Why are there so many species? Since Hutchinson addressed the American Society of Naturalist in 1958 (“Homage to Santa Rosalia or why are there so many kinds of animals”), ecologists have long sought to answer this question. Recently, scientists from UCLA examined this basic question (report in PLOS ONE) and asked why are there three species of giraffe - Masai Giraffe (Giraffa tippelskirchi), Reticulated Giraffe (G. reticulata), and Rothschild’s Giraffe (G. camelopardalis) - instead of one in East Africa. Below are the three species of giraffes.
Giraffe’s are large, mobile, herbivores that rely on plants that grow during seasonal rains. For example, giraffe’s in Niger have an average home range of more than 90 square km. In East Africa, there are 3 distinct populations of giraffes even though they have the ability to come into contact and breed. Yet the giraffes living in East Africa are not one species, but probably three distinct species. Recent research suggests that there is a distinct genetic structure in their mitochondrial DNA and nuclear microsatelllies and minimal gene flow among the three adjacent populations. Furthermore, these giraffes interbred in zoos. In the absence of any appreciable geographic barriers that reproductively isolates each population, how do these highly mobile giraffes, that regularly interbred in zoos, remain distinct as three species as opposed to a single species? Below is a map that shows the geographic range of the three giraffe species.
Thomassen et al. (2013) examined 4 factors that are hypothesized to keep the Rothschild’s, Reticulated, and the Masai giraffe isolated from one another and prevent interbreeding. Ultimately, their results found that only differences in seasonal rainfall and growing season, influenced the time of year that giraffes breed. (In East Africa, there are three geographically distinct seasonal patterns of rain). Thus, these giraffes are only reproductively active during the growing season, the growing season is different in these three parts of East Africa, and natural selection acts on the timing of reproduction so it corresponds with the seasonal rains. Simply, giraffes that breed and reproduce when food is plentiful will grow better and faster and pass more of their genes into the next generations - voila, there are three species of giraffes instead of one. This is a great example of the simple power of divergent natural selection.
This figures shows the population range of Giraffe's in Africa. Notice how the population range of the Rothschild's, Masai, and Reticulated giraffe abut. Thomassen et al. (2013) showed that seasonal patterns of rainfall are the likely reason there are three distinct species of giraffe in East Africa as opposed to one. From Wikipedia Commons
In case you wanted to know:
Here is the citation to the study:Thomassen HA, Freedman AH, Brown DM, Buermann W, Jacobs DK (2013) Regional Differences in Seasonal Timing of Rainfall Discriminate between Genetically Distinct East African Giraffe Taxa. PLoS ONE 8(10): e77191. doi:10.1371/journal.pone.0077191
The 4 hypotheses Thomassen et al. 2013 examined were: 1) Isolation by distance, 2) Barriers to Dispersal (limit or prevent gene flow among populations), 3) Spatial habitat differences that are not due to phenology (seasonal differences), and 4) Seasonal differences related to the “greening up” of the habit - basically differences in rainfall that influence the growing season of the plants that the giraffes eat. Using multivariate, spatially non-explicit, and spatially explicit analyses, Thomassen et al. 2013 only found that only seasonal patterns in rainfall and corresponding “green up” of plants were responsible for the reproductive isolation in the three species of giraffes.
“Sailor of the atmosphere;
Swimmer through the waves of air;”
~ Emerson: The Humble-bee
Throughout the winding streets of Somerville, Cambridge, and Boston, the flowering trees and gently falling blossoms announce spring. The city’s thoroughfares buzz with activity and people eagerly greet the sun’s warm rays. Kayaks, crew shells, and sailboats glide along the Charles River, signaling an end to a “hibernating” city. As my daughter eagerly points out, the streets buzz not only with people, but with one of the first and most important native wild pollinators to emerge. The furry, booming bumble bees.
Bumble bees are in the genus, Bombus, translating to “booming”, which captures the cacophonous and frenzied pace of a bumble bee racing from flower to flower. It's called flight of the bumble bee for a reason. In the spring, you’ll probably notice that the bumble bees are much larger than you would see in the summer because the bumble bees flying around, searching for a nest site, and collecting nectar and pollen at cherry, apple, or holly flowers - to name a few - are queen bees, beginning their epic quest to start their own colony.
Before I talk more about the booming queens you see and hear whizzing by you this spring, let’s back up to last fall. In the summer and early fall when these nascent queens, who were carefully reared by the colony the previous summer, emerged, they had three critical jobs: 1) mate with male drones, 2) build up fat reserves by foraging on pollen and nectar, which allowed them to survive the winter and emerge the following spring, and 3) hibernate (it's more accurate to say they are dormant during the winter). To hibernate, most bumble bees are poor diggers and a queen will usually dig a hole in loose disturbed soil to create a cavity where it will remain during the winter.
In the spring, once queens emerge from hibernation (dormancy), they need to stock up on nectar and pollen and find a suitable nesting site to form a colony. However, it can be really cold in the spring and life is very precious for newly emerged queens. Bumble bees can warm themselves up and get moving on cool spring days. Check out this fantastic BBC video narrated by Sir David Attenborough and watch the thermal images of a bumble bee on a cold spring morning:
Bumble bee species vary, but most species will either nest in 1) pre-existing holes and cavities in the ground, like abandoned mouse nests, 2) some will nest just above the ground in the grass, under moss, or in tussocks, or 3) some species will use old bird nests, cavities in the foundations of houses or under sidewalks (like at my apartment where there is a Bombus bimaculatus colony), tree cavities, or bird boxes. A common characteristics of many nest sites is the presence of insulating material, like feathers, leaves, and moss.
Once a suitable nest is found, the queen bumble bee lays her first batch of eggs (usually between 8-16). During this time, the queen will care for and keep the brood warm, while continuing to forage for pollen and nectar that she’ll use to feed the developing larvae. After 4 to 5 weeks, the first set of workers will emerge (you’ll start to notice smaller bees flying around - the B. bimaculatus colony just had their first workers emerge this week) and the queen will lay eggs while the workers will care for the brood and forage for nectar and pollen.
To Learn More:
"The bee buzz'd up in the heat...
For me, spring starts when one of the earliest flying bee emerges, the plasterer bee Colletes inaequalis (Colletidae). Colletes inaequalis are solitary, like all species in the genus Colletes, and nest in the ground, usually in aggregations. The pictures below of the Colletes female emerging from her nest was from an area where dozens of Colletes nests were in a meter squared and the entire nesting area was about the size of a basketball court. Enjoy the pictures of this great sign of spring.
More pictures and info about Colletes inaequalis
The magnolia trees were first planted by Laura Dwight in 1963.
The Bee and The Flower by Alfred Tennyson
I am curious about nature and the wonderful species interactions I read about and see on a daily basis.