What are carotenoids signaling? Immunostimulatory effects of dietary vitamin E, but not of carotenoids, in Iberian green lizards

Abstract

In spite that carotenoid-based sexual ornaments are one of the most popular research topics in sexual selection of animals, the antioxidant and immunostimulatory role of carotenoids, presumably signaled by these colorful ornaments, is still controversial. It has been suggested that the function of carotenoids might not be as an antioxidant per se, but that colorful carotenoids may indirectly reflect the levels of nonpigmentary antioxidants, such as melatonin or vitamin E. We experimentally fed male Iberian green lizards (Lacerta schreiberi) additional carotenoids or vitamin E alone, or a combination of carotenoids and vitamin E dissolved in soybean oil, whereas a control group only received soybean oil. We examined the effects of the dietary supplementations on phytohaemagglutinin (PHA)-induced skin-swelling immune response and body condition. Lizards that were supplemented with vitamin E alone or a combination of vitamin E and carotenoids had greater immune responses than control lizards, but animals supplemented with carotenoids alone had lower immune responses than lizards supplemented with vitamin E and did not differ from control lizards. These results support the hypothesis that carotenoids in green lizards are not effective as immunostimulants, but that they may be visually signaling the immunostimulatory effects of non-pigmentary vitamin E. In contrast, lizards supplemented with carotenoids alone have higher body condition gains than lizards in the other experimental groups, suggesting that carotenoids may be still important to improve condition.

Dietary constraints can preclude the expression of an honest chemical sexual signal

“…Thus, it is unlikely that the enhanced responses of females were explained by compounds that decreased slightly in relative abundance, leaving the high proportions of VE as the most likely candidate to explain the increased chemosensory responses. This hypothesis is in line with the hypothesis that VE may function as an honest chemical sexual signal 24–26 in this species too. Nevertheless, because VE is needed in metabolism and must be acquired from dietary sources, female preferences could be sensory biased.…”

“…As in the case of I. cyreni and VE, striking divergences in chemical profiles of scents might be unleashed by ecological factors rather than phylogenetic differences per se . Namely, due to the antioxidant and immunostimulatory properties of VE 24, 31 , lizards must trade-off between the associated costs and benefits of diverting this vitamin from metabolism to chemical secretions. The result of such trade-off is indicative of male quality 25, 26, 31 .…”

“…However, although VE participates in many metabolic and physiological pathways, lizards of some species allocate large amounts of this vitamin in chemical secretions that are deposited outside of the body. The magnitude of such allocation has been correlated with the quality of males in some species, which may explain the preference of females for the scent of those males 24–26 . Only best-quality males can obtain enough VE in their diet to support physiological functions (for example, to maintain and enhance the immune response 24, 26 ) and, at the same time, allocate high levels of VE to chemical signals.…”

“…The magnitude of such allocation has been correlated with the quality of males in some species, which may explain the preference of females for the scent of those males 24–26 . Only best-quality males can obtain enough VE in their diet to support physiological functions (for example, to maintain and enhance the immune response 24, 26 ) and, at the same time, allocate high levels of VE to chemical signals. Nevertheless, in addition to a signalling function of male quality, a pre-existing sensory bias in females for a chemosensory search of VE in their food might simply explain why females also preferred chemical secretions of males with higher proportions of VE 27, 28 .…”

Effects of energy and thermoregulation time on physiological state and sexual signal in a lizard

“…We used phytohemagglutinin (PHA)-induced skin swelling reaction to estimate immunocompetence (Kopena, López, & Martín, 2014;Martin et al, 2006;Salaberria, Muriel, de Luna, Gil, & Puerta, 2013). PHA activates cellular immune response, offering information about cellmediated immune activity (Tylan & Langkilde, 2017).…”