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Behavior

Why Mom Cares.

Pike, David A. et al. “Surf and Turf: Predation by Egg-Eating Snakes Has Led to the Evolution of Parental Care in a Terrestrial Lizard.” Scientific Reports 6 (2016): 22207. PMC. Web. 4 May 2016.  do: 10.1038/srep22207

Introduction:

The evolution of maternal care is a mystery for many species. Logically, maternal care increases the likeliness of offspring survival, and thus the renewal of a species with each mating season.

Maternal care can be defined as anything that strengthens and protects an offspring. Examples range from simply protecting a nest, or literally feeding offspring directly from the parent’s mouth to the young’s mouth.

Past laboratory studies have concluded that caring evolves as threats of predation increase.   There is however limited information of this evolution in natural systems.

Long-tailed_Skink_(Mabuya_longicaudata)_長尾南蜥

Figure 1: Long-tailed Sun Skink

mothersday3

FIgure 2: Taiwan, Green Island, and Orchid Island

It is a difficult topic to study, often achieved with models or by focusing on the level of care provided rather than the impact of a complete lack of maternal care.   At least one instance where scientists are able to study maternal care evolution in natural systems is in the long-tailed sun skink (Eutropis longicaudata, Figure 1) populations on Taiwan mainland, Green Island, and Orchid Island (Figure 2).

The long-tailed sun skink is a cosmopolitan tropical lizard. Typically mother skinks lay and bury their eggs (~6) in grassy areas and then move on with their existence, leaving the eggs behind for good. Eggs incubate for around 26 days before hatching and then have an 18-month journey to adulthood.   During the 26-day incubation period the main threats of mortality to the skink eggs are fungus, ants, and snakes.

Taiwan mainland, Green Island, and Orchid Island are similar except that on Orchid Island there is an 110 meter long sandy beach where sea turtles come to lay their eggs between May and September, annually.   600 meters inland from the beach is a long-tailed sun skink-nesting site. The skink-nesting site is a 2km-long concrete retaining wall with 1200 10 centimeter wide holes (think Swiss cheese), in which sun skinks nest; apparently, the concrete retains an optimal temperature for embryonic development.   Green Island and Taiwan mainland also have retaining walls that are nesting sites for sun skinks, but neither has nesting sea turtles.

On all three islands skinks lay their eggs in the holes in the retaining walls.   On Taiwan and Green Island the skinks lay their eggs and then leave, like they would if they had buried their eggs.   On Orchid Island though, long-tailed sun skinks are unique because they violently protect their eggs against predation by kurki snakes (Oligodon formosanus, Figure3); the duration of nest guarding is dependent on the snake threat level.

Figure 3: Kurki snake

Figure 3: Kurki snake

Kurki snakes do not have interest in the skinks; they are only interested in the eggs.   Their favorite meal, however, is green sea turtle eggs (Chelonia mydas, Figure 4).   This has a lot to do with the nutritional value of sea turtle eggs, which are 4 times bigger and 15 times more abundant than lizard eggs (compare 4,515 grams from a clutch of turtle eggs verses only 60 grams from a clutch of skink eggs). Sea turtle eggs are only available seasonally though from May-September so for the remainder of the time skink eggs are the available food source.

figure 4: Sea turtle egg

figure 4: Sea turtle egg

Researchers wanted to determine if the skink moms on Orchid Island protect their eggs as a direct response to the danger brought upon by nesting sea turtles and the hungry snakes they attract.      Their specific questions were: Do the source of sea turtle eggs drive snake density on Orchid Island? , And does the snake density increase pressure on the lizard population?

Lab and Field Methods

Scientists made laboratory and field observations during their investigation.

Lab observations were setup to study the expression of anti-snake behavior from captive skinks. Researchers collected and raised in captivity 100 sun skink eggs from Taiwan, Green Island, and Orchid Island. Hatchlings were raised in housing mimicking the field sites with respect to drainpipes and temperature. Mating males and females that had matured from hatchlings increased the captive skink population. Pairs were allowed to breed, then the male was removed, and the mom skink remained in the cages.   After eggs were laid, an egg-eating snake was introduced into the cage and the mother’s protective behavior, or lack there of, was observed.

In the field researchers wanted to understand how evolution of skink maternal care may be linked to the interaction between terrestrial and marine habitats.  To achieve their goals scientists investigated sources of mortality of skink eggs and the relationship, if any, between green sea turtle nesting, snake density, and predation rates on skink nests.

 Field observations at the sea turtle nesting beach were six times nightly between May and October over 11 years (1997-2008- after 2008 sea turtles nests were relocated to protect them from predators).  During observations, the number of eggs laid and hatched was recorder, and observed snakes were captured, tagged, and released.   Inland observations on the skink nests were made every six hours during the same months and years as the beach observations. Inland information gathered included the number of snakes, whether or not skinks guarded the nests, and the success of eggs hatching.   Scientists used the tags to monitor movements of snakes between the two areas. One field experiment preformed was aimed to determine the effectiveness of maternal care. Scientists removed the guarding female and then replaced her with a net that would allow predation by ants and fungus while excluding snakes.

Results:

Lab experiments aimed to study the reaction of captive females with eggs during their first encounters with preying snakes concluded that only the descendants of the Orchid Island eggs guarded their eggs by attacking the snakes. The offspring from the other two islands did not.

On Taiwan mainland and Green Island an equal percentage of skink eggs hatched in protected and unprotected nests, ~60% and ~55%, respectively.   Orchid Island had nearly 80% of the skink eggs hatched in protected areas compared to only ~20% in unprotected areas.

On Taiwan mainland and Green Island the major killers of eggs were ants and fungus. On Orchid Island however, ants and fungus had a shadowed role in egg mortality compared with the overwhelming impact from snakes (figure 1a,b original article).  When predation by snakes was removed as a factor by replacing guarding moms with nets, it only had an effect on the Orchid Island hatching success.

On Orchid Island the number of egg eating snakes increased with the number of sea turtle nests, the number of snakes inland decreased with the number of snakes on the beach, and the number of snakes inland decreased with the number of green sea turtle nests (figure 3 original article).

Discussion:

Positive correlation between snake density and sea turtle nesting suggests that the sea turtle presence does drive the snake behavior.   More so, the negative correlation between snakes inland with the number of green sea turtle nests suggests that the snakes are leaving the inland to go to the beach, presumably to snack on some sea turtle eggs.   Finally, because the number of snakes inland also negatively correlates with the number of snakes on the beach, it is further evidence that snakes are migrating between the two areas.

Interestingly, the proportion of skink nests was nearly constant over the time span from May and September. During that same time period, proportion vs. time plots of turtle nests and snakes almost overlap with increases from May to a peak in August and then a rapid decline by September (Figure 4a original article).    Scientists also found that increases in snakes inland correlated with an increase in the number of skink nests preyed upon (Figure 4b original article).  This information suggests that regardless of the snakes focus on sea turtles or skink eggs, the skink mothers will always protect their nests, whether the threat level be high, or low.

That being said, there is an evident relationship between snake presence and sea turtle nesting, but it does not appear to impact the proportion of skink nests on Orchid Island.

The evolution of the maternal care trait in the skink population of Orchid Island where snake predation is common does not seem to appear in populations not threatened by snakes.   The evolution on Orchid Island is attributed to the sea turtles nesting; sea turtles on Orchid Island attract the snakes, which has forced the skink population to develop maternal care traits in order to hinder the demise of their population. Researchers note that it is possible that an absence of adult skink predators has aided in the evolution of egg-guarding behaviors. Future work involves determining if the evolution is gene mutation, or perhaps a maternal care gene that becomes turned on or off.

Importance

Studies like this are important because they aim at learning about the complexity of nature. For instance, these scientists looked at the role of the sea turtle on the dynamics and evolution of the terrestrial population; just one example of how the life habits of a marine specie can force evolution of a terrestrial specie without having any direct interaction.

 

 

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