Pupping season, perinatal sex ratio and natality rates of the Mediterranean monk seal (Monachus monachus) from the Cabo Blanco colony

Abstract

Pupping seasonality, annual pup production, reproductive rate and sex ratio at birth were determined for the Mediterranean monk seal Monachus monachus colony at Cabo Blanco, the only surviving population of the species which still shows a colony structure. Data were collected by means of a monitoring programme that operated continuously in the main reproductive areas of the population during the period March 1993-May 1997. Most (84%) of the pups observed were born in two caves separated by a short distance. Although both caves were used by females to bear the pups, preferences greatly fluctuated from season to season with no apparent trend. Reproduction occurred throughout the year with a small peak of births in October. This unusual lack of seasonality is associated with the low latitudinal location of the colony and the effect of the upwelling that provides constant food throughout the year. The neonatal sex ratio did not differ significantly from 1:1 and the estimated annual productivity ranged from 44-58 pups. Taking into account population numbers, the annual birth rates were estimated to range from 0.3 to 0.43 pups per reproductive female, an extremely low rate when compared to other pinniped populations, particularly those of a similar size. Such low rates may be either an evolutionary determined characteristic of the species or a demographic consequence of the low population density over the last few centuries. Coupled with high pup mortality rates, this may account for the observed non-recovery of the population.