Reproductive endocrinology of the Black-browed albatross Diomedea melanophris and the Grey-headed albatross D. chrysostoma
Gonadal size and the circulating concentrations of two pituitary hormones (luteinizing hormone and prolactin) and three gonadal steroids (testosterone, progesterone and oestradiol‐17β) were measured in two closely related Diomedea albatrosses at South Georgia. The Grey‐headed albatross D. chrysostoma, if successful in rearing a chick, usually breeds biennially, whilst the Black‐browed albatross D. melanophris normally breeds annually. Direct examination (by laparoscopy) of the gonads showed that the testes of both species underwent annual cycles, whilst endocrine data confirmed that those male Grey‐headed albatrosses at the colony in the pre‐laying period but not breeding in that year (having bred successfully the previous year) were apparently in full reproductive condition with elevated testosterone levels typical of breeding birds. However, the females of the two species differed markedly. Grey‐headed albatrosses, in a year following successful breeding, had undeveloped ovaries with low levels of circulating oestradiol but high levels of progesterone, whereas the Black‐browed albatrosses showed a pattern consistent with annual ovarian development. The profiles of gonadal steroids through the breeding season were similar for the males of both species but differences existed between the females. In the female Grey‐headed albatrosses, transient peaks of progesterone were present throughout chick rearing but these were absent from Black‐browed albatrosses. Prolactin had a similar profile in both species, with uniformly high levels throughout incubation and a rapid fall near the end of the brood‐guard period. It is suggested that Grey‐headed, like Black‐browed, albatrosses are intrinsically annual breeders. However, if a female Grey‐headed albatross breeds successfully in one year, then nutritional factors operate to ensure that in the following year the female does not show ovarian development, although the ovary is active in terms of progesterone secretion.