San Benito's project goals

The general goal of the research project can be achieved only by the accurate measurement of the phenotypic, social, and physical factors that may affect male mating tactics. Therefore, the specific goals of the field works are as follows:

Topography of breeding areas and mapping of harems

Breeding beachesare mapped using GPS receivers at the beginning of the breeding season, to reduce disturbance. From maps the perimeter, area, and shape of the breeding beaches is estimated. Mapping will is repeated at minimum and maximum tide level to estimate the effect of tides on breeding space availability, and the consequent variation in female density. Harems are mapped repeatedly during the season, using laser rangefinder when possible to reduce disturbance, to estimate female density. Digital pictures of the breeding beaches are taken from vantage points of view at regular interval, including in the frame reference points established by GPS static surveying. The reference points are used for rectification of digital pictures, from which female distribution and spacing inside harem is estimated. This data is combined with female counts to estimate harem crowding, and its variation with tides and fluctuations in microclimate.

Position of individuals

Positions of harem holders, peripheral males and moving females is collected during behavioral observations at 5 minutes interval using the laser rangefinder. This require two observers to carry out the work, and, therefore, the number of harems subject to laser rangefinder mapping depend on personnel availability. Our goal is to have at least one weekly estimate of female density and crowding for each harem in the study area.

Demography and socionomy of harems

Harem size, number of associated males, males age structure, and breeding sex ratio are calculated from daily census. An estimate of number of females in estrus is derived from parturition dates, and the operational sex ratio is calculated accordingly.

Climate data

To study the effect of climate on the mating systems data from data loggers and hand handled instruments are collected and correlated to measures of female density and to female spatial distribution at small scale. Microclimate seems to have a significant effect on female spatial distribution at San Benitos, in particular in the central island, due to the presence of land harems without easy access to the sea. These movements from harems to sea, and the reverse, offer many alternative mating opportunities for peripheral males. Dye marking of females combined with laser rangefinder positioning permits to estimate the effect of female mobility on their likelihood to be intercepted by peripheral males.

Structural traits that affect mating success

The measurement of structural traits of male phenotype is carried out with standard methodologies already used in our Falklands research project. Males are classified in age categories based on the morphology of the frontal shield and trunk. This method has been tested and found effective in both the northern (Clinton 1994) and the southern elephant seals. Males are also classified in age-specific size categories. Age and size classification are independently estimated by the different observers, and the concordance of different classification is routinely checked. Male size is estimated using a photogrammetric method, that we have extensively used, with excellent results, on southern elephant seals. This method permits to get good estimates of body length and weight without any restrain of the animal, and with little disturbance.

Behavioral traits that affect mating success

The number of days of presence on land and an index of male tenure (number of days in which the male is active in breeding) is calculated from the daily records of marked males. From the results of dominance interactions among males we calculate the local male rank, at weekly interval, and a cardinal index of competition success. From male-female interaction we calculate an index of male mating effectiveness (proportion of actual mating on total mating attempts). Previous breeding experience, that may have a significant impact on male mating tactics, is evaluated from the records of males tagged during the previous breeding seasons.

Estimation of mating success and paternities

Three measures of breeding success are calculated from census and behavioral observation data: - the female days index, i.e., the total number of females held by a male during the breeding season - a mating success index, the number of copulations carried out by the male per 100 hours of observation - a fertilization success index, the ENFI (estimated number of females inseminated), calculated from the proportion of copulations carried out by a male in each harem and the number of females breeding the harem. These measures show a high positive correlation, and, therefore, ENFI is used in the following analysis as a summary measure of breeding success. The microsatellite analysis of skin samples permits the estimation of the actual genetic paternities.