The cost of parental care and egg cannibalism in the river bullhead,Cottus gobio L. (Pisces,Cottidae)

Summary Male river bullheads guard and care for egg masses during a single brood cycle every breeding season. A study of two bullhead populations demonstrated that nesting males show a strong reduction in food intake rate and that their physical condition deteriorates during parental care. The estimated weight loss for the average guarding male was 18.8% in one population and 13.5% in the other. This could in part be responsible for the peak of male mortality observed during the second part of the breeding season. A high incidence of egg cannibalism was observed in males guarding eggs. Analysis of the developmental stage of individual egg masses demonstrated that heterocannibalism is very rare in this species and that the observed rate of egg cannibalism is mainly due to guarding males preying upon their own eggs (filial cannibalism). In both populations the frequency of filial cannibalism was negatively correlated with the male's chance of getting other food items. The probability of a male cannibalizing its own eggs was also significantly influenced by the time elapsed since the beginning of parental care. The observed limited cannibalism of progeny in the river bullhead cannot be explained as a male's strategy for obtaining energy to be used in subsequent brood cycles, as suggested for other fishes which show filial cannibalism. Rather, it can be interpreted as a behaviour aimed at avoiding the risk of dying of starvation before the eggs hatch. The observed criteria of female mate choice, i.e. a preference for males in good physical condition and for males that already have eggs in their nests, are consistent with the prediction of Rohwer's filial cannibalism theory, although other hypotheses cannot be excluded.     

Manipulation of sex differences in parental care

Summary In a species with biparental care two parents cooperate to provide the appropriate amount of care for the young. Recent theoretical treatments consider the evolutionarily stable investment strategy. Under most conditions, the parental investment of the two partners should be negatively correlated, with the shortfall of one partner being partially compensated for by the other. Previous experimental manipulations of biparental care have involved removal of one partner, yet the response of a widowed bird may differ from that of a mated bird whose partner is doing less than its fair share of parental care. We present the first data involving subtle manipulations of sex differences in parental care where both partners continue to care for the young. This study involves pairs in a nestbox colony of european starlings (Sturnus vulgaris L.) with all brood sizes manipulated to five chicks. Pairs were randomly assigned to three groups: (i) male parental care reduced; (ii) female parental care reduce; and (iii) control pairs. Parental care was manipulated by attaching small weights to the base of a bird's tail feathers. Regardless of sex, nest visitation rate was reduced in the weighted birds with an incomplete compensatory increase by their unweighted partner. Additional parental duties were also considered, including shifts in prey type delivered to the nest, in both weighted birds and their partners. The shift in diet and the overall lower total visitation rate in experimental nests contributed to slower chick growth and lower chick weights than in control nests. The data accord with models suggesting that equality of invesment in biparental species is evolutionarily stable, but reveal new dimensions of parental response that need to be taken into account in theoretical treatments.     

Parental care of replacement clutches in common terns (Sterna hirundo)

In long-lived seabirds with low annual reproductive output, the renesting decision after breeding failure is critical, and the parents have to weigh benefits of replacement clutches against possible future reproductive costs. In this study, we investigated factors influencing renesting decisions in common terns (Sterna hirundo) and compared aspects of breeding biology and body mass between two breeding attempts by the same pairs in each of 4 years of heavy losses due to predation. Renesting birds were characterized by early laying dates and by a high age. Among early breeders, high egg mass reduced the probability of renesting. A long relaying interval coincided with low mass of replacement eggs in one year, and short intervals with high egg mass in another. Further, egg mass decreased and relaying intervals increased the later the predation events occurred. Evidence of high levels of parental care of replacement clutches came from body mass data: female mass increase prior to egg laying was higher in the second attempt than in the first, whereas male mass was lower during the second courtship period than during the first. Male mass also affected relaying intervals and mass of replacement eggs. We conclude that common terns expend high levels of parental care of replacement clutches. Intrinsic factors related to individual quality (age, body condition) seemed most important for renesting decisions and for the degree of parental care, but foraging conditions seemed to have modifying effects. Received: 13 August 1999 / Received in revised form: 5 February 2000 / Accepted: 13 March 2000     

Food selection by laboratory-reared larvae of the scaled sardine Harengula pensacolae (Pisces,Clupeidae) and the bay anchovy Anchoa mitchilli (Pisces,Engraulidae)

Food selection by laboratory-reared larvae of scaled sardines Harengula pensacolae, and bay anchovies Anchoa mitchilli, was compared. Natural plankton was fed to the larvae during the 22 days following hatching. Food levels in the rearing tanks were maintained at an average of 1,600 to 1,800 potential food organisms per liter. Larvae of both species selected as food copepod nauplii, copepodites, and copepods; initial feeding was on organisms of 50 to 75 body width. Larvae of H. pensacolae averaged 4.2 mm in total length at hatching and those of A. mitchilli about 2.0 mm. H. pensacolae larvae grew about 1.0 mm per day and A. mitchilli 0.70 mm per day. The mean number of food organisms in each digestive tract was greater in H. pensacolae than in A. mitchilli, and the difference in number increased as the larvae grew. Average size of food organisms eaten increased for both species with growth, because of selection by the larvae; the average size of copepodites and copepods in digestive tracts increased at a faste rate in H. pensacolae than A. mitchilli. A. mitchilli longer than 8 mm did not eat copepod nauplii.Contribution No. 170, Bureau of Commercial Fisheries Tropical Atlantic Biological laboratory, Miami, Florida 33149, USA.     

Parental condition, brood sex ratio and differential young survival: an experimental study in gulls (Larus fuscus)

Empirical evidence is growing that the offspring sex ratio in birds can be biased in relation to the body condition of parents during breeding. The sex ratio bias may come about because (1) the actual production of the two sexes may be skewed and/or (2) there may be a sex bias in early nestling mortality contingent on parental condition. By manipulating parental condition and giving them a control brood to rear, thereby eliminating effects operating via the eggs, we examined the extent to which parental condition influences the post-hatching survival of male and female lesser black-backed gulls, Larus fuscus. We found that the pre-fledging survival of male chicks was strongly reduced in all-male broods reared by parents in poor condition. Pre-fledging survival of female chicks was, however, unaffected by parental condition or brood sex composition. Thus, independently of any production biases, sex differences in nestling mortality alone can bias the offspring sex ratio at fledging in relation to the prevailing rearing conditions. In other studies on gulls we have, however, also shown that females in poor condition at laying preferentially produce female eggs. Clearly a bias in fledging sex ratio can occur within the same species due to a combination of differential production and differential post-laying mortality; the latter can involve a differential effect of poor egg quality on male and female offspring, differential effects of brood sex composition on their survival and a difference in the capacity of parents to rear males and females. All of these processes need to be taken into account in attempting to understand offspring sex ratios. Received: 15 February 2000 / Revised: 7 August 2000 / Accepted: 26 August 2000     

Lifetime reproductive success in the spotted sandpiper (Actitis macularia) : sex differences and variance components

Summary Lifetime reproductive success (LRS) of male (N=103) and female (N= 66) spotted sandpipers (Actitis macularia) was studied for 13 years of a 17-year study at Little Pelican Island, Leech Lake, Minnesota. There was no sex difference in longevity, but females had significantly more mates, eggs, chicks, fledged young, and young returning in subsequent years than did males. Variance in LRS was partitioned into five life-history components: longevity (L), mates per year (M), eggs per mate (E), proportion eggs hatched (H), and proportion of chicks fledged (F). For both sexes, F accounted for the greatest proportion of variance in LRS (males, 43%; females, 47%), followed by L (males, 26%; females, 43%) and H (males, 21%; females, 28%). Positive covariance between H and F was consistent with predator-caused clutch and brood loss. Contrary to our expectations, males had a higher coefficient of variation in reproductive success than did females. This was because males were relatively more likely than females to produce no young.Offprint requests to: L.W. Oring at the current address     

Parental investment and the secondary sex ratio in northern elephant seals

Summary Data on northern elephant seals, Mirounga angustirostris, bearing on sex ratio theory were collected at Año Nuevo, California, and other Californian and Mexican Islands, during the period 1967 to 1988. The mass of males exceeded that of females by 7–8% at birth and at weaning. The sex ratio was biased to males at birth (51.2%) and was near unity at weaning (49.6% males). The sex ratio did not vary as a function of maternal age or maternal mass except in 6-year-old females, who produced significantly more males. Although sons cost more to rear in energetic terms than daughters, and mothers were more successful weaning the latter, the sex of the pup reared exerted no significant effect on the mother's reproductive performance the following year or on her subsequent survival. These data suggest that parents invest equally in sons and daughters when investment is measured in terms of future reproduction (Fisher 1930) and provide no support for the theory of adaptive shifts in sex ratio (Trivers and Willard 1973). The small sex difference in mass due to maternal effort reflects the fact that females fast during lactation and all energy transferred is from limited body stores. Because of these circumstances, selection for superior condition at the end of the period of parental investment may act more strongly on pups, who have the opportunity to steal milk, than on their mothers.     

Parental investment, adult sex ratios, and sexual selection in a socially monogamous seabird

Although most birds are monogamous, theory predicts that greater female parental investment and female-biased adult sex ratios will lower the polygyny threshold. This should result in polygynous mating, unless obligate biparental care or the spatial and temporal distribution of fertilizable females constrains a male’s ability to take advantage of a lowered polygyny threshold. Here we present data on the extent of male sexually dimorphic plumage, adult sex ratios and breeding season synchrony in three populations of a socially monogamous seabird, the brown booby Sula leucogaster. For one of these populations, San Pedro Mártir Island, we also present data on differences in male and female parental investment, mortality and probability of pairing. The extent of plumage dimorphism varied among populations. Sex ratios were female biased in all populations. On San Pedro Mártir Island, parental investment was female biased, females failed more often than males to find a mate, but there was no polygyny. We suggest that on San Pedro Mártir: (1) a period of obligate biparental care coupled with a relatively synchronous breeding season constrained the ability of males to take advantage of a high environmental polygamy potential and (2) the resulting socially monogamous mating system, in combination with the female-biased adult sex ratio, caused females to be limited by the availability of males despite their greater parental investment. Received: 18 November 1999 / Accepted: 24 January 2000     

Karyotypes in two species of the genusHippocampus (Pisces: Syngnatiformes)

Diploid modal numbers of chromosomes 2n=48 forHippocampus ramulosus and 2n=44 forH. hippocampus, collected in the Gulf of Palermo in May 1985, are established. No sex-linked heteromorphic pairs are observed in the two species analyzed. The karyotype ofH. hippocampus seems to be a derived condition.     

Notes on the occurrence of Tilapia zillii (Pisces) in Suez Bay

Tilapia zillii Gervais is recorded for the first time from Suez Bay off the Institute of Oceanography and Fisheries, Ataqa (UAR), during October and November 1965 and 1966. Salinities and temperatures of surface sea water ranged from 42 to 42.79 and 23° to 24.4°C, respectively. The occurrence of specimens of T. zillii in the Bay indicates that it can tolerate high salinities and survive in the sea.     

Allozyme variation in global populations of striped mullet, Mugil cephalus (Pisces: Mugilidae)

The striped mullet, Mugil cephalus Linnaeus, 1758, is one of the few species of marine shore fish with a worldwide circumtropical distribution. Because of this distribution and the dependency of M. cephalus on coastal waters during various phases of its life cycle, as well as nearshore living habits, questions have been raised regarding levels of genetic divergence and gene flow among transoceanic populations. To cast more light on this, allozyme variation at 27 presumptive gene loci was investigated in ten globally diverse populations. The observed heterozygosity ranged from 0.018 (Hawaii) to 0.081 (Florida), averaging 0.050. The proportion of polymorphic loci showed a similar trend. Several populations were characterised by fixed allelic differences. Estimated gene diversities were very high, the allele frequency variation among populations was found to be 68%; genetic distances reached 0.242, with an average of 0.117. Estimated rates of gene flow were high among Mediterranean populations (Nm = 7.26), and between Mediterranean and East Atlantic populations (Nm= 2.86), but extremely low between non-contiguous populations within the Indian, Pacific and Atlantic Oceans, where Nm ranged from 0.03 to 0.05. Received: 7 April 1997 / Accepted: 4 February 1998     

Factors affecting buoyancy in the eggs of Bairdiella icistia (Pisces: Sciaenidae)

Several aspects of buoyancy were studied in the normally pelagic eggs of the sciaenid fish Bairdiella icistia (Jordan and Gilbert). Spawning fish acclimated to a low salinity (15‰) produced larger, more buoyant eggs which had a higher water content than eggs produced by fish living in ordinary sea water (33‰ S). Eggs fertilized in lower salinities were larger and more buoyant than eggs fertilized in higher salinities. The salinity of the medium during the first 5 to 7 min after fertilization had a lasting effect on egg buoyancy, but subsequent transfer to a different salinity also influenced buoyancy. Although egg buoyancy in this species can be influenced by the ambient salinity both before and after spawning, the capacity for adjusting buoyancy is limited, and eggs spawned in salinities lower than 30‰ would probably sink.     

Aggressiveness and foraging mode of young-of-the-year brook charr,Salvelinus fontinalis (Pisces,Salmonidae)

Summary Young-of-the-year brook charr in streams use either an active or a sit-and-wait foraging tactic and exhibit a range of resource defense from territoriality to tolerating conspecifics. We use simple graphical models, based on encounter rate with drift and the theory of economic defendability, to predict qualitative changes in the aggressiveness and mobility of brook charr in relation to current velocity. Aggressiveness (percent of conspecifics eliciting an overt response) initially increases with increasing current velocity, as does drift rate and foraging rate. However, aggressiveness decreases at high current velocities, probably because of increased costs of defense at these velocities. In standing water areas, brook charr use primarily an active foraging tactic, but mobility (percent time spent moving) decreases rapidly as current velocity increases. These results are generally consistent with the simple graphical models. A literature survey suggests that the models can be generalized for most species of stream salmonids.     

Growth of Chaetodon larvatus (Chaetodontidae: Pisces) in the southern Red Sea

Growth and age of Chaetodon larvatus were studied using growth bands in otoliths and length-frequency analyses. Otoliths of 180 C. larvatus were extracted and measured. Polished sections of sagittae revealed alternating opaque and translucent bands corresponding with a seasonal growth pattern. Both mass and size of the otoliths continue to grow steadily throughout life. Length-at-age data revealed very fast growth during the first year. Growth proceeded at a decreasing rate during the second and the third year; fishes older than 3 years did not grow noticeably. No difference in growth patterns between males and females could be detected. The growth parameters obtained for the whole population are: the asymptotic length (L _∞)=10.64 cm, growth constant (K)=1.14 year⁻¹ and the theoretical age at length zero (t ₀)=−0.30 year. The maximum age recorded was 14 years. Length frequency data collected at a recruitment site confirmed the fast growth of juveniles.     

Diet of orange roughy,Hoplostethus atlanticus (Pisces: Trachichthyidae) on the Challenger Plateau,New Zealand

Stomach contents of 687 orange roughy [Hoplostethus atlanticus (Collett, 1889)] from the Challenger Plateau, sampled in March, July and November 1984, were examined. The relative importance of different components of the diet was assessed using three feeding indices which combine, in different ways, percentage frequency of occurrence, percentage number, and percentage weight of prey categories. For both sexes, for all size classes and at any time of the year, the most frequent and abundant components of the diet were prawns, followed by fish, squid, amphipods and mysids. The main natant decapod families were Oplophoridae, Pasiphaeidae, and Sergestidae. Fish, mainly Chauliodontidae and Myctophidae, were the most important prey by weight. Most prey species were benthopelagic and mesopelagic organisms which move towards the surface at night. However, H. atlanticus can be caught by bottom trawl (between 750 and 1 200 m) during any 24 h period, and there was no evidence of vertical migration in search of their prey. When the fish grow in length, there is a transition in the diet from prawns, mysids, and fish, to prawns, fish and squid. Squid were not found in the stomach contents of fish smaller than 20 cm. These dietary changes may be linked to modifications in morphology with growth. The relative proportions of the main dietary items, and in particular the natant decapod families, varied with time of the year. Also, the stomach data seem to indicate an increasing importance of fish and squid in the diet in deeper water. H. atlanticus appears to be an opportunistic predator, consuming a wide variety of invertebrates and fishes. Our results provide evidence to support the notion that benthopelagic predators which consume vertically migrating mesopelagic fish, have an important role in the transfer of energy to the benthos.     

A comparative study of the blood of four Tilapia species (Pisces)

The blood of 4 species of Tilapia has been examined. Mean erythrocyte count and size, hemoglobin content, and hematocrit value varied markedly among the species examined. A tendency towards a reciprocal relationship between the number of erythrocytes and their size was observed. In general, their is a correlation between the erythrocyte counts, hemoglobin content, and hematocrit value among the species examined.     

Mating patterns and a test for female mate choice in Etheostoma spectabile (Pisces,Percidae)

Neither size nor breeding color correlated with spawning success of male orangethroat darters, Etheostoma spectabile (Pisces, Percidae), under natural field conditions. When females were presented experimentally with a simultaneous choice they spent no more time in proximity to large than small males, and were subsequently no more likely to spawn with large than with small males. Females also displayed no preference for bright versus dull males. Males and females did not differ significantly in size. Etheostoma spectabile may lack sexual size dimorphism as a result of the lack of female choice for size and the ineffectiveness of male attempts to monopolize females, or selection may be for increased size of females. Males are not dwarfs because of sperm competition. Contest competition among males appeared to be important in initiating spawnings but many males obtained spawnings by participating in ongoing spawning events. Etheostoma spectabile is an example of a sexually dimorphic species with no evident female preference for male size or color.