Mo Healey

Seeing red: morph-specific contest success, and survival rates, in a colour-polymorphic agamid lizard

Red coloration in multicoloured or polymorphic species has been associated with dominance in a number of case studies, including probability of winning in human sports. However, it is not clear at what stage during ontogeny the association between red and the probability of winning contests occurs (or being perceived as more threatening by a contradistinct rival), for example, at conception, early ontogeny or at maturation. We analysed such coloration effects in a polymorphic (red versus yellow) species of lizard, the Australian painted dragon, Ctenophorus pictus. Red males were more likely to win dyadic contests with yellow males when competing for receptive females. When contestants were repainted with a rivals colour, there was a more than 30-fold increase in contest duration. Furthermore, when we tested for red-enhanced contest success in immature males, red was still significantly associated with winning. Thus, the association between red coloration and dominance seems to be set long before a male is naturally involved in sexual contests and could be an innate response to aid facultative fight or submission decisions, particularly in young males. To assess in situ selective benefits of the polymorphism, we released males in polymorphic and monomorphic groups. Males from polymorphic groups survived better, implying that polymorphism among neighbouring territorial males in the wild results in selective benefits.

Free radicals run in lizard families

In the ageing individual, the production of reactive oxygen species (ROS) accelerates with cell senescence. Depending on the heritability of the underlying processes that determine net ROS levels, this may influence ageing per se and its evolutionary direction and rate of change. In order to understand the inheritance and evolution of net ROS levels in free-ranging lizards, we used flow cytometry together with ROS-sensitive fluorogenic probes to measure ROS in lizard blood cells. We measured basal levels of (i) non-specific ROS (superoxide, singlet oxygen, H2O2 and peroxynitrite), (ii) superoxide specifically and (iii) superoxide after CCCP treatment, which elevated ROS production in the mitochondria. The cumulative level of non-specific ROS was higher in adults than juveniles and superoxide level showed high heritability and variability among families. We suggest that the evolution of ROS dynamics may be ROS species specific and perhaps depend on the relative degree of uni- or biparental inheritance of ROS main regulatory pathways.

Mating system variation and morph fluctuations in a polymorphic lizard

In polymorphic male painted dragon lizards (Ctenophorus pictus), red males win staged contests for females over yellow males, and yellow males have greater success in staged sperm competition trials than red males. This predicts different reproductive strategies in the wild with red males being more coercive or better mate guarders than yellow males. Yellow males would be expected to sire more offspring per copulation and have a greater proportion of offspring from clutches with mixed paternity. However, here we show using microsatellites that the frequency of mixed paternity in the wild is low (< 20% on average across years), that all morphs on average have the same number of offspring sired per year, and that mating system variation (polyandry vs. monandry) is strongly correlated with perch density on male territories. Furthermore, a logistic regression on male successful vs. unsuccessful mate acquisition showed that red males were under negative selection when they dominated the population, which suggests ongoing frequency dependent selection on male colouration.

Afternoon t: Testosterone level is higher in red than yellow male polychromatic lizards.

Recent work on within-species polymorphism across a broad range of taxa has renewed and considerably increased the attention to this classic evolutionary area, notably in lizard species where colors covary with reproductive strategies. We demonstrate elsewhere that red-headed males beat yellow-headed males in staged contests for females in the Australian painted dragon lizard Ctenophorus pictus. This morph difference in behaviour is linked to what appears to be a convention of red dominance in male-male interactions set very early in ontogeny, long before coloration has developed. In the current note, we investigate the relationship between time of day, which is directly linked to vigilance time in territorial males, and plasma levels of testosterone and corticosterone. We show that red males have higher testosterone levels in late afternoon following a day of territory patrolling and a non-significant trend in plasma corticosterone levels that decline with time of day. In conclusion, there are significant differences in testosterone profile between the two color morphs, providing a potential proximate link to the behavioural differences between them.

Sons are made from old stores: sperm storage effects on sex ratio in a lizard

Sperm storage is a widespread phenomenon across taxa and mating systems but its consequences for central fitness parameters, such as sex ratios, has rarely been investigated. In Australian painted dragon lizards (Ctenophorus pictus), we describe elsewhere that male reproductive success via sperm competition is largely an effect of sperm storage. That is, sperm being stored in the female reproductive tract out-compete more recently inseminated sperm in subsequent ovarian cycles. Here we look at the consequences of such sperm storage for sex allocation in the same species, which has genetic sex determination. We show that stored sperm have a 23% higher probability of producing sons than daughters. Thus, shifts in sex ratio, for example over the reproductive season, can partly be explained by different survival of son-producing sperm or some unidentified female mechanism taking effect during prolonged storage.

Effects of sperm storage and male colour on probability of paternity in a polychromatic lizard

Sexual selection may take place before or after mating and may involve a large number of different mechanisms, for example, overt male aggression, mate choice, sperm competition and cryptic female choice. In most species, males show similar reproductive tactics and, hence, achieve their reproductive success in the same or a similar way. Sometimes, however, males evolve alternative reproductive tactics. One such example is the polychromatic Australian painted dragon lizard, Ctenophorus pictus, in which red males beat yellow males in staged contests for females and show different emergence patterns post-hibernation in the wild with red males emerging to establish territories before yellow males do (at least in some years). Here we show that yellow males have significantly larger testes in relation to body size and condition than red males and copulate for a shorter period of time. Our mating experiments further showed that sperm storage played a significant role in male reproductive success (i.e. males sired offspring in later ovarian cycles than the one in which they actually mated). Furthermore, yellow males had a three times higher probability of paternity in some situations of sperm competition than red males, suggesting that male polymorphism may be associated with different reproductive tactics.

Digit ratio, color polymorphism and egg testosterone in the Australian painted dragon.

Variation in exposure to sex hormones during early development contributes to phenotypic plasticity in vertebrate offspring. As a proposed marker for prenatal sex hormone exposure and because of their association with various physiological and behavioral characteristics, digit ratio and/or digit length have received notable interest within the field of evolutionary ecology. However, the validity of digit measures as a proxy of prenatal sex hormone exposure is controversial and only few studies have provided direct evidence for the link between digit development and prenatal sex hormones. Here, we report morph- and sex-specific variation in digit ratio in wild painted dragon lizards (Ctenophorus pictus). Lizards expressing a yellow bib have significantly larger third-to-fourth toe ratios (3D:4D) than lizards without a bib. Males have significantly smaller 3D:4D than females. Furthermore, we show that experimental elevation of yolk testosterone significantly increases 3D:4D in hatchling painted dragon lizards, but has no influence on hatchling size. Our results provide direct and indirect evidence for the involvement of prenatal sex steroids in digit development and it is suggested that digit ratio may be used as a biomarker for prenatal steroid exposure in this reptilian species. As such, digit ratio may provide a useful tool to study temporal or spatial differences in the proximate hormonal mechanisms modulating physiological and behavioural phenotypes.

A SIGNIFICANT COMPONENT OF AGEING (DNA DAMAGE) IS REFLECTED IN FADING BREEDING COLORS: AN EXPERIMENTAL TEST USING INNATE ANTIOXIDANT MIMETICS IN PAINTED DRAGON LIZARDS

A decade ahead of their time, von Schantz et al. united sexual selection and free radical biology by identifying causal links between deep‐rooted physiological processes that dictate resistance to toxic waste from oxidative metabolism (reactive oxygen species, ROS), and phenotypic traits, such as ornaments. Ten years later, these ideas have still only been tested with indirect estimates of free radical levels (oxidative stress) subsequent to the action of innate and dietary antioxidants. Here, we measure net superoxide (a selection pressure for antioxidant production) and experimentally manipulate superoxide antioxidation using a synthetic mimetic of superoxide dismutase (SOD), Eukarion 134 (EUK). We then measure the toxic effect of superoxide in terms of DNA erosion and concomitant loss of male breeding coloration in the lizard, Ctenophorus pictus. Control males suffered more DNA damage than EUK males. Spectroradiometry showed that male coloration is lost in relation to superoxide and covaries with DNA erosion; in control males, these variables explained loss of color, whereas in EUK males, the fading of coloration was unaffected by superoxide and unrelated to DNA damage. Thus, EUKs powerful antioxidation removes the erosion effect of superoxide on coloration and experimentally verifies the prediction that colors reflect innate capacity for antioxidation.

Testing the Quality of a Carrier: a Field Experiment on Lizard Signalers

In the Australian painted dragon lizard (Ctenophorus pictus), males occur in two different morphs with respect to gular color, with or without a yellow bib. Males without a bib lost within-clutch paternity significantly more often to rivals than bibbed males. Thus, it appears that bibs identify some phenotypic advantage linked to competitive ability. To test whether this could be related to whole-organism capacity to withstand an increased workload (due to better health and vigor, or evolved differences in self-maintenance), we implanted males with a lead pellet (loaded), Styrofoam pellet (controls), or sham-operated males without implants (shams), and compared male categories with respect to how they maintained body mass during the mating season. Somewhat unexpectedly, bibbed males consistently lost more body weight across all treatments and controls, although we could not verify that this translated into higher mortality in this short-lived animal (about 80% survive for one year only). However, bibbed males may invest more into “mating success” than nonbibbed males, which agrees with our experimental results and paternity data.

Basal superoxide as a sex- specific immune constraint

There is increasing evidence that reactive oxygen species (ROS), a group of unstable and highly reactive chemical molecules, play a key role in regulating and maintaining life-history trade-offs. Upregulation of ROS in association with immune activation is costly because it may result in an imbalance between pro- and antioxidants and, hence, oxidative damage. Previous research aimed at quantifying this cost has mostly focused on changes in the pro-/antioxidant balance subsequent to an immune response. Here, we test the hypothesis that systemic ROS may constrain immune activation. We show that systemic, pre-challenge superoxide (SO) levels are negatively related to the strength of the subsequent immune response towards the mitogen phytohaemagglutinin in male, but not female painted dragon lizards (Ctenophorus pictus). We therefore suggest that systemic SO constrains immune activation in painted dragon males. We speculate that this may be due to sex-specific selection pressures on immune investment.