Non-breeding members of a number of species of birds display a fascinating behavior: they help with the raising of young that are not their own. Several hypotheses have been proposed to explain why members of cooperatively breeding species may display this behavior, which is called alloparental care. First, if helpers are helping young that share some of their genes, and the costs of helping are relatively low, then what the helpers gain in terms of inclusive fitness makes helping worthwhile. Second, it has been suggested that helpers gain experience while helping that later makes them more successful parents. In contrast to these adaptionist hypotheses, other authors have proposed that helping is not a result of selection for helping at all, but is instead a byproduct of selection for expressing parental behavior whenever in the presence of young (so that the birds have the instincts to be effective parents when they do eventually breed). This is a hypothesis based on what Steven Jay Gould calls exaptation (as opposed to adaptation), a trait that was selected for in one context, and then subsequently provides a secondary benefit in another.

There has been some definitive support for the adaptationist inclusive fitness hypothesis in some species of cooperative breeders. For example, in white-fronted bee eaters (Merops bullockoides) not all non-breeders help, and those that do help tend to concentrate on helping relatives. However, the complete answers to questions about adaptation, or the lack of adaptation, as explanations of alloparental care remain murky. Clearly one way to shed more light on our understanding of these ultimate questions about helping behavior will be to learn more about the proximal mechanisms behind the behavior. It is exactly this type of inquiry that has occupied Program in Animal Behavior scientist Stephan Schoech.

Dr. Schoech has researched the Florida Scrub-jay (Aphelocoma coerulescens), a cooperatively breeding corvid living in scrub oak habitat in central Florida, for the past eleven years, first while at Arizona State University, then while at the University of Washington, and then while serving as a research associate at Indiana University. Whereas most of the research efforts on avian alloparental care in the past several decades have focused on adaptationist questions, Schoech has examined several hypotheses about the mechanisms of helping behavior.

One mechanistic hypothesis concerning helpers is the hypothesis that helpers are physically unable to reproduce. Schoech looked into the simple question: Are helpers capable of breeding? Schoech and his colleagues looked at this question in a number of ways. First, they looked at levels of luteinizing hormone, a hormone from the pituitary necessary for gonadal function, in both breeders and helpers. They found that luteinizing hormone levels were statistically the same in the two groups. Second, they compared the levels of testosterone, a steroid hormone required for reproductive function in males, in male breeders and male helpers. Testosterone levels were higher in breeders, but helpers still showed a pronounced testosterone spike at the beginning of the breeding season. Third, Schoech and his colleagues examined levels of estradiol, a steroid hormone required for reproduction in females, in breeders and helpers; they found that estradiol levels were statistically the same in both groups. Fourth, Schoech examined the possibility that helpers could not breed because they were in poorer physiological condition than breeders; he did this by providing an experimental group of helpers with a supplemental food mixture of dog food, meal worms, and peanuts. His finding: the proportion of helpers that became breeders did not increase when helpers were provided with supplemental food.

Next, Schoech examined whether or not stress was suppressing reproduction in helpers. He compared levels of the "stress hormone" corticosterone, a steroid secreted from the adrenal cortex in response to stress-related hormonal signals from the pituitary, in breeders and helpers. He found corticosterone levels to be statistically indistinguishable. Given all of the above results, and observations that helpers can opportunistically breed very quickly if a breeding territory becomes available, Schoech concluded that helpers are completely capable of breeding, but they do not do so simply because they do not have the opportunity; habitat saturation forces them, in effect, to "bide their time."

Schoech also explored the following question: Does the pituitary hormone prolactin play a mechanistic role in triggering helping behavior? Prolactin has been observed to be related to parental-type behavior in a number of species, so Schoech thought it would be a good hormonal candidate to examine in scrub-jays.

Schoech found that prolactin levels increased in both breeders and helpers in the segment of the breeding season when care was being provided, and that prolactin levels were higher in females than in males, and were higher in breeders than in helpers. Breeders had higher levels of prolactin than helpers, but this finding could in part be explained by the fact that breeders may have more triggers to initiate prolactin release than helpers do (just being around eggs and a nest may trigger the release of the hormone). He did find a significant positive correlation in helpers between the intensity with which food was supplied to young and the level of prolactin in the body. No such positive correlation was found, however, between feeding intensity and prolactin levels in breeders (perhaps because all breeders had high levels of stimuli that may trigger prolactin release, such as proximity to eggs, young, and a nest.

The results on prolactin suggest a possible relationship between prolactin and parental behavior. As for the obvious question of what comes first, the hormone or the behavior, Schoech says, "Yes!" In other words, as Schoech (1998) writes, "a hormone may increase the likelihood that a behavior will occur, and engaging in the behavior often induces increased secretion of the same hormone." Therefore, understanding the mechanistic relationship among hormones and behaviors is a complex task, but it is a task that is invaluable for refining our in our understanding of the proximal aspects of helping behavior. These advances, in turn, will help scientists make more refined conclusions about the ultimate causes for this behavior. This summer, Schoech is starting up a new lab and beginning a professor position at the University of Memphis in Tennessee.


REFERENCES

Brown, J. L. 1987. Helping and Communal Breeding in Birds: Ecology and Evolution. Princeton University Press, Princeton.

Mumme, R. L., and Koenig, W.D. 1991. Explanations for avian helping behavior. Trends in Ecology and Evolution 6: 343-344.

Jamieson, I. G., and Craig, J. L. 1987. In: Perception and Ethology, Volume Seven (Bateson, P. and Klopfer, P., eds.), pp. 79-98, Plenum Press.

Schoech, S. J. 1996. The effect of supplemental food on body condition and the timing of reproduction in a cooperative breeder, the Florida scrub-jay. Condor 98: 234-244.

Schoech, S. J. 1998. Physiology of helping in Florida scrub-jays. American Scientist 86: 70-77.

Schoech, S. J., Mumme, R. L., and Moore, M. C. 1991. Reproductive endocrinology and mechanisms of breeding inhibition in cooperatively breeding Florida scrub-jays (Aphelocoma c. coerulescens). Condor 93: 354-364.

Schoech, S. J., Mumme, R. L., and Wingfield, J. C. 1996. Prolactin and helping behavior in the cooperatively breeding Florida scrub-jay (Aphelocoma coerulescens). Animal Behavior 52: 445- 456.

Schoech, S. J., Mumme, R. L., and Wingfield, J. C. 1997. Corticosterone, reproductive status, and body mass in a cooperative breeder, the Florida scrub-jay (Aphelocoma coerulescens). Physiological Zoology 70: 68-73.