Hans Hofmann started by outlining the historical roots of neuroethology, in particular Niko Tinbergen’s four pillars of ethology: Development, survival value (function), causation  and evolution. In his own lab, they study the three most important decisions in any animals’s life: the decision to fight, to flee and to mate, They study these processes using Cichlid fish from Africa. Some of these species are highly social. For instance, in Astatotilapia burtoni, about 30% of males are dominant and territorial, whereas the remaining subordinate males school in open water and show a number of physiological differences to the dominant males, among them dull coloration and reduced gonad size. Dominant males will mate with the females while the subordinate fish do not. Interestingly, which animals are dominant and which subordinate can change: subordinates can become dominant and vice versa multiple times in the lifetime of an individual, with all the physiological changes this entails.

The dominant fish not only get to mate with the females, they also face, due to their conspicuous coloration, a higher risk of predation by Kingfisher birds. At this point Hans introduced yet another difference between dominant and subordinate males: dominant males have a quicker escape response towards auditory stimuli mimicking the sound a Kingfisher makes when it dives into the water. As with the other physiological differences, these escape response parameters also change when the social status of an individual changes. These so-called C-start escape responses are very well studied and mediated by a very famous neuron, the Muthner cell, the largest vertebrate neuron. These Mauthner cells show an increased excitability in dominant males, consistent with what can be observed on the behavioral level. Administering serotonin to dominant males reduces their escape probability to the level of subordinates. Conversely, serotonin antagonists increased escape probabilities an dominant and in subordinate males. When the Mauthner cell was recorded from in these treated animals, excitabilities were observed which matched the behavioral results, suggesting that the Mauthner cell’s change in properties is responsible for the change in behavior. Doing single-cell RT-PCR on Mauthner cell mRNA, they showed the presence of a range of serotonin receptors, although not the one they antagonized in the experiments above, suggesting that serotonin may act both directly on the Mauthner cell and on inhibitory neurons which make synaptic connections with the Mauthner cell.

After escape, the next decision Hans talked about was whether to mate or not. Studying swordtails, Hans showed data that sexual and social stimuli in a mate-choice situation elicit rapid and contrasting genomic responses in these animals only minutes after the encounter, affecting about 10% of the protein-coding genome. Translating these experiments to Hans’ Cichlid fish, they established a mate-choice paradigm for their model species. Interestingly, dominant males are not preferred all the time by the females (even though dominants do get the overwhelming majority of matings). Prostaglandin F2alpha facilitates the decision for the dominant males in the female. Using this and other hormones involved in mate-choice, Hans now plans to study the evolutionary conservation of hormonal, neuronal and developmental mate choice mechanisms.