I wrote this article as part of my Bachelor of Science study at the University or Groningen in 2009. You can download the whole PDF below for free.
Abstract of the article
Although testosterone plays an important role in complex social behaviour, the effects of this hormone on developing avian chicks remain rather unclear. Previous research on pied flycatcher chicks shows an increase in begging behaviour after experimentally increasing testosterone plasma levels in the short term, while implantation with testosterone in black headed gull chicks results in a decrease in begging behaviour. This opposite role of testosterone in these two avian species could be an intriguing species-specific difference, but could also be the result of the different experimental methodology adopted by the two studies. This study manipulates plasma testosterone levels of black headed gull chicks by means of injections, raising the circulating hormone levels of the chicks only in the short-term (i.e. a few hours). Injections of testosterone resulted in a decrease in begging behaviour, an increase in aggression and an increase in sibling competition. Therefore it can be concluded that the begging intensity of black headed gull chicks is not decreased by testosterone, which is the opposite of the effect found in pied flycatcher chicks.
Introduction: effects of hormones on begging behavior in birds
Gonadal steroids play a very important role in the regulation of complex behaviour in vertebrates. Much research has been done on the effects of various hormones on adults, while only a few studies have been done on young dependent offspring. In precocial and semi precocial avian species social behaviour in this stage of development is very important for survival. In a semi-precocial species like the black-headed gull (Larus ridibundus) the most important social behaviours are begging behaviour towards the parents to solicit food and territorial behaviour towards non-sibling (Groothuis, 1989). The begging behaviour consists of various conspicuous displays and calls and it serves to solicit food from the parents. The displays are head pumping, in which the chick moves its head up and down, begging calls, which are frequent high-pitched sounds and begging pecks, which are quick non-aggressive pecks towards the bill of the parent. The territorial behaviours consist of threat postures and calls and also actual pecking and fighting. The most common treat postures are the oblique and the choking display. The oblique display is a posture were the chicks is standing up tall with the head raised and wing somewhat open and is typical for older chicks. The choking display is a posture were the chick lies down and pecks into the ground. Loud alarm calls are uttered in all aggressive displays. These behaviours are displayed to protect the food brought to the nest by the parents from non-siblings in the colony (Groothuis, 1989). It is surprising that up to now only a few studies have tried to unravel the hormones behind these behaviours.
Recent evidence suggests a strong role for steroid hormones, mainly testosterone, in the regulation of social behaviour of chicks. Correlative studies on passerines, for example on canaries (Serinus canaria), show a correlation between circulating testosterone levels and begging intensity (Buchanan et al., 2007). Previous research on black headed gull chicks involved implantation of testosterone capsules that constantly released testosterone into the blood throughout the whole period of the experiment. These chicks showed more aggressive behaviour than their controls as well as a reduction in begging behaviour (Groothuis and Ros, 2005). These findings are contradictory to research performed in pied flycatchers (Ficedula hypoleuca), in which begging behaviour was increased after administrating testosterone (Goodship and Buchanan, 2007). The difference between these studies could be because of a species specific role for testosterone in the regulation of begging behaviour. However, these studies differ not only in species but also in way of hormone administration. Flycatcher chicks were given testosterone orally just before performing the begging tests, while the black headed gull chicks were implanted with crystalline testosterone pellets. Therefore, the testosterone plasma concentration of the canaries therefore shows a steep and short peak, while the black headed gull chicks perceive a long term, constant elevated testosterone plasma level. The opposition in the effect of testosterone on the begging behaviour of these two species could be related to the way testosterone was administered. A continuous elevated level of testosterone in the blood could have an entirely different effect compared to a short rise in testosterone plasma levels. If the way of administering the testosterone is not responsible for the difference in the effect of this hormone, then the altricial flycatcher might have an opposite response to testosterone as the semi-precocial black headed gull. This difference in the two species could reflect a very interesting divergent evolutionary path for the function of testosterone. To find more information about this interesting possibility, black headed gulls are injected with testosterone to reproduce short term rises in testosterone plasma levels in this study.
Corticosterone is also a hormone that could be involved in regulating begging behaviour in young birds (e.g. Love et al., 2003; Kitaysky et al., 2001). In black-legged kittiwake (Rissa
tridactyla) chicks implantation of a silastic tube filled with crystallized corticosterone resulted in an increase of begging frequency (Kitaysky et al., 2001). Injections with corticosterone dissolved in sesame oil did not result in elevated corticosterone plasma levels in a pilot done on black headed gull chicks. However, injections with pure sesame oil did result in an increase in circulating corticosterone plasma levels. In this study corticosterone levels are therefore manipulated by injection with oil in black headed gull chicks. It is expected that increasing the circulating corticosterone levels will result in a higher begging intensity.
This study should give insight in the hormonal control of begging behaviour, aggression and sibling competition in black headed gull chicks.
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