Working hard and providing for many offspring takes a lot of energy. Even jackdaws experience the consequences of this, behavioural biologist Martijn Salomons discovered. He studied a jackdaw colony in Haren, a town in the province of Groningen, and concluded that there is a clear connection between the number of chicks in a brood and their health and survival chances. He also examined the effects of brood size on the life expectancy of parents. Salomons will be awarded a PhD for his jackdaw study by the University of Groningen on 16 October 2009.
Salomons studied the large jackdaw colony for years. Not only did this yield important scientific information, he also enjoyed the watching the busy birds’ behaviour. ‘Once you start watching, it’s impossible to stop', according to Salomons, who studied the Haren jackdaw colony throughout his student years. ‘It’s like I have been following a soap opera all these years’, he says. ‘There are lots of things going on in such a colony, and the birds’ behaviour patterns are intriguing.’ Jackdaw males appear to be henpecked by their bossy wives, and monogamy is the norm among jackdaw couples. ‘Extramarital affairs are rare. Fascinating’, says Salomons.
In addition to these entertaining personal observations, Salomons’s research also focused on the effects of manipulation of brood size on the health of the parents and their young. Salomons varied the brood sizes by moving newborn chicks to different nests and examined the effects of this on the basis of the difference in DNA damage, measured in telomere length. Telomeres are long stretches of DNA at the end of each chromosome. Hard work and so-called oxidative stress possibly damage and consequently shorten these telomeres.The length of telomeres, and in particular the speed at which they shorten, turns out to be predictive of an individual’s chances of survival. DNA damage is thus indicative of the life history and life expectancy of individual jackdaws. One of the reasons why Salomons’s study is of importance is that he is the first to demonstrate this in animals living in the wild.
Salomons stresses that his PhD research bears relevance for studies on ageing and health in humans. ‘A lot of research is being done into telomere length and its effects on life expectancy’, he explains. ‘There are indications that telomere length may be predictive of life expectancy in ageing humans as well. Recent research by the UMCG, for example, has shown that there is a connection between telomere length and the risk of heart failure. My findings are in line with these research results and supplement them.’
Another notable finding in Salomons’s study was that jackdaws that nurture larger numbers of chicks make greater efforts but are less successful than parent couples with smaller broods. Chicks in increased broods do not grow as much and weigh less than their brothers and sisters who are moved to smaller broods. This has a negative effect on their future reproductive success. At the same time, there are indications that sons and daughters deal with deteriorated circumstances in different ways. When there are more chicks in a nest and less food is brought in for each chick, sons opt for growth, which results in an increased decrease in telomere length. Daughters, on the other hand, choose to invest less energy in growing.Therefore, although daughters may seem to be affected more strongly by worsened conditions, the prediction on the basis of the effects on telomere length is that sons may be equally affected by the manipulation of brood sizes but that these effects manifest themselves in different ways.
In addition, there appears to be a connection between social dominance and the number of sons and daughters in a brood. Dominant jackdaw couples produced more sons than daughters between 1998 and 2002, whereas couples that were lower on the social ladder produced more daughters. This connection, however, is not constant – in 2004, 2005 and 2007 the dominant couples were the ones that produced more daughters. Salomons: ‘We have not yet been able to find a good explanation for why or how investing in sons or daughters can be linked to social hierarchy.’Salomons also discovered that jackdaws who were high up in the hierarchy were less successful in producing offspring. ‘This indicates that dominance comes at a price’, he explains. ‘The costs may even exceed the benefits in some cases.’ According to the researcher, this finding will contribute to the body of knowledge of the evolution of social dominance hierarchies.
Martijn Salomons (Dronten, 1975) studied Biology in Groningen and conducted his PhD research at the Department of Behavioural Biology of the School of Behavioral and Cognitive Neurosciences (BCN) of the Faculty of Mathematics and Natural Sciences. The research was financed by NWO. Salomons’s supervisor was Prof. Verhulst. His PhD thesis is entitled Fighting for fitness.
Contact: e-mail: Martijnsalomons@hotmail.com
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