Date Published: June 18, 2018
Publisher: Public Library of Science
Author(s): Raïssa A. de Boer, Marcel Eens, Wendt Müller, Cheryl S. Rosenfeld.
Inbreeding is a central topic in evolutionary biology and ecology and is of major concern for the conservation of endangered species. Yet, it remains challenging to comprehend the fitness consequences of inbreeding, because studies typically focus only on short-term effects on inbreeding in the offspring (e.g. survival until independence). However, there is no a priori reason to assume that inbreeding has no more effects in adulthood. Specifically, inbred males should have lower reproductive success than outbred males among other things because of inbreeding depression in attractiveness to females and a reduced lifespan. Such differences in future reproductive value should affect male mating behaviour, such that an inbred male of a given age should be more motivated to seize a current mating opportunity than an outbred male of the same age. We used an inventive experimental set-up that enabled us to assess male behaviour in relation to an apparent mating opportunity while excluding potential confounding effects of female preference. Age-, weight-, and size-matched inbred and outbred male canaries (Serinus canaria) were presented with a female that only one male at a time could access visually via a ‘peephole’ and thus when both males were equally interested in seizing the apparent mating opportunity this would result in contest. We find that inbred males spent more than twice as much time ‘peeping’ at the female than outbred males, suggesting that inbreeding indeed causes different behavioural responses to an apparent mating opportunity. Our study is among the first to highlight that inbreeding affects male mating behaviour, and therewith potentially male-male competition, which should be taken into account in order to understand the full range of inbreeding effects on fitness.
Mating between related individuals often leads to negative effects on fitness (‘inbreeding depression’) , which has been shown in a wide variety of animal species . The negative effects of inbreeding on fitness result in selection pressures which can affect reproductive behaviour and dispersal strategies among other things. Inbreeding depression is therefore a central topic in evolutionary biology and ecology [3,4]. Moreover, due to human-induced changes to the environment many populations become fragmented, which increases the occurrence of inbreeding and ultimately extinction risk. This further underscores the importance of understanding the potential fitness consequences of inbreeding [5,6]. Yet, research on inbreeding typically focuses on short-term effects of inbreeding in the offspring (e.g. survival until independence), which currently limits our understanding of the reproductive costs of inbreeding .
The PCA analysis resulted in three principal components that had eigenvalues larger than 1, which together explained 69% of the total variance (Table 2). The first component, accounting for 29% of the total variance, was defined by the occurrence of beak wipes, preening and feather shuffles. This PC was interpreted as displacement behaviour. The second component, explaining 24% of the total variance, was primarily loaded with the occurrence of escapes and moving away from the opponent (= avoidance behaviour). The last component, that was responsible for 17% of the variance, was defined by the occurrence of chases and threat displays (= aggressive behaviour).
In this study we examined the behaviour of inbred and outbred males while exposed to an apparent mating opportunity and rated the time each male occupied the position in front of a peephole to look at a female. We have previously shown that inbreeding in these birds led to negative effects on growth , song expression and attractiveness . Here, we show that inbred males spent on average twice the amount of time peeping at the female compared to outbred males. In line with our hypothesis, this may suggest that inbreeding causes a behavioural change with inbred males having a higher motivation to seize an apparent mating opportunity than outbred males. These differences in mating and reproductive behaviour could be due to differences in future reproductive value [9,24], which is expected to be lower in inbred than in outbred males.