Laboratory experiments were conducted with female guppies to determine the effects of intraspecific competition for food on fecundity, fry size, and gestation period. Two groups of females which had been raised at different levels of intensity of competition were compared. Gestation period was significantly longer in the high competition group. The effects on fecundity and fry size were dependent on the size of the adult female. The largest females in the high competition group did not differ with respect to these parameters from individuals of the same size in the low competition treatment; the smaller females produced fewer but larger fry in each brood under high competition. The total weight of fry per brood produced by females of a given size was not different between treatments. These results are discussed from the standpoint that the reproductive characteristics of the individuals of a population can be viewed as tactical components of a strategy whose objective is to maximize the present value of the number of surviving progeny produced by each individual in the course of its lifetime. The optimal distribution over time of energy devoted to reproductive ends and the optimal distribution at one point in time of a given amount of energy among many or few offspring are expected to vary among populations according to the particular age- and size-specific mortality rates faced by each population. Both magnitude and variability of mortality are important. It is suggested that by measuring the appropriate reproductive parameters of the adult members of a population, it should be possible to make predictions about the shape and expected variability of the stock/recruit relationship. / Science, Faculty of / Zoology, Department of / Graduate

This thesis attempts to elucidate some adaptive modifications of behaviour in response to environmental influences in geographically isolated populations of the guppy. Poecilia reticulata, Peters, I examined courtship behaviour of ♂guppies from three Trinidad streams which differ in several physical and biotic parameters. Males of two head-stream populations (the Paria and Upper Aripo Rivers) are larger and more brightly colored than ♂♂ of a lowland stream (the Guayamare River). Headstream ♂♂ are more conspicuous in their courtship than downstream ♂♂ in a one ♂ -one ♀ encounter: they exhibit more display behaviour and move around the ♀ more. It was concluded that differences in behaviour are genetic. I then investigated effects of ♂ interactions on courtship. Paria ♂♂ were much more aggressive than Upper Aripo or Guayamare ♂♂ upon encountering other ♂♂of the same race. Increased aggressiveness in P ♂♂ occurred in the presence of ♀♀, suggesting that ♂♂ fight over ♀♀. Display behaviour was reduced (especially in P ♂♂, who fought) when two ♂♂ were introduced to one another and a ♀ after an isolation period. However, no decrease was noted without an isolation period. Display behaviours decreased with the addition of a third ♂ in the Paria and Upper Aripo races, but not in the Guayamare. Increasing the number of ♂♂ generally heightened the level of ♀-oriented activity and reduced the distance between ♂♂ and the ♀. Preferences for fish of the same race occurred. When three ♂♂, one of each race, were presented to a virgin ♀ simultaneously, Paria ♂♂ fought only in the presence of Paria ♀♀, Upper Aripo displays were more successful in eliciting sexual responses, especially from Upper Aripo ♀♀, and Guayamare ♂♂ thrust more at Guayamare. Choice behaviour experiments revealed that virgin Paria and Guayamare ♀♀ respond selectively to ♂♂ of their own race. In another experiment, Upper Aripo ♀♀ responded much more readily to displays from Upper Aripo males. Females seem more likely to complete full sexual responses with ♂♂ of their own race. It appears that relatively light predation and good visibility have resulted in the evolution of displaying and fighting as the primary mating strategies in headstreams. Heavy predation and poor visibility have resulted in selection for downstream ♂♂ which display less frequently and rely more heavily on tactile signals to insure insemination. A simple model is presented to suggest how behavioural differences evolved. The results are discussed in light of other studies on geographic variation in color and mating behaviour. / Science, Faculty of / Zoology, Department of / Graduate

Thesis (M.Sc.). / Written for the Dept. of Biology. Title from title page of PDF (viewed 2008/12/07). Includes bibliographical references.

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The role of pituitary and gonadal steroids in the development and maintenance of the testis and secondary sex characters of the guppy Poecilia reticulata Peters has been studied by the technique of surgical hypophysectomy and chemical inhibition of gonadotropic action using 'methallibure’. Hypophysectomy of juvenile or adult guppies completely blocks mitosis in the spermatogonia and their transformation into spermatocytes. However, spermatocytes, spermatids and sperm already present in the adult testis at the time of operation transform into spermatophores. In the absence of the pituitary, the spermatophores rupture after eight weeks and the resulting sperm are phagocytosed within the sperm ducts. Sertoli cells, interstitial cells and the epithelial cells lining the sperm ducts regress in hypophysectomized fish. Testosterone treatment of the hypophysectomized adult guppy initiates spermatogonial multiplication and the transformation of spermatogonia into spermatocytes; the regressed Sertoli cells, interstitial cells and the epithelial cells lining the sperm ducts resume their normal appearance. Testosterone treatment of hypophysectomized juvenile guppies does not initiate spermatogenesis but the sperm ducts become well differentiated. Of two particularly well differentiated secondary sex characters of the adult male guppy, the gonopodium (modified anal fin) remains unaffected after hypophysectomy whereas the lipophores (yellow and red pigments) present on the sides of body become obscure or entirely disappear in the absence of pituitary; the lipophores reappear after testosterone treatment. Secondary sex characters never appear in guppies hypophysectomized as juveniles. When hypophysectomized juveniles are treated with testosterone, secondary sex characters (gonopodium and lipophores) become evident. The regression of the gametogenetic and the steroidogenetic tissues in the testis of 'methallibure'-treated (1:10⁶ parts) adult guppy is not as complete as in the hypophysectomized fish. This indicates that the release of pituitary gonadotropins is not completely blocked. With the same dose of 'methallibure', however, the gonadotropin release in the juveniles is apparently blocked. In both adult and juvenile guppies 'methallibure' brings about a clear decrease in both the number and mean cell diameter of gonadotrophs. The gonadotropic hormone blocking activity of the compound seems to occur at the level of hormone synthesis. From these studies it has been concluded that mitotic division of spermatogonia and their transformation into spermatocytes are dependent on pituitary, but the transformation of spermatocytes, spermatids and sperm into spermatophores are pituitary-independent. The release of spermatophores is under the control of pituitary. The regressed Sertoli cells, interstitial cells and the epithelial cells lining the sperm ducts of hypophysectomized fish assume normal appearance with testosterone treatment. The appearance of secondary sex characters in hypophysectomized juveniles treated with testosterone indicates that secondary sex characters are directly controlled by testosterone. 'Methallibure' completely blocks the synthesis of gonadotropic hormones in the juvenile guppies but not in adults. / Science, Faculty of / Zoology, Department of / Graduate

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Digestive tracts of wild guppies, Poecilia reticulata (Pisces, Poeciliidae) contained mainly benthic algae and invertebrates. In the laboratory, guppies matured on diets consisting solely of Chlorococcum (Chlorophyceae), Daphnia, or dried fish food, but rates were slowest on algae. Fish grazing benthic algae performed rapid pecks with jaws maximally protracted to cover a relatively large area. Pecks were performed in series at intervals of 0.55 s. Jaw movements required 0.17 s, while substrate contact lasted 0.03 s. Ingestion/peck varied with fish size, sex and algal density. Males fed on lower densities and ingested more per peck than females of similar weight. Ingestion rates were high and showed no consistent daily pattern. Females had longer feeding bouts than males. / Dans la nature, l’appareil digestif du guppy, Poecilia reticulata (Pisces, Poeciliidae), contient principalement invertébrés et algues benthiques. En laboratoire, le taux de croissance des guppys nourris avec Chlorococcum (Chlorophyceae) fut plus lent que celui des poissons nourris avec la nourriture sèche ou des Daphnia. Les poissons se nourissant d’algues, accomplissent des mouvements d’ingestion rapides, avec leurs mâchoires maximalement ouvertes pour couvrir le plus d e sur f ace possible. Ce s mouvements se répétent en série, a intervalles de 0.55 s. Le mouvement d’ouverture des mâchoires dure 0.17 s, le temps de contact avec le substrat 0.03 s. Le rapport ingestion/bouchée varie avec la taille du poisson, son sexe et la densité des algues. Les mâles se nourrissent sur des plus faibles densités et ingérent plus par bouchée que les femelles du même poids. Les taux d’ingestion sont élevés et ne suivent pas de cycles journaliers. Les femelles ont des périodes d’ingestion plus longues que les mâles. fr

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