How certain tree smells boost male bird testosterone levels

Greg Howard
11th February, 2024

How certain tree smells boost male bird testosterone levels

Great Tit (Parus major)

Photo adapted from: Полина Полежанкина / CC BY (Source)
Trees under attack from caterpillars release airborne chemical signals, known as Herbivore-Induced Plant Volatiles (HIPVs). These signals aren’t just for the plants’ own defence; they’re also detectable by other organisms, including the birds that prey on the caterpillars. Researchers at Univ Montpellier[1] investigated whether these scents influence not only where birds forage, but also their reproductive behaviour. For some time, it’s been known that birds can locate caterpillar-infested trees by smelling the HIPVs they release[2]. This allows insectivorous birds to efficiently find food sources. Furthermore, plants actively employ indirect defences, releasing these volatiles to attract predators of the herbivores that are damaging them[3]. The timing of these signals is also important, as the concentration of HIPVs in early spring may indicate the availability of insects, a crucial factor for breeding birds. However, it wasn’t clear if birds use these scents to time their breeding, or to adjust the intensity of their reproductive effort. The study focused on great tits, a common songbird species. Researchers exposed male and female great tits, prior to the breeding season, to the odours of trees infested with caterpillars. This exposure took place in a controlled environment, allowing the researchers to isolate the effect of the scent. They then monitored several reproductive parameters: when the females started laying eggs, the size of the clutch (number of eggs laid), and the size of the eggs themselves. Crucially, they also measured hormone levels – testosterone in males and 17β-estradiol in females – as these hormones are key drivers of reproductive behaviour. The results showed that exposure to caterpillar-scented air didn’t cause females to lay eggs earlier, lay more eggs, or lay larger eggs. Nor did it affect their 17β-estradiol levels. This suggests that the scent alone isn’t a strong enough cue to directly influence female reproductive decisions. However, the study revealed a significant effect on males. Males exposed to the HIPVs exhibited higher testosterone levels during the egg-laying period. Testosterone is linked to male reproductive effort, including behaviours like mate guarding and territorial defence. This finding indicates that males are responding to the scent as a signal of increased food availability, and are increasing their reproductive investment accordingly. It doesn’t necessarily mean they are anticipating a larger brood, but rather that they are preparing to compete more intensely for mating opportunities, given the potential for abundant food resources. Interestingly, this research builds upon earlier work showing that the timing of breeding in birds is often closely linked to vegetation phenology – the timing of leafing and flowering[4]. Birds appear to use these visual cues to assess insect availability, as insects emerge and reproduce alongside the plants they feed on. The current study suggests that scent provides an additional, and potentially earlier, signal that males can use. Recent research has also highlighted the challenges climate change poses to the synchronisation of breeding seasons across different species[5]. As temperatures shift, the timing of budburst, caterpillar emergence, and bird breeding can become mismatched, leading to reduced reproductive success. The ability of birds to respond to cues like HIPVs could be crucial in maintaining this synchronisation in a changing climate. While the study didn’t directly address climate change, it provides a foundation for understanding how birds might adapt to altered seasonal cues. The study demonstrates that songbirds possess a remarkable sensitivity to plant odours, capable of detecting even minute amounts. While the scent alone isn’t enough to dramatically alter female reproductive behaviour, it does provide males with valuable information, triggering an increase in reproductive effort. This adds another layer of complexity to our understanding of how birds perceive and respond to their environment.

WildlifeEcologyAnimal Science

References

Main Study

1) Odours of caterpillar-infested trees increase testosterone concentrations in male great tits.

Published 9th February, 2024

https://doi.org/10.1016/j.yhbeh.2024.105491

Related Studies

2) Birds exploit herbivore-induced plant volatiles to locate herbivorous prey.

https://doi.org/10.1111/ele.12177

3) Herbivore-induced, indirect plant defences.

Journal: Biochimica et biophysica acta, Issue: Vol 1734, Issue 2, May 2005

4) Spring vegetation phenology is a robust predictor of breeding date across broad landscapes: a multi-site approach using the Corsican blue tit (Cyanistes caeruleus).

https://doi.org/10.1007/s00442-009-1545-0

5) Climate change and unequal phenological changes across four trophic levels: constraints or adaptations?

https://doi.org/10.1111/j.1365-2656.2008.01458.x


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