Wednesday, September 18, 2013

Timing of reproduction

Ring-tailed lemurs and brown lemurs have begun to give birth here in Berenty. I’ve seen two infant ring-tailed lemurs clinging to their mothers’ bellies. I saw my first brown lemur infant briefly today (September 5). All I could see was a tiny head peeking out from a mother’s leg as she walked quadrupedally across the forest floor. Most primate infants are specially adapted to cling tightly to their mothers. Human infants have lost this ability, likely because humans don’t have nearly as much hair to cling to as other primates.
Sifaka infant and mother resting

Sifakas in Berenty typically give birth towards the end of July. Here, that corresponds with the start of the dry season. Why give birth during the dry season, when the leaves are falling and food is harder to come by? Why give birth during the most difficult time of the year? Well, giving birth and nursing an infant during the dry season certainly presents its challenges. But on the flip side of this coin, sifakas wean their infants when fruits are much more available, likely giving their offspring a better chance of surviving this period (if they made it through the dry season that is).
Ring tailed mother and infant nursing

Sifaka infants have reached the stage where they are a little bit stronger and more adventurous. I’ve seen them practicing their clinging and leaping onto tree branches a little bit. They’re venturing further and further from their mothers. Sometimes they even ride on their mother’s back for a bit. They have a bit more fur now, but quite a few still have those long tails that resemble a rat’s tail, if you ask me. I’m sure they resemble rats less when all of their fur has grown in.

Brown lemur mother and infant
There are multiple strategies when it comes to timing reproduction. Gestation of any infant is energetically costly. Just think about how much more a pregnant woman eats than a woman who is not pregnant. Lactation is also costly, as infants are growing at a rapid pace. Some primates produce milk that is rich in nutrients and fat so that infants are weaned quickly. Other primates have milk with a higher composition of water, and their infants are generally weaned over a longer period of time. There are pros and cons to each strategy. As a mother, it is in her best interest to produce as many infants that survive to adulthood as possible over the course of her lifetime. So, on the one hand she wants to wean her infant as soon as possible so that she can prepare for the next mating season. On the other hand, she wants her infant to survive and the juvenile period of development is a risky time for a primate. Timing reproduction so that birth is during a period of optimal fruit availability may make sense for some primates; timing infant weaning during a period of optimal fruit availability makes sense for others. Having dilute milk or having fatty milk is beneficial in different scenarios. Multiple factors come into play. I’m just happy I get to see more cute infants, although I have to confess, I think sifakas have the least attractive infants of the lemurs I’ve seen so far.

True or false: Primate infants consuming milk high in fat are generally weaned over a long period of time.

Critical thinking: How does this post connect back to the concept of biological fitness discussed earlier?

Critical thinking: During what season do humans give birth? Why do you think this is?
Infant sifaka grooming

Answer: False

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