CAROL M. VLECK and THERESA L. BUCHER*, Department of Zoology and Genetics, Iowa State University, Ames, Iowa 50011
*Present address: University of California-Los Angeles, Department of Biology, Tarzana, California 91356
The population dynamics of penguins in Antarctica have been the focus of several studies (Ainley, LeResche, and Sladen 1983; Trivelpiece et al. 1990). Our research focuses on the physiological underpinnings of individual differences in reproductive behavior that contribute to population-level performance. We concentrate on the reproductive hormones that influence reproductive behavior. Adélie penguins are a particularly tractable species in which to study the hormonal control of reproductive behavior and individual variance. Birds can be caught easily and repeatedly in the breeding colonies to take blood samples for hormone analysis, and it is easy to observe their behavior and reproductive success.
One of our first objectives was to determine the hormonal and body-mass changes that accompany the reproductive cycle of penguins on Torgersen Island near Palmer Station (64°46'S 64°04'W), Antarctic Peninsula. Adélie penguins begin to arrive at the breeding colonies in mid-October. After arrival they generally do not leave until their two-egg clutch is complete. Pairs defend a small nest site within the colony and construct a nest of rocks. Egg laying peaks in mid-November; chicks hatch in mid to late December. Males take the first shift of incubation. Females return to the sea to forage after egg laying is complete. On Torgersen Island, females return approximately 8-10 days later, and males leave the colony for the first time to forage. During mid to late incubation and once the eggs have hatched, males and females trade off in attendance at the nest every few days. Adélie chicks are brooded or guarded nearly continuously until they are about 3 weeks old. After reaching thermal independence, the chicks join crêches while both parents forage simultaneously, returning every day or so to feed the chicks. By mid-February, some chicks begin leaving the colony.
During the 1995-1996 field season, we banded 62 focal pairs of Adélie penguins as they arrived in the colony and set up territories. These birds were followed through the course of the breeding season. Birds were weighed and a small blood sample taken by jugular venipuncture every 10-15 days. Hormone levels were later assayed by radioimmunoassay at our home institution. We surveyed the colonies daily, weather permitting, to determine which birds were present and their reproductive stage. Of the 62 focal pairs, 56 laid eggs, nine lost their eggs during incubation, three lost the chicks after hatching, and 44 raised at least one chick to the crêche stage. The mean date on which the first eggs were laid for these pairs was 14 November 1995, and the mean date of the first hatching was 20 December 1995. The mean interval from first egg to first hatching was 36 days [standard deviation (SD)=4], from lay of the second egg to its hatching was 33 days (SD=4), and between laying of the first and second eggs was 3 days (SD=1).
Plasma testosterone was elevated in both males and females during the courtship (pre-egg laying) stage, although the levels of testosterone were approximately an order of magnitude higher in males than in females, and there was substantial variability between individuals (figure 1). A distinct drop in testosterone occurred prior to the day the first egg was laid. This decrease in testosterone may be necessary for the birds to shift their behavioral focus from courtship and nest-site defense to incubation of the eggs (Wingfield et al. 1990). Likewise, estradiol was elevated in both sexes during courtship but was about threefold higher in females than in males. Prolactin is the hormone most strongly associated with parental care in birds (Buntin 1996). Plasma prolactin was low when the birds arrived in the colony and rose during the courtship period. It reached its highest levels by mid-incubation and thereafter remained elevated through the brooding stage. During the parental phase, prolactin levels were significantly higher in females than in males (F=78.2, P<0.001), even though the two sexes share equally in parental duties.
Adélie penguins arrive on the breeding colony with large fat reserves. Body mass decreases as the birds fast through the courtship phase (figure 2). The body mass of focal males in October after arrival from the winter foraging grounds was 5.01 kilograms (kg) [SD=0.34, sample size (n)=44]. Females were slightly smaller; the body mass of focal females arriving in October was 4.55 kg (SD=0.32, n=43). The body mass of those males that did not successfully acquire a mate who laid eggs was not different from the body mass of males that mated with females that did lay eggs (figure 2 A ). Those females that never laid eggs were smaller on average than females that laid eggs (figure 2 B ). Of birds weighed in October, two female nonlayers weighed 0.5 kg less than those females that did lay eggs during the season. These nonlaying females may be unsuccessful because they do not have sufficient fat stores to undergo reproduction and/or they may be young, pre-breeding birds that have returned to the colony and keep company with another bird but do not lay eggs (Ainley et al. 1983). During the courtship fast, focal females lost approximately 38 grams (g) of body mass per day or about 25 percent of the body mass over a 1-month fast. Males lost approximately 43 g of body mass per day or approximately 39 percent of their body mass over an approximate 45-day fast. When the birds were alternating between foraging at sea and tending eggs or chicks, their mean body masses were 4.20 kilograms (SD=0.43) for males and 3.85 kilograms (SD=0.36) for females (figure 2).
We thank our field assistants, Asrun Kristmundsdottir and Wendy Reed, for their able assistance with all aspects of this research, and the support staff at Palmer Station. This research was supported by National Science Foundation grant OPP 93-17356 to C.M. Vleck.
Ainley, D.G., R.E. LeResche, and W.J.L. Sladen. 1983. Breeding biology of the Adélie penguin . Berkeley: University of California Press.
Buntin, J. 1996. Neural and hormonal control of parental behavior in birds. Advances in the Study of Behavior , 25, 161-213.
Trivelpiece, W.Z., S.G. Trivelpiece, G.R. Geupel, J. Kjelmyr, and N.J. Volkman. 1990. Adélie and chinstrap penguins: Their potential as monitors of the southern ocean marine ecosystem. In K.R. Kerry and G. Hempel (Eds.), Antarctic ecosystems, ecological change, and conservation . Berlin: Springer-Verlag.
Wingfield, J.C., R.E. Hegner, A.M. Dufty, Jr., and G.F. Ball. 1990. The "challenge hypothesis": Theoretical implications for patterns of testosterone secretion, mating systems, and breeding strategies. The American Naturalist , 136, 829-846.