Thursday, November 25, 2010

Mating Systems & Climate Change

A night time boat trip on Bueng Boraphet, one of Thailand's largest wetlands, was expected to produce frogs and snakes. Instead we found chicks of the Pheasant-tailed Jacana (Hydrophasianus chirurgus). They were little balls of feathers with long legs running from lotus leaf to lily pad leaf, and would occasionally disappear under the water. Jacanas are found around the world in the tropics and they have an unusual and relatively rare mating system for a bird, the males incubate the eggs and guard the chicks, while the females move onto another male and produce another clutch of eggs. Jacanas have a classic polyandrous mating system. Hypotheses for polyandry are variable but in the case of the jacana their eggs have an exceptionally high mortality rate because they are laid at the water's edge. Rising water and predators destroy a high percentage of the bird's reproductive effort. Chen et al. (2008) found jacana behavior most parsimonious with the idea that females produced eggs with many males for increasing the survival rate of their offspring. Animal mating systems are diverse and adaptive, and reptiles have an added variable not found in birds, they are ectotherms.

Lacerta agilis. Photo Credit: Rolf Gebhardt
Since 1984, Matt Olsson and colleagues have studied the evolutionary biology of Swedish Sand Lizard (Lacerta agilis). They observed male and female active every day during the mating season from late April to early June. The lizards were active whenever spring temperatures and cloud cover allowed the lizards to thermoregulate. They found variation in the date’s females laid eggs, confirming the impact temperature has on activity and reproduction. Males are more active than females and subsequently have a larger home range than that of the more sedentary females. Male mating tactics vary with body size. Larger males are bright green on the sides of their bodies and are more aggressive; overtly signal their presence to other males. Smaller males are more cryptically colored (less green) and use mating tactics that are “sneaky.”  Sand Lizards are polyandrous with females mating with multiple males and sperm competition is continuous with a “raffles/lottery” effect set by sperm investment. Males regulate sperm transfer during copulation and cryptic female choice on male genotypes. Olsson's previous research found males less related to the female do better in sperm competition. Under the assumption that male activity is enhanced by higher temperatures, the research group made the predictions that warmer years would result in (1) higher mating rate in both sexes, and (2) stronger sperm competition and/or cryptic female choice. During the study period the mean annual temperature increased by 2ºC. The increase in temperature resulted in a greater mate encounter rate, increasing the degree of polyandry and polygyny, and an increase in the number of sires per clutch in the free-ranging Sand Lizards. The increased number of partners produced greater variation in mate quality, and resulted in a decline in the number of sires per clutch. The authors found this agrees with previous lab studies in which females exercised stronger cryptic female choice when male quality varied. Thus, females had some control of decreasing the risk of having malformed offspring - and controlling which males fertilized their eggs. Olsson and colleagues concluded that, “Ultimately, such variation may contribute to highly dynamic and shifting selection mosaics in the wild, with potential implications for the evolutionary ecology of mating systems and population responses to rapidly changing environmental conditions.”

Literature

Chen T. C., Y-S. Lin, and T-S Ding. 2007 (2008). Time Budget of Polyandrous Pheasant-Tailed Jacana (Hydrophasianus chirurgus) during Breeding Season in Taiwan. Taiwania, 53:107-115.

Olsson, M., E. Wapstra, T. Schwartz, T. Madsen, B. Ujvari, and T. Uller. 2010. In hot pursuit: fluctuating mating system and sexual selection in Sand Lizards. Evolution, no. doi: 10.1111/j.1558-5646.2010.01152.x

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