy Island and South Georgia, there is no significant winter haul-out.

At Signy Island, because the breeding population is small, the highest numbers are ashore molting in the last week of January, representing immigrants from South Georgia. But at Macquarie Island and South Georgia, the largest numbers are present in the breeding season.

Thus, the general haul-out cycles for northern and southern elephant seals are broadly similar in the breeding season, but there are significant differences in detail throughout the rest of the year. The overall breeding season is longer in the southern species, mainly due to earlier haul-out of the bulls, but the pupping season is more synchronous with a more pronounced peak in numbers of cows; weaned pups depart to sea earlier (fig. 1.4). The female and juvenile molt is slightly shorter and later at Macquarie Island than at Año Nuevo. The period when males are ashore for the molt is twice as long at Macquarie as at Año Nuevo, starting earlier and ending earlier at Macquarie; also, few adult males molt at Año Nuevo (i.e., only the males that breed there). The juvenile haul-out is similar in the two locations, although slightly longer and earlier at Macquarie. At South Georgia and Signy Island, south of the Antarctic Convergence, it would be expected to be earlier than at Año Nuevo or Macquarie owing to lower air temperatures (and pack ice around Signy Island) from April onward. The limited data confirm this (Laws 1956a ).

Some colony and species differences in the shape of the annual cycle of total animals are to be expected given that there are differences in molting locations, in the time that food is abundant, and in the distance of prey from the rookery. By and large, the presence of elephant seals on traditional rookeries and hauling grounds is predictable by age and sex.