结论：吃水果

There is growing concern about theeffects of man-made noise on marinelife. In particular, marine mammals that use sound to communicate, navigate, and detect predators and prey may try to avoid loud sound sources up to tens of kilo- metres away1. Here, in a study conducted in cooperation with the US Navy2, we show that the singing behaviour of male hump- back whales was altered when they were exposed to LFA (low-frequency active) sonar. As the song of these whales is associ- ated with reproduction3, widespread alter- ation of their singing behaviour might affect demographic parameters, or it could represent a strategy to compensate for interference from the sonar. During the breeding season male hump- back whales sing long, complex songs that are thought to be sexual displays3. Songs consist of a series of themes, progressing in a predictable order, that may repeat for sev- eral hours4. We used a small observation vessel to find singing humpbacks and con- duct focal sampling5, recording behaviour before, during and after playback. (Strictly speaking, we have evaluated the additional impact that LFA sounds have on a singing whale that is already being followed.) We recorded the vocal behaviour of each focal singer continuously for several hours using a towed, calibrated hydrophone array6. When the whale was at the surface, observers sampled visible behaviour. Pho- tographs of fluke and dorsal fin features confirmed the whale’s identity throughout each follow7. At least two songs were recorded before the observation vessel requested the US Navy R/V Cory Chouest to transmit ten (in one case four) 42-s LFA signals at 6-min intervals. The sonar was broadcast at less than full strength, and no focal singer was exposed to a signal louder than 150 dB (with respect to 1 mPa). Sixteen singers were followed during 18 playbacks. In nine follows, the whale sang continuously throughout the playback; in four the singer stopped when he joined other whales (typical of normal social inter- action); and in five the singer stopped, pre- sumably in response to the playback. We recorded at least one complete song in all conditions from six individuals, and pooled the songs of each of the two individuals that were subjects in two experiments. For these six whales, we measured the duration and theme structure of song spectrograms, comparing song duration in the three con- ditions using analysis of variance8. On average, humpback whales’ songs were 29% longer during LFA playbacks (Fig. 1) — a particularly strong result, given the low power of the test and small sample size9. Song duration returned to normal after exposure, suggesting that this response has a limited duration. There was little dif- ference in the likelihood of an aberrant theme transition across exposure condi- tions (x243.273, P40.195), indicating that long songs resulted from longer themes within a normal song structure. Across the six singers, maximum received level of the sonar at the whale did not correlate posi- tively with either the increase in mean song duration from pre-exposure to exposure condition (r410.90) or with the subse- quent decrease from exposure to post-expo- sure condition (r410.63). We suggest that humpbacks sang longer songs during LFA sonar transmissions to compensate for acoustic interference. Our study shows that it is possible to measure the behavioural responses of individual whales in controlled experiments at sea. Patrick J. O. Miller*, Nicoletta Biassoni*, Amy Samuels*†, Peter L. Tyack* *Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA †Daniel F. and Ada L. Rice Conservation Biology and Research Center, Chicago Zoological Society, Brookfield, Illinois 60513, USA 1. Richardson, W. J., Greene, C. R. Jr, Malme, C. I. & Thomson, D. H. Marine Mammals and Noise (Academic, San Diego, 1995). 2. Waters, M. A. US Federal Register 61, 37452–37453 (1996). 3. Tyack, P. L. Behav. Ecol. Sociobiol. 8, 105–116 (1981). 4. Payne, R. & McVay, S. Science 173, 585–597 (1971). 5. Altmann, J. Behaviour 49, 227–267 (1974). 6. Miller, P. J. & Tyack, P. L. Deep-Sea Res. II 45, 1389–1405 (1998). 7. Katona, S. et al. in The Behavior of Marine Mammals (eds Winn, H. E. & Olla, B. L.) 33–44 (Plenum, New York, 1979). 8. Zar, J. H. Biostatistical Analysis (Prentice-Hall, Englewood Cliffs, NJ, 1984). 9. Lehmann, E. L. Ann. Math. Stat. 29, 1167–1176 (1958). brief communications NATURE | VOL 405 | 22 JUNE 2000 | www.nature.com 903 Whale songs lengthen in response to sonar Male humpbacks modify their sexual displays when exposed to man-made noise. Pre- During Post- 0 5 10 15 20 Exposure condition S on g d ur at io n (m in ) Figure 1 Duration of songs (5s.e.m.) produced by humpbacks before, during and after exposure to LFA sonar transmissions (bold, filled diamonds, mean of all six singers; other symbols, indi- vidual singers). The maximum received level of the sonar at the whale ranged from 130 to 150 dB re 1 mPa. Songs were grouped in the exposure condition if a sonar transmission occurred at any point during the song. The average number of songs per singer in the pre-exposure, exposure and post-exposure conditions was 3.2, 4.7 and 3.8, respectively. Differences were assessed using a mixed-model analysis of variance treating exposure condition as a fixed factor, whale identity as a random factor, and each song duration as an independent observation. The effect of exposure condition on song duration was statistically significant at P40.047 (F2,1044.200, power40.50, n46). Nutrition Antioxidant activity of fresh apples Vitamin C is used as a dietary supple-ment because of its antioxidant activi-ty, although a high dose (500 mg) may act as a pro-oxidant in the body1,2. Here we show that 100 g of fresh apples has an anti- oxidant activity equivalent to 1,500 mg of vitamin C, and that whole-apple extracts inhibit the growth of colon- and liver- cancer cells in vitro in a dose-dependent manner. Our results indicate that natural antioxidants from fresh fruit could be more effective than a dietary supplement. Apples of the Red Delicious variety were extracted using 80% acetone and their con- tent of phenolics and flavonoids deter- mined3,4: the extracts contained 290.254.2 and 219.851.8 mg phenolics, and 142.753.7 and 97.653.9 mg flavonoids per 100 g apples with and without skin, respectively. There are known to be more phenolics in the skins of apples than in the flesh, and quercetin glycosides are found only in the skins5. We measured the total antioxidant activ- ity of apples by using the total oxyradical- scavenging capacity (TOSC) assay6 and found that apples with skin had a higher TOSC value than apples without skin (Fig. 1a). The total antioxidant activity of 1 g apples with skin was 83.358.9 TOSC (mmol vitamin C equivalents) — that is, the © 2000 Macmillan Magazines Ltd Marian V. Eberhardt, Chang Yong Lee, Rui Hai Liu Department of Food Science, 108 Stocking Hall, Cornell University, Ithaca, New York 14853-7201, USA e-mail: RL23@cornell.edu 1. Podmore, I. D. et al. Nature 392, 559 (1998). 2. Herbert, V. et al. J. Nutr. 126, 1213S–1220S (1996). 3. Singleton, V. L. & Rossi, J. A. Jr Am. J. Enol. Viticult. 16, 144–158 (1965). 4. Zhisen, J., Mengcheng, T. & Jianming, W. Food Chem. 64, 555–559 (1999). 5. Burda, S., Oleszek, W. & Lee, C. Y. J. Agric. Food Chem. 38, 945–948 (1990). 6. Winston, G. W., Regoli, F., Dugas, A. J., Fong, J. H. & Blanchard, K. A. Free Rad. Biol. Med. 24, 480–493 (1998). 7. USDA Nutrient Database for Standard Reference (1998). 8. Wang, H., Cao, G. & Prior, R. L. J. Agric. Food Chem. 44, 701–705 (1998). 9. Lunec, J. J. Int. Fed. Clin. Chem. 4, 58–63 (1992). 10.Cory, A. H., Owen, T. C., Barltrop, J. A. & Cory, J. G. Cancer Commun. 3, 207–212 (1991). and on nutrient availability and uptake during summer, rather than on cambial cell division. From tree-ring analysis across several sites in the Siberian subarctic, Vaganov et al.1 provide evidence that annual variability in mean ring width is determined by the date of the thaw through its influence on the date of cambial initiation, as well as by temperature during the subsequent early growing season. Variability in annual net ecosystem production (NEP) is also largely determined by the timing of the thaw3, which enables the NEP to switch immedi- ately and rapidly from a daily loss to a daily gain of CO2. Our own observations on black spruce trees at the Boreas Southern Study Area4 show that when air temperature exceeds 11 °C and the overlying snow starts to melt, meltwater percolates down into the soil, the temperature of the upper soil hori- zons rises towards 0 °C, and the switch from a small net daily loss of carbon to a large net gain occurs over just a few days. In boreal conifers, the availability of soil water is a prerequisite for the recovery of photosynthetic capacity in spring and early summer2. The effect of frozen soils on annual CO2 uptake by Norway spruce at 64° N is partic- ularly dramatic because, before the thaw, daily solar radiation is already substantial and effectively being wasted from the per- spective of CO2 capture 5. Thus, CO2 uptake is synchronized and strongly stimulated by the thaw, and afterwards, once a critical temperature sum is reached, cambial activi- ty and NEP increase together as the tem- perature rises. Such observations related to the poor growth of trees in boreal forests have led to the presumption that their growth is con- strained by temperature. By contrast with temperate and tropical forests6, boreal for- est trees are small in relation to their age and coniferous boreal forests have a very low net primary production of about 2.5 tonnes of carbon ha11 yr11 (refs 7,8). To investigate the extent to which low temper- ature is the primary controlling variable, a long-term nutrient-optimization and irri- gation experiment on Norway spruce was set up at Flakaliden (64° N) in Sweden9. Since 1987, we have applied complete fertil- izer daily through every growing season either in irrigation water or as a single solid dose at the start of the growing season. We found that growth on the fertilized plots (with or without irrigation) increased by a factor of four10 (Fig. 1a), so air temperature cannot be the major direct constraint on tree growth. However, temperature may be influenc- ing tree growth indirectly through the length of the growing season and by its effects on decomposition of soil organic 904 NATURE | VOL 405 | 22 JUNE 2000 | www.nature.com antioxidant value of 100 g apples is equiva- lent to 1,500 mg of vitamin C. Given that the average vitamin C content in fresh apples with skin is 5.7 mg per 100 g (ref. 7) and that the total antioxidant activity of 0.057 mg vitamin C (in 1 g of whole apples) is only 0.32 TOSC (Fig. 1a), then almost all of the antioxidant activity in apples must be due to phytochemicals. We treated a colon-cancer cell line, Caco- 2, with extracts equivalent to 0, 5, 10, 20, 30, 40 and 50 mg ml11 apples for 96 hours (the treatment time for maximal response). Cell proliferation was inhibited in a dose-depen- dent manner after exposure to apple-extract concentrations above 20 mg ml11 (Fig. 1b): at 50 mg ml11, inhibition was 4351% and 2954.1% for apples with skin and for apples without skin, respectively. We also tested the effect of apple extracts on the proliferation of another cancer-cell line, HepG2 human liver-tumour cells. We found that apple extracts at 50 mg ml11 inhibited the proliferation of these cells as well, by 5750.21% and 4050.64% for apples with and without skin, respectively (Fig. 1c). The extracts of apple with skin could thus significantly (t-test, P*0.031) reduce tumour-cell proliferation compared with extracts of apples without skin. No cytotoxicity of the apple extracts was seen at any of the concentrations tested (data not shown). We suggest that this strong inhibition of tumour-cell proliferation in vitro could be due to apples’ combination of phytochemi- cals (phenolic acids and flavonoids), as these are natural antioxidants. It has been proposed that the consumption of whole fruits may provide the antioxidant balance needed to quench reactive oxygen species8 which have been implicated in tumorigen- esis9. Phytochemicals in apples other than ascorbic acid seem significantly to enhance their antioxidant properties and their capacity to inhibit the proliferation of tumour cells in vitro. brief communications 100 80 60 40 20 0 120 100 80 60 110 100 90 80 70 60 50 40 30 83.3 0.32 46.07 M ea n to ta l a nt io xi d an t ac tiv ity C el l p ro lif er at io n (% ) C el l p ro lif er at io n (% ) 0 10 20 30 40 50 Apple extracts (mg ml–1) 0 10 20 30 40 50 Apple extracts (mg ml–1) a b c Vi ta m in C fro m (A +S ) (A +S ) (A –S ) Figure 1 Antioxidant activity of apples and their effect on tumour-cell proliferation in vitro. a, Mean total antioxidant activity expressed by total oxyradical-scavenging capacity (TOSC; mmol vitamin C equivalents per g) assay for 1 g apple with skin (A&S), and for apple without skin (A1S). b, Inhibition of proliferation of Caco-2 colon-tumour cells by extracts of apple with and without skin. c, Inhibition of prolifera- tion of HepG2 liver-tumour cells. Control samples were assayed as apple extracts, but they contained only vitamin C at the same concen- tration that exists in apples (0.057 mg g11). Cell proliferation was determined by using the MTS assay10. Blue circles, apple with skin; yellow circles, apple without skin; orange triangles, control extracts. Botany Constraints to growth of boreal forests Understanding how the growth of treesat high latitudes in boreal forest iscontrolled is important for projec- tions of global carbon sequestration and timber production in relation to climate change. Is stem growth of boreal forest trees constrained by the length of the growing season when stem cambial cells divide1, or by the length of the period when resources can be captured2? In both cases, the timing of the thaw in the spring is critical: neither cambial cell division nor uptake of nutri- ents and carbon dioxide can occur while the soil is frozen. Here we argue, on the basis of long-term observations made in northern Saskatchewan and Sweden, that the time between the spring thaw and the autumn freeze determines the amount of annual tree growth, mainly through temperature effects on carbon-dioxide uptake in spring © 2000 Macmillan Magazines Ltd