The Effects of Androgen on the Mate-finding Behavior of Stingray - Research Paper Example

?First Last 30 May The Effects of Androgen on the Mate-finding Behavior of Stingray Stingrays are cartilaginous fishes which are close relatives of sharks and skates. Geographically, they are widely distributed throughout the tropical and subtropical marine waters of the world. The mating and courtship behaviors of different species of stingrays have been well-documented and studied. Several factors such as seasonal changes and fluctuations in hormone levels highly affect the mating and courtship behaviors of stingrays. Stingrays (Dasyatis sabina) have well-defined reproductive periods with specific sequence of events that lead to reproductive development. The mating season begins in August and continues until April of the following year. Female stingrays begin to mature upon reaching approximately 22-23cm. Development of secondary female characteristics and other morphological changes such as development of large disc width occurs because males would grasp on the females’ discs for copulation (Chapman et al. 241). Males, on the other hand, mature at approximately 21cm. Their claspers calcify and production of sperm begins (Johnson and Snelson Jr. 74). During breeding seasons, reproductively active female stingrays proceed to the shore lines or in the shallow freshwater grounds in the case of the fresh water stingrays (Potamotrygonidae) and burry themselves to the sandy bottom. Females would usually burry themselves next to the other female stingrays. Solitary males then search the area for the females hidden under the sand (Tricas et al. 129). Upon location of a potential mate, the male stingray excavates the female in the sand and pursues the female closely. An aggressive courtship behavior follows. The courtship behavior in the wild species of southern stingrays, Dasyatis americana (Dasyatidae) was described in detail by Chapman. The male orally grasp the female’s pectoral disc leaving the female with mating scars and wounds. If the male is successful, copulation and internal fertilization will occur. The male will keep copulating with other females until the end of the breeding period. The female keeps the embryos inside her body, in the womb without a placenta. The embryos absorb nutrients from the yolk sac, and later directly from the mother. The pregnancy period lasts from three to nine months depending on the species (Charvet-Almeida et al. 165). The mating and reproductive behaviors of stingrays can be affected by several factors. For example, certain changes to the gonadal steroid hormones in male stingrays have been shown to coincide with the reproductive behavior, aggression and morphological changes in them (Wilkens and Hofman). One of the many hormones shown to affect mating behavior of male stingrays is androgens. Androgens are naturally occurring hormones that stimulates the development of masculine characteristics. This hormone is common to males of all species, including humans. In stingrays, one of the most important roles of these hormones in stingrays is the development of gonads. Androgens also signal and control the production of sperm (Oliveira et al. 985). Androgen levels of stingrays have been shown to naturally fluctuate throughout the year. Tricas (2000) has reported four phases of annual fluctuation in the levels of androgens. The first phase occurs in April to July in between the reproductive seasons. In this phase, called the androgen suppression (AS) phase, the testis is inactive and the androgen levels are very low to nearly undetectable. In August, androgen levels increase constantly until November (primary androgen increase, PIA). This is the longest time when androgen levels in male stingrays are at the highest, spermatocytes begin to develop and gonad sizes are at the maximum (Tricas et al.). The surge of androgen levels is followed by a period of androgen decrease (AD) in the months of December to January. Though the stingrays are still actively copulating and producing sperm during these months, the testis size is decreased as well as the number of spermatocytes. A secondary increase in androgen (SIA) levels marks the end of the breeding period. It happens at around February to early March. Male stingrays produce spermatocytes again and the peak of sperm maturation is reached (Tricas et al.). Aggressiveness is also evident in male stingrays during the escalation of androgen levels. Tricas (2000) reported the cases of chasing, biting and nipping at the fins of the females by the male stingrays during the seasonal elevation of androgens. Aside from these effects, androgens have also been known to affect the mate-finding capability of stingrays (Tricas et al. 129). As stated earlier in this paper, during the mating season, the female stingrays burry themselves next to other females in the shallow sandy waters. The question is: how do they know where the other female stingrays are hiding? Second, how can the males accurately locate the females hidden in the sand? The act of accurately finding the hiding grounds of the females is positively shown by the observations of Tricas when the male stingrays that are cruising the shallow flats make a sharp turn to the direction of them. This only means that the stingrays manifest other sensing abilities for specific localization. The ability of the male stingrays to accurately find the females buried under the sand is the same as the prey-finding ability of other cartilaginous fishes such as sharks and skates. When hunting for prey, sharks and stingrays do not need visual contact to locate the prey. Instead, they rely on weak electrical impulses radiated by the prey through movements. In these creatures, detection of weak electrical impulses is very sensitive that even subtle movements such as breathing through gill splits were also detected (Kalmijn), making them very efficient predators in the dark or in murky waters. When they are the ones being hunted, they also use capability in order to escape predators. The stingrays apply the same technique when locating for mates. However, studies have also shown that prey-hunting as well as mate finding in stingrays are not only based on the movement of the creature they are looking for. Tricas has convincingly shown that stingrays can detect electric signals from artificial electric dipoles. On their experiment, they used plastic stingray models emitting electric charges similar to the electric discharge of female stingrays. Majority of the stingrays responded to the electric field. The males were confused by the plastic models as a potential mate. The males approached the plastic model while the females buried themselves next to the dummies. This demonstrated that even without very subtle movements, such as breathing, stingrays can identify the unique bioelectric field from other stingrays. This phenomenon is known as electoreception. Electroreception is the capacity to emit and detect electrical charges from the environment. Unlike eels and other electric rays, stingrays lack an active electric organ that emits electric charges. Instead, stingrays use bioelectric sources which originate from differential standing ionic potentials at various sites on the skin and buccal epithelia in order to generate weak electrical impulses (Tricas et al. 129). These electric organ discharges (EOD) are sexually dimorphic and are distinct between individual stingrays. In general, males release EOD at a lower frequency than females (Zupanc and Bullock 14). In turn, other stingrays perceive the electric impulses generated by other fishes through special receptor organs known as electroreceptors. There are two types of electroreceptors. First are the tuberous receptors which are found in non-teleost fishes and even in some amphibians. In sharks, stingrays and skates, these receptors are specifically called as ampullary organs (Albert and Crampton 616). The ampullary organs are sensitive to wide array of frequency but are most sensitive to DC and low frequency AC electric fields (2 - 4 Hz) (Zupanc and Bullock 14). These organs are located in the invaginations of the fish’s epidermis and are distributed all over the fish’s body with increased densities in the head region. Structures of the receptors vary between species of stingrays and increased electrosensory capabilities was observed to those with greater ventral pore numbers and densities (Jordan et al.). The key organs for electroreception in stingrays are first, sensory organs with electroreceptor cells, the ampullary organs, embedded in the skin; second, nerve fibers connecting to the; third, specialized brain ganglia that processes the information and senses gathered by the ampullary organs (Albert and Crampton 616). One characteristic feature of the electroreceptors is that they are “tuned” or most sensitive to a specific EOD frequency. Studies refer to it as best frequency or BF (Sisneros and Tricas 2000). The tuning of these electroreceptors is easily varied with hormones, such as androgens (Zupanc and Bullock 14). The effect of androgens in the electroreception of stingrays was successfully verified by Sisneros and Tricas. In their study, they observed wild male Atlantic stingrays throughout the four phases of androgen level fluctuations (AS, PIA, AD and SIA). Two factors were carefully observed in each phases. First, they determined the regularity of resting electrical discharge activity in the males. The results show that during the PIA, resting discharge rates exhibited a more regular pattern compared to the other times of the year. Next, they observed the changes in the sensitivity of afferent neurons to electrosensory stimuli when androgen levels fluctuate. No significant difference was recorded in the response of the males to stimuli during the AS, AD and SIA phases. However, the responsiveness of males to stimuli of lower frequency (0.01to 4Hz) during PIA was significantly increased (Sisneros and Tricas 8586). Natural increase in androgen levels and increased sensitivity to low frequency stimuli could be regarded by other scientist as coincidental, and not as an effect of the former to the later. Thus, another experiment was conducted in a controlled environment inside the laboratory in order to confirm the observed trait in the wild species. A similar upward shift in the sensitivity to lower frequency stimuli was also observed in male stingrays which are treated with artificial androgens. Sisneros tested the effect of androgen in the electroreception of male stingrays which are not undergoing the breeding periods. Male stingrays were captured and after three days, were implanted with dihydrotestosterone (DHT), a testosterone derivative that binds with androgen receptors and is also naturally present in stingrays. Another group of male stingrays were captured and was given empty tablets to serve as the control group. After the treatments, the resting discharge activity was recorded from a total of 131 electrosensory primary afferent neurons in both groups of stingrays. The resulting data were consistent with the results from the previous observation with wild samples. The group given with DHT were found to be more sensitive to low frequency stimuli between 0.5 to 2 Hz, while the control group remained in the original sensitivity at 2 to 4Hz (Sisneros and Tricas 8586). The significance of these results is that, increased sensitivity in lower frequency stimuli might increase the probability of conspecific detection in the mate-finding ritual of the stingrays. Female stingrays buried in the ground might be impossible to be detected by visually searching the field. However, the females emit a significant standing DC fields that is partially modulated by ventilatory movements of the mouths and gill slits. These DC fields, both the static and modulated, are being used by the males to locate the females. The static and modulated components of the bioelectric fields generated by the females are both very attractive to the males because it varies at around 0.25 to 2Hz. Therefore, androgen-induced changes in the sensitivity of the electoreceptor of male stingrays to lower frequency increase the probability of mate-finding and successful copulation in stingrays (Sisneros and Tricas 8586). Though androgens were proven to cause masculine type of changes, androgens are also reported to be naturally occurring in the female population but at lower concentrations compared to the males. Fluctuations in androgen levels in females are also seen to have significant effects in their breeding and mating behavior (Sisneros and Tricas 8586). Brief spikes of androgen levels in female stingrays could enhance mate choices by females between the competing males. Tricas has shown that an increase in the levels of androgen occur in female stingrays during the ovulation period in March. Increase in androgen levels in females, like in males, leads to enhanced aggression making the female able to flee pursuing males. Since females do not store sperm, the males that copulate with female nearest the time of ovulation will most likely fertilize the eggs that the females carry (Sisneros and Tricas 8586). In conclusion, stingrays follow a specific, complex mating behavior during breeding periods. These mating behaviors are easily affected by hormones such as androgens. Naturally occurring or induced fluctuations in androgen levels greatly affects both male and female stingrays. Androgen levels directly affect the morphology (i.e. formation of gonads and development of claspers), functionality (production of sperms), and sexual behavior of the male population. Importantly, androgens increase the electrolocation efficiency of male stingrays making them more competent in looking for female mates which are hidden in the sandy shores. This is being accomplished by an increase in the sensitivity of males in receiving signals of low frequency stimuli. Fluctuations in androgen levels may also affect the sexual behavior of the female population by becoming more aggressive and increasing the capacity to flee persistent males. Works Cited Albert, James S., and William G. R. Crampton (2005). “Electroreception and electrogenesis.” In D. H. Evans, and J. B. Claiborne, eds. The Physiology of Fishes. Taylor and Francis Inc., Bosa Roca, pp. 616. Chapman, Demian D., Mark J. Corcoran, Guy M. Harvey, Sonita Malan, and Mahmood S. Shivji. “Mating Behavior of Southern Stingrays, Dasyatis americana (Dasyatidae).” Environmental Biology of Fishes 68 (2003): 241-245. Charvet-Almeida, Patricia, Maria Lucia G. De Araujo, and Mauricio P. De Almeida. “Reproductive Aspects of Freshwater Stingrays (Chondrichthyes: Potamotrygonidae) in the Brazilian Amazon Basin.” Journal of Northwest Atlantic Fishery Science 35 (2005):165-171. Johnson, Michael R., and Franklin F. 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