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.
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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