Condition-dependent trade-offs between sexual traits, body condition and immunity: the effect of novel habitats

Abstract

Background: The optimal allocation of resources to sexual signals and other life history traits is usually dependent on an individual's condition, while variation in the expression of sexual traits across environments depends on the combined effects of local adaptation, mean condition, and phenotypic responses to environment-specific cues that affect resource allocation. A clear contrast can often be drawn between natural habitats and novel habitats, such as forest plantations and urban areas. In some species, males seem to change their sexual signals in these novel environments, but why this occurs and how it affects signal reliability is still poorly understood.

Results: The relative size of sexual traits and level of immune responses were significantly lower for male palmate newts Lissotriton helveticus caught in pine and eucalyptus plantations compared to those caught in native forests, but there was no habitat-dependent difference in body condition (n = 18 sites, 382 males). The reliability with which sexual traits signalled body condition and immune responses was the same in all three habitats. Finally, we conducted a mesocosm experiment in which males were maintained in pine, eucalypt or oak infused water for 21 days. Males in plantation-like water (pine or eucalypt) showed significantly lower immune responses but no change in body condition. This matches the pattern seen for field-caught males. Unlike field-caught males, however, there was no relationship between water type and relative sexual trait size.

Conclusions: Pine and eucalyptus plantations are likely to be detrimental to male palmate newt because they are associated with reduced immune function and smaller sexual traits. This could be because ecological aspects of these novel habitats, such as high water turbidity or changes in male-male competition, drive selection for reduced investment into sexual traits. However, it is more probable that there are differences in the ease of acquisition, hence optimal allocation, of resources among habitats. Our mesocosm experiment also provides some evidence that water toxicity is a causal factor. Our findings offer insights into how plantations affect amphibian life histories, and how novel habitats might generate long-term selection for new resource allocation strategies in native species.

Keywords: Lissotriton helveticus; Novel habitats; PHA; Resource allocation; Sexual selection.

Fig. 1

Map showing the 18 capture sites. White: natural oak forest; grey: pine plantations; black: eucalyptus plantations

Fig. 2

The relationship between relative sexual trait expression and: (a) PHA immune response was positive in all three habitats, model R ²: 0.62; (b) body condition was positive in all three habitats, model R ²: 0.42. The strength of the relationship did not differ among forests for either PHA immune response or body condition (see main text). Oak forest: white circles, dashed line; pine plantations: grey squares, grey solid line; eucalyptus plantations: black circles, black solid line

Fig. 3

In males in mesocosms there was (a) no relationship between sexual trait expression and PHA immune response in any of the three treatments, model R ²: 0.51; (b) a significant positive relationship with body condition that did not differ in strength among the three treatments (see text), model R ² : 0.12. Oak forest: white circles, dashed line; pine plantations: grey squares, grey solid line; eucalyptus plantations: black circles, black solid line