Mare Lõhmus

Effects of egg yolk testosterone on growth and immunity in a precocial bird

In oviparous vertebrates, maternal steroid allocation to eggs can have important fitness consequences for the offspring. However, elevated testosterone levels are not only associated with beneficial postnatal effects, such as enhanced growth and high social status, but may also entail costs by suppressing the immune system. In this study, testosterone levels in eggs of Chinese painted quail (Coturnix chinensis) were experimentally manipulated to evaluate its effects on growth and immunocompetence. Testosterone did not affect embryonic development, body size or growth during the first 20 days. However, elevated testosterone levels during embryonic development were immunosuppressive for chicks with inherently higher growth rate. Adaptive scenarios where only beneficial effects of increased testosterone levels are considered may therefore need to be re‐evaluated.

Growth hormone transgenic salmon pay for growth potential with increased predation mortality

Recent advances in gene technology have been applied to create fast–growing transgenic fish, which are of great commercial interest owing to their potential to shorten production cycles and increase food production. However, there is growing concern and speculation over the impact that escaped growth hormone (GH)–transgenic fish may have on the natural environment. To predict these risks it is crucial to obtain empirical data on the relative fitness of transgenic and non–transgenic fish under nature–like conditions. Using landscaped stream aquaria with live food and predators, we show that the predation mortality of newly hatched GH-transgenic coho salmon fry (Oncorhynchus kisutch) is much higher than in non–transgenic conspecifics, and that this difference is amplified when food abundance decreases. The growth rate of transgenic and non–transgenic fish is similar at high food levels, whereas transgenic fish grow more slowly than non-transgenic fish when food abundance is reduced. Our results suggest that the fitness of young GH–transgenic coho salmon in the wild will be determined by both predation pressure and food availability.

SELECTION ON INCREASED INTRINSIC GROWTH RATES IN COHO SALMON, ONCORHYNCHUS KISUTCH

Abstract Substantial evidence from the animal kingdom shows that there is a trade‐off between benefits and costs associated with rapid somatic growth. One would therefore expect growth rates under natural conditions to be close to an evolutionary optimum. Nevertheless, natural selection in many salmonid species appears to be toward larger size and earlier emergence from spawning redds, indicating a potential for increased growth rate to evolve. We tested how selection for genetic variants (growth hormone transgenic coho salmon, Oncorhynchus kisutch, with more than doubled daily growth rate potential relative to wild genotypes) depended on predator timing and food abundance during the early period of life (fry stage). In artificial redds, fry of the fast‐growing genotypes showed a highly significant developmental shift, emerging from gravel nests approximately two weeks sooner, but with an 18.6% reduced survival, relative to wild‐genotype fry. In seminatural streams, fry of the fast‐growing genotypes suffered higher predation than those of wild genotypes when predators were present at the time of fry emergence, but this difference was less pronounced when food was scarce. In streams where predators were introduced after emergence, fry survived equally well regardless of food availability. Surviving fry grew faster in habitats provided with more food, and fast‐growing genotypes also grew faster than wild genotypes when predators arrived late and food was abundant. Fewer fish migrated downstream past a waterfall when food availability was high and in the presence of predators, and wild‐genotype fry were more likely to migrate than fry of the fast‐growing genotypes. After being returned to the experimental streams after migration, fast‐growing genotypes survived equally well as those of the same genotypes that did not migrate, whereas migrating wild genotypes experienced higher mortality relative to those of the same genotypes that did not migrate. Comparisons of growth rates between siblings retained under hatchery conditions and those from habitats with the fastest growth in the experimental stream revealed that growth rates were similar for wild genotypes in both environments, whereas the fast‐growing genotypes in the streams only realized 90% of their growth potential. The present study has shown that a major shift in developmental timing can alter critical early stages affecting survival and can have a significant effect on fitness. Furthermore, ecological conditions such as food abundance and predation pressure can strongly influence the potential for fast‐growing variants to survive under natural conditions. The large‐scale removal of many predatory species around the world may augment the evolution of increased intrinsic growth rates in some taxa.

Investment in territorial defence depends on rearing environment in brown trout (Salmo trutta)

In many animals, territoriality will arise or cease depending on environmental factors such as intruder rate and resource availability. We investigated the effect of rearing environment on territorial behaviour in ~1.5-month-old brown trout. In the laboratory, wild-caught (reared at a low density) and hatchery-reared (high density) trout were allowed to defend a territory against a size-matched intruder reared in the same or the other environment. Because territorial behaviour should be relaxed at high-rearing densities, we hypothesized that hatchery-reared trout should value their territories less and therefore invest less in defence compared with wild-caught trout. However, in all cases, territory owners were more likely to win the contest and hatchery-reared trout were just as likely as wild-reared to win mixed contests. Furthermore, pairs of hatchery-reared trout initiated contests sooner, fought longer and were more aggressive during the contest compared with pairs of wild trout. When hatchery-reared owners met wild intruders, the contest ended sooner compared with when the roles were reversed. We conclude that territorial behaviour in brown trout is largely innate, but that the hatchery environment has promoted more aggressive individuals. These results suggest that hatchery-reared trout invest more time and energy to obtain the same contest success as wild trout. In conclusion, the lack of experience of territorial defence in a high-density rearing environment seems to reduce the efficiency of territorial behaviour. In turn, this may have negative consequences for the performance of released hatchery fish in the wild.

Leptin depresses food intake in great tits (Parus major).

Food availability for wild organisms typically varies both in time and space, requiring a mechanism that regulates the storage of excess energy and makes it possible to use stores during energy shortfall. Leptin, a protein hormone encoded by an obesity gene, has been suggested to be the signal mediator for this flux of energy. In a controlled laboratory experiment on caged great tits (Parus major) we evaluated the effect of leptin on food intake and behaviour. Experimental birds were given an intramuscular injection of 10 microg leptin dissolved in phosphate buffered saline (PBS), while the control birds were injected with PBS only at 09:00 h after a nights fasting. Within the first 20 min after injections we observed a significant difference in food intake between groups: control birds initially fed at higher rates compared to leptin treated birds. The cumulative food intake suggested that the effect of leptin disappeared after approximately 40-50 min post-injections. Similar results have previously been found in domesticated chickens. To our knowledge, this is the first study to show that leptin depresses food intake in wild birds.

Leptin and social environment influence the risk-taking and feeding behaviour of Asian blue quail

Optimality models predict that hungry animals should be more willing to take risks than satiated individuals; thus, the decision whether to forage is influenced by the physiological process mediating the sensing of energy reserves. An important substance involved in this sensing process is leptin, a protein hormone known to down-regulate appetite and increase energy expenditure. However, feeding and risk-taking behaviours are also influenced by the behaviour of surrounding individuals. By increasing leptin levels in a focal bird and/or in its groupmates, we studied the combined effects of physiological and social factors on the risk-taking and feeding behaviour of Asian blue quail, Coturnix chinensis. Risk-taking behaviour, estimated by the time taken to start feeding, was dependent on social factors and a birds own weight but not on whether it was treated with leptin; focal birds with leptin-treated companions took longer before feeding than did focal birds with control companions. Total feeding time, on the other hand, was physiologically governed with no effect of the companions; leptin-treated focal birds spent less time feeding than did control focal birds. To our knowledge, this study is the first to investigate the interaction between leptin and social factors in animal behaviour.