Mating behaviour and parapatry in two species of Australian reptile tick

Summary Few quantitative studies have examined the ecological consequences of similarities and/or differences in mating behaviour of parapatric species. Reproductive interference occurs between several parapatric species of Australian reptile tick, due to similarities in their mating behaviour (Andrews et al. 1982a). Attempts to determine whether reproductive interference serves to maintain parapatry between Amblyomma limbatum and Aponomma hydrosauri have been hindered because of difficulties in providing conditions conducive to conspecific mating in Amb. limbatum. The present study examined whether off-host and/or onhost temperature influenced the subsequent mating behaviour (i.e. the proportion of females that mate and the time when mating occurs) of these two species. Irrespective of the temperature experienced by ticks prior to host attachment, specific on-host temperatures were needed to induce mating in Amb. limbatum (i.e. host cloacal temperatures >32° C prior to the time of peak mating activity). Significantly more Amb. limbatum females were mated and the time taken by females to mate decreased with increasing on-host temperatures. mating in Ap. hydrosauri occurred over a wider range of on-host temperatures and the time when mating occurred did not alter at different on-host temperatures. In addition, significantly more Ap. hydrosauri males moved and each male made more moves on hosts than did Amb. limbatum males. It is suggested that Ap. hydrosauri may in consequence have a competitive mating advantage over Amb. limbatum at a boundary. Similarities in mating behaviour, on the other hand, increase the probability of reproductive interference, hence reduce the reproductive fitness of colonizing females of both species. We propose that similarities and differences in mating behaviour could play a critical role in the maintenance of parapatric boundaries.     

The impact of tick load on the fitness of their lizard hosts

A survey was conducted of natural populations of the sleepy lizard Tiliqua rugosa in South Australia to determine whether infestation by ectoparasitic ticks reduced their fitness. Between 1982 and 1990, 2183 captures of 824 individual lizards were made in an area where they were infested by the tick Aponomma hydrosauri, and 3668 captures of 586 individual lizards were made in an area where they were infested with the tick Amblyomma limbatum. Lizards with high tick loads in one year tended to have high loads the next year. Longevity of lizards in the study was either not correlated with tick load, or positively correlated. Size achieved was greater amongst lizards with greatest tick load, and lizards in mating pairs had higher tick loads than those never found in pairs. The data do not support the hypothesis that tick load diminishes host fitness.     

Organization of ascending spinal projections in Caiman crocodilus

Summary Ascending spinal projections in the caiman (Caiman crocodilus) were demonstrated with Nauta and Fink-Heimer methods following hemisections of the third spinal segment in a series of twelve animals. These results were compared with earlier data in the literature obtained from a turtle, a snake, and a lizard using the same experimental and histological procedures. The results show remarkable similarities considering that each species represents a different reptilian order with different evolutionary history and habitat. However, the caiman displays several important peculiarities.Although the dorsal funiculus of the caiman contains the largest number of ascending spinal projections of the four species examined, this funiculus has not differentiated into cuneate and gracile fasciculi as is the case in the tegu lizard. The ventro-lateral ascending spinal projections follow a fundamentally similar general morphologic pattern in the four species with only minor variations. The anatomical arrangement in the caiman and tegu lizard appears most similar in the high cervical and the medullary regions; however, this is not the case in midbrain and thalamic regions where considerably more extensive projections are seen in the caiman. In the caiman an extensive spinal connection to the ventro-lateral nucleus of the dorsal thalamus is present; this connection is reminiscent of the mammalian spinal projection to the ventro-basal complex. The caiman has in common with the other three reptilian species a small projection to another dorsal thalamic region that is apparently homologous to the mammalian intralaminar nuclei, which are the destination of the mammalian paleospinothalamic tract.     

Effect of methallibure (ICI 33,828) on the pituitary-gonad and the pituitary-thyroid axes of the Indian garden lizard,Calotes versicolor (Daudin)

Summary The effect of methallibure (ICI 33,828) on the pituitary-gonad and the pituitary-thyroid axes of the Indian garden lizard, Calotes versicolor was studied. Following administration of methallibure B₂ (FSH) and B₃ (ICSH) cells of the pars distalis exhibited significant alterations with concomitant changes in the seminiferous tubules, and in the interstitial cells and epididymis respectively. B₁ (TSH) cells exhibited conspicuous hypertrophy and degranulation which was reflected in dramatic changes in thyroid morphology. The caudally localized acidophilic A₁ (PRL) cells also showed hypertrophy, hyperplasia and intense granulation. The rostrally located A₂ (STH) acidophils and the B₄ (ACTH) cells were unaffected by the treatment. The possible mode of action of this drug is discussed in the light of available literature.Supported by a grant from the University Grants Commission of India. The generous supply of methallibure by Dr. A.L. Walpole of the Imperial Chemical Industries Limited, England, through Professor C.J. Dominic, Banaras Hindu University, is gratefully acknowledged     

Effects of melatonin on the behavioral and hormonal responses of red-sided garter snakes (Thamnophis sirtalis parietalis) to exogenous corticosterone

We investigated possible interactions between melatonin and corticosterone in modulating the reproductive behavior of male red-sided garter snakes (Thamnophis sirtalis parietalis) following spring emergence. We also examined whether melatonin's modulatory actions could be explained by its potential properties as a serotonin receptor antagonist. Exogenous corticosterone significantly reduced courtship behavior of male snakes in a dose-dependent manner. Melatonin also significantly reduced courtship behavior of male garter snakes. Pretreatment with melatonin before administering corticosterone treatments further suppressed courtship behavior of red-sided garter snakes. These results indicate additive inhibitory effects of melatonin and corticosterone in modulating reproductive behavior. Snakes receiving ketanserin, a serotonergic type 2A receptor antagonist, followed by corticosterone also showed reduced courtship behavior; this serotonin receptor antagonist followed by treatment with vehicle did not significantly influence courtship behavior of male snakes. Neither melatonin nor corticosterone treatments significantly influenced testosterone + 5-alpha-dihydrotestosterone concentrations of male garter snakes, supporting a direct effect of melatonin and corticosterone on courtship behavior that is independent of any effect on androgen concentrations. We propose that a serotonin system is involved in the modulation of male courtship behavior by melatonin and corticosterone. In addition, our data support the hypothesis that melatonin may function as a serotonin receptor antagonist. Further research is necessary to discern whether the actions of melatonin and corticosterone are converging on the same pathway or if their effects on different pathways are having additive inhibitory effects on courtship behavior.     

Repeatable intra-individual variation in plasma testosterone concentration and its sex-specific link to aggression in a social lizard

Individual hormone profiles can be important generators of phenotypic variation. Despite this, work on the consequences of hormone profiles has traditionally ignored the large inter-individual variation within natural populations. However, recent research has advocated the need to explicitly consider this variation and address its consequences for selection. One of the key steps in this process is examining repeatability in hormone profiles and their links to behavioral traits under selection. In this study we show that individuals within a free-ranging population of the Australian lizard Egernia whitii exhibit temporal repeatability in their circulating baseline testosterone concentrations as well as their aggressive response towards conspecific intruders. Furthermore, we show significant, sex-specific links between testosterone and aggression. Specifically, testosterone and aggression is negatively linked in males, while there is no relationship in females. As conspecific aggression has significant consequences for fitness-related traits (parental care, mating strategies) in this species, inter-individual variation in testosterone concentrations, through their effects on aggression, could have important implications for individual fitness. We discuss the potential causes and consequences of hormonal repeatability as well as provide explanations for its sex-specific links with aggression. Specifically, we suggest that these patterns are the result of alternative hormonal pathways governing aggression within Egernia and may indicate a decoupling of aggression and testosterone across the sexes.     

Regulation of ventilation in the caiman (Caiman latirostris): effects of inspired CO2 on pulmonary and upper airway chemoreceptors

In order to study the relative roles of receptors in the upper airways, lungs and systemic circulation in modulating the ventilatory response of caiman (Caiman latirostris) to inhaled CO₂, gas mixtures of varying concentrations of CO₂ were administered to animals breathing through an intact respiratory system, via a tracheal cannula by-passing the upper airways (before and after vagotomy), or via a cannula delivering gas to the upper airways alone. While increasing levels of hypercarbia led to a progressive increase in tidal volume in animals with intact respiratory systems (Series I), breathing frequency did not change until the CO₂ level reached 7%, at which time it decreased. Despite this, at the higher levels of hypercarbia, the net effect was a large and progressive increase in total ventilation. There were no associated changes in heart rate or arterial blood pressure. On return to air, there was an immediate change in breathing pattern; breathing frequency increased above air-breathing values, roughly to the same maximum level regardless of the level of CO₂ the animal had been previously breathing, and tidal volume returned rapidly toward resting (baseline) values. Total ventilation slowly returned to air breathing values. Administration of CO₂ via different routes indicated that inhaled CO₂ acted at both upper airway and pulmonary CO₂-sensitive receptors to modify breathing pattern without inhibiting breathing overall. Our data suggest that in caiman, high levels of inspired CO₂ promote slow, deep breathing. This will decrease dead-space ventilation and may reduce stratification in the saccular portions of the lung.     

Physiological and hormonal control of thermal depression in the tiger snake, Notechis scutatus

Plasma sodium concentrations in field-caught Western tiger snakes, Notechis scutatus, from semi-arid Carnac Island (CI) varied seasonally, with snakes exhibiting significant hypernatraemia during summer and normal concentrations following autumn rain. In contrast, field-caught tiger snakes from a perennial fresh-water swamp (Herdsman Lake, HL) exhibited no significant increase in plasma sodium concentrations during summer. Laboratory-induced hypernatraemia caused thermal depression in both populations; there was a weak negative relationship between plasma sodium concentration and temperature selection that was significant for CI snakes. Hypernatraemia significantly elevated circulating concentrations of the neuropeptide arginine vasotocin (AVT) in both CI and HL snakes. CI snakes injected with a physiological dosage of AVT also evidenced thermal depression. Despite the positive correlation between AVT and both plasma sodium concentration and osmolality for laboratory snakes, field samples from CI snakes indicate that circulating levels of AVT may be influenced more by plasma osmolality than sodium levels. The data suggest that, in CI snakes, chronic dehydration in the field leads to hypernatraemia which may lead to elevated levels of AVT if plasma osmolality also increases. This will in turn invoke a depression in thermal behaviour that may improve the water economy and survival of snakes on semi-arid CI. Although HL snakes do not experience seasonal dehydration, physiological changes away from the stable homeostatic state appear to prompt the same behavioural shifts, illustrating the intrinsic nature of the thermal behaviour in different populations of the same species of snake.