Evolutionary explanations for showing off in females.

Supervisors:
Dr Jonathan P Green, Department of Zoology, Oxford
Prof Ashleigh S Griffin, Department of Zoology, Oxford
Dr Natalie Cooper, Natural History Museum, London

Individuals of many species signal aspects of their quality to conspecifics, but to date research has focused largely on quality signalling by males, while signalling by females remains little explored. For males, which produce large numbers of relatively inexpensive sperm and typically invest little in parental care, maximising reproductive success usually depends upon maximising the number of matings by attracting females and repelling rival males. Because males are relatively unconstrained by investment in reproductive function, they are consequently free to invest heavily in traits that signal their quality to potential mates and rivals. Females too may frequently be under selection to advertise their quality to conspecifics, but at the same time face increased reproductive costs in the forms of egg production and, in many species, offspring care. Understanding both the trade-offs that females face between investment in signalling and reproduction and the contexts in which females are selected to advertise their quality remain major challenges in behavioural ecology. This DPhil project will address these questions by investigating quality signalling by females in two distinct vertebrate taxa (fish and birds) across two different contexts: (1) quality signalling by females during competition, and (2) quality signalling by breeding females in cooperatively-breeding societies.

(1) Quality signalling by females during competition
Female investment in quality signalling during competitive interactions with same-sex rivals will be examined in the cichlid Lamprologus ocellatus. In this species, female reproductive success depends on access to empty snail shells, in which females lay their eggs. When mature, females settle in vacant shells within a male’s territory, and invest time and energy in defending their shells from other females. The opportunity to manipulate the position and number of shells within a male’s territory allows precise control over the intensity of competition among females. The student will carry out behavioural experiments using a captive population of L. ocellatus housed in the Zoology Department at the University of Oxford to investigate (a) the potential reproductive costs to females of investment in quality signalling when competing with other females and (b) how investment strategies change with increasing competition and the effect of this on female (and male) reproductive success.

(2) Quality signalling by females in cooperatively-breeding societies
In many cooperative breeders, offspring delay dispersal and remain with their parents to help them rear further offspring (the helper’s siblings). By enhancing the breeding success of their parents, helpers gain indirect fitness, but the size of this payoff will depend upon the quality of the breeders, specifically their ability to keep producing offspring. If breeder fecundity declines, helpers may achieve greater fitness by breeding themselves, either within the group (which may involve the eviction of the current breeder) or outside the group. To prevent mutiny or desertion by helpers, breeders may therefore be under selection to advertise their continuing fecundity. This idea has been explored extensively in eusocial insects, where queen pheromones are thought to signal fecundity to workers, but nothing is known about the potential for female breeders in cooperatively-breeding vertebrates to signal their fecundity to helpers. To explore this possibility, the student will collect data on female plumage coloration across a range of cooperative and non-cooperative bird species, making use of the Natural History Museum’s extensive collection. Using these and other data on mating system, social structure and the tenure and fate of breeders and helpers, the student will then undertake comparative analyses to test the prediction that females in cooperatively-breeding species advertise their quality in order to retain offspring helpers.

As outlined above, the DPhil project will involve both experimental study of fish behaviour and measurement and analysis of bird plumage coloration. The student will receive extensive training both in statistics, including comparative methods, and in designing and performing behavioural experiments with fish and will have significant opportunities to contribute their own ideas. The project would suit a student with strong interests in the evolution and ecology of animal behaviour.