Corticosterone levels predict survival probabilities of Galapagos marine iguanas during El Nino events

Abstract

Plasma levels of corticosterone are often used as a measure of "stress" in wild animal populations. However, we lack conclusive evidence that different stress levels reflect different survival probabilities between populations. Galápagos marine iguanas offer an ideal test case because island populations are affected differently by recurring El Niño famine events, and population-level survival can be quantified by counting iguanas locally. We surveyed corticosterone levels in six populations during the 1998 El Niño famine and the 1999 La Niña feast period. Iguanas had higher baseline and handling stress-induced corticosterone concentrations during famine than feast conditions. Corticosterone levels differed between islands and predicted survival through an El Niño period. However, among individuals, baseline corticosterone was only elevated when body condition dropped below a critical threshold. Thus, the population-level corticosterone response was variable but nevertheless predicted overall population health. Our results lend support to the use of corticosterone as a rapid quantitative predictor of survival in wild animal populations.

Figure 1

Body conditions of iguanas captured on six different islands during 1999 and during the El Niño of 1998. Each bar represents the mean ± SE for each year on each island for the sample sizes indicated. The body condition index = (body mass/snout-vent length³) × 10⁶.

Figure 2

Corticosterone responses to the stress of capture and handling in marine iguanas captured on six different islands during 1999 and during the El Niño of 1998. Each point represents the mean ± SE. Sample sizes for El Niño and 1999 are, respectively: n = 28 and 35 on Fernandina; n = 11 and 8 on Seymour; n = 18 and 26 on Genovesa; n = 36 and 10 on Santa Fe; n = 13 and 11 on Isabela; and n = 20 and 8 on Santa Cruz.

Figure 3

Baseline corticosterone levels depend on the body condition of iguanas, as determined by (body mass/snout-vent length³) × 10⁶. At body conditions above ≈35, corticosterone is apparently low in all individuals. When body condition drops below 35, corticosterone levels increase dramatically. Vertical dashed line represents the proposed threshold in body condition. Lines represent the linear regression for body conditions above and below 35.

Figure 4

Example of iguanas with a body condition near 60 in 1999 (Upper) and a body condition of ≈30 during El Niño (Lower).

Figure 5

Corticosterone levels measured after 15 min of handling stress predict the survival of Galápagos marine iguanas. Data show mean ± SE of local survival as determined by counts (circles without vertical error bar) or survival analysis (circles with vertical bars, for Santa Fe and Genovesa island only). Horizontal bars indicate SE of corticosterone measurements. ●, data for the El Niño period 1998 to 1999; ○, data for the subsequent La Niña period (1999 to 2000; islands Genovesa, Santa Fe, and Seymour only). The line represents the linear regression for the El Niño period only.