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Title: ENDOCRINE PATTERNS DURING AGING IN THE COMMON TERN (STERNA HIRUNDO)

Author
item NISBET, I.C.T. - NISBET & CO.
item FINCH, C - UNIV OF SOUTHERN CALIF
item THOMPSON, N - UNIVERSITY OF MARYLAND
item RUSSEK-COHEN, E - UNIVERSITY OF MARYLAND
item Proudman, John
item OTTINGER, M - UNIVERSITY OF MARYLAND

Submitted to: General and Comparative Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: The effect of aging on reproductive hormones in birds is poorly understood. Most studies of reproductive aging in birds have been conducted on laboratory or domestic species, and show a decline in fertility and changes in endocrine function. However, these birds have relatively short reproductive life spans. Seabirds generally mature slowly, have low reproductive rates, and a long reproductive lifespan. Individuals may liv for 20 to 50 years, and breed for 20 or more reproductive season. This study measured the plasma levels of reproductive hormones in wild common terns of known age during two successive breading seasons. Afes ranged from 2 to 21 years. Results showed a strong inverse correlation between age and nesting date, with older birds of both sexes nesting earlier in the year. Plasma luteinizing hormone (LH) increased in males up to 5 years, and was correlated with plasma androgen levels. After maturity, male plasma LH levels did not change. In females, plasma LH levels were constant up to 5 years, and increased steadily as age increased. Since only laying (nesting) birds were sampled, these results suggest that birds require higher levels of LH as they age in order to maintain reproduction. Plasma progesterone and estradiol did not change in an age-dependent manner. There was some indication of a reduction in homone levels among the oldest individuals (16-21 years), which might indicate the beginning of reproductive senescence. These results show that some birds can maintain reproduction in very old age, and that plasma levels of reproductive hormones are constant or rising as such birds age. This information will be useful to scientists in understanding the effects of aging on reproduction in birds.

Technical Abstract: Blood samples were taken from breeding common terns (Sterna hirundo) of known age during two successive breeding seasons to determine if plasma concentration of estradiol (E2), progesterone (P), androgen, and luteinizing hormone (LH) reflect the process of aging. Males and females were trapped and sampled once between laying of the first and second eggs in the clutch. The transient capture and sampling did not disrupt incubation or the timing of the subsequent oviposition. Ages ranged from 2 to 21 yr, with most in the range 4-11 yr. There was a strong inverse correlation (p<0.0001) between age and laying date in both sexes, with older birds nesting earlier in the season. In older (>5 yr) females, plasma LH increased (p<0.005). Plasma P and E2 levels were positively correlated manner (p<0.03) in females, but did not change in an age- dependent manner. In males, plasma P levels showed no age-related changes. .Plasma androgen levels increased in males between 3 and 5 yr of age (slope=2.73; p<0.01), but remained unchanged thereafter. Plasma LH levels increased in males younger than 5 yrs of age and were correlated (p<0.005) with increasing androgen levels. These data provide evidence for changing endocrine status with age, but the patterns differ for younger individuals (<5 yr) and adults. It appears that in younger individuals, nesting experience and maturity of the pair greatly affect reproductive performance and endocrine status. In adults, individual variability due to different rates of aging and a decline in the proportion of the population that continued to reproduce may have masked age-related changes. In summary, common terms continue to breed successfully until at least age 21 yr and these individuals maintain relatively stable endocrine status during aging.